diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index a403d6f..55f5351 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -3057,6 +3057,197 @@
       always produces a new object, even if no changes were made.
 
 
+.. _bytes-formatting:
+
+``printf``-style Bytes Formatting
+----------------------------------
+
+.. index::
+   single: formatting, bytes (%)
+   single: formatting, bytearray (%)
+   single: interpolation, bytes (%)
+   single: interpolation, bytearray (%)
+   single: bytes; formatting
+   single: bytearray; formatting
+   single: bytes; interpolation
+   single: bytearray; interpolation
+   single: printf-style formatting
+   single: sprintf-style formatting
+   single: % formatting
+   single: % interpolation
+
+.. note::
+
+   The formatting operations described here exhibit a variety of quirks that
+   lead to a number of common errors (such as failing to display tuples and
+   dictionaries correctly).  If the value being printed may be a tuple or
+   dictionary, wrap it in a tuple.
+
+Bytes objects (``bytes``/``bytearray``) have one unique built-in operation:
+the ``%`` operator (modulo).
+This is also known as the bytes *formatting* or *interpolation* operator.
+Given ``format % values`` (where *format* is a bytes object), ``%`` conversion
+specifications in *format* are replaced with zero or more elements of *values*.
+The effect is similar to using the :c:func:`sprintf` in the C language.
+
+If *format* requires a single argument, *values* may be a single non-tuple
+object. [5]_  Otherwise, *values* must be a tuple with exactly the number of
+items specified by the format bytes object, or a single mapping object (for
+example, a dictionary).
+
+A conversion specifier contains two or more characters and has the following
+components, which must occur in this order:
+
+#. The ``'%'`` character, which marks the start of the specifier.
+
+#. Mapping key (optional), consisting of a parenthesised sequence of characters
+   (for example, ``(somename)``).
+
+#. Conversion flags (optional), which affect the result of some conversion
+   types.
+
+#. Minimum field width (optional).  If specified as an ``'*'`` (asterisk), the
+   actual width is read from the next element of the tuple in *values*, and the
+   object to convert comes after the minimum field width and optional precision.
+
+#. Precision (optional), given as a ``'.'`` (dot) followed by the precision.  If
+   specified as ``'*'`` (an asterisk), the actual precision is read from the next
+   element of the tuple in *values*, and the value to convert comes after the
+   precision.
+
+#. Length modifier (optional).
+
+#. Conversion type.
+
+When the right argument is a dictionary (or other mapping type), then the
+formats in the bytes object *must* include a parenthesised mapping key into that
+dictionary inserted immediately after the ``'%'`` character. The mapping key
+selects the value to be formatted from the mapping.  For example:
+
+   >>> print(b'%(language)s has %(number)03d quote types.' %
+   ...       {b'language': b"Python", b"number": 2})
+   b'Python has 002 quote types.'
+
+In this case no ``*`` specifiers may occur in a format (since they require a
+sequential parameter list).
+
+The conversion flag characters are:
+
++---------+---------------------------------------------------------------------+
+| Flag    | Meaning                                                             |
++=========+=====================================================================+
+| ``'#'`` | The value conversion will use the "alternate form" (where defined   |
+|         | below).                                                             |
++---------+---------------------------------------------------------------------+
+| ``'0'`` | The conversion will be zero padded for numeric values.              |
++---------+---------------------------------------------------------------------+
+| ``'-'`` | The converted value is left adjusted (overrides the ``'0'``         |
+|         | conversion if both are given).                                      |
++---------+---------------------------------------------------------------------+
+| ``' '`` | (a space) A blank should be left before a positive number (or empty |
+|         | string) produced by a signed conversion.                            |
++---------+---------------------------------------------------------------------+
+| ``'+'`` | A sign character (``'+'`` or ``'-'``) will precede the conversion   |
+|         | (overrides a "space" flag).                                         |
++---------+---------------------------------------------------------------------+
+
+A length modifier (``h``, ``l``, or ``L``) may be present, but is ignored as it
+is not necessary for Python -- so e.g. ``%ld`` is identical to ``%d``.
+
+The conversion types are:
+
++------------+-----------------------------------------------------+-------+
+| Conversion | Meaning                                             | Notes |
++============+=====================================================+=======+
+| ``'d'``    | Signed integer decimal.                             |       |
++------------+-----------------------------------------------------+-------+
+| ``'i'``    | Signed integer decimal.                             |       |
++------------+-----------------------------------------------------+-------+
+| ``'o'``    | Signed octal value.                                 | \(1)  |
++------------+-----------------------------------------------------+-------+
+| ``'u'``    | Obsolete type -- it is identical to ``'d'``.        | \(7)  |
++------------+-----------------------------------------------------+-------+
+| ``'x'``    | Signed hexadecimal (lowercase).                     | \(2)  |
++------------+-----------------------------------------------------+-------+
+| ``'X'``    | Signed hexadecimal (uppercase).                     | \(2)  |
++------------+-----------------------------------------------------+-------+
+| ``'e'``    | Floating point exponential format (lowercase).      | \(3)  |
++------------+-----------------------------------------------------+-------+
+| ``'E'``    | Floating point exponential format (uppercase).      | \(3)  |
++------------+-----------------------------------------------------+-------+
+| ``'f'``    | Floating point decimal format.                      | \(3)  |
++------------+-----------------------------------------------------+-------+
+| ``'F'``    | Floating point decimal format.                      | \(3)  |
++------------+-----------------------------------------------------+-------+
+| ``'g'``    | Floating point format. Uses lowercase exponential   | \(4)  |
+|            | format if exponent is less than -4 or not less than |       |
+|            | precision, decimal format otherwise.                |       |
++------------+-----------------------------------------------------+-------+
+| ``'G'``    | Floating point format. Uses uppercase exponential   | \(4)  |
+|            | format if exponent is less than -4 or not less than |       |
+|            | precision, decimal format otherwise.                |       |
++------------+-----------------------------------------------------+-------+
+| ``'c'``    | Single byte (accepts integer or single              |       |
+|            | byte objects).                                      |       |
++------------+-----------------------------------------------------+-------+
+| ``'b'``    | Bytes (any object that follows the                  | \(5)  |
+|            | :ref:`buffer protocol <bufferobjects>` or has       |       |
+|            | :meth:`__bytes__`).                                 |       |
++------------+-----------------------------------------------------+-------+
+| ``'s'``    | ``'s'`` is an alias for ``'b'`` and should only     | \(6)  |
+|            | be used for Python2/3 code bases.                   |       |
++------------+-----------------------------------------------------+-------+
+| ``'a'``    | Bytes (converts any Python object using             | \(5)  |
+|            | ``repr(obj).encode('ascii','backslashreplace)``).   |       |
++------------+-----------------------------------------------------+-------+
+| ``'%'``    | No argument is converted, results in a ``'%'``      |       |
+|            | character in the result.                            |       |
++------------+-----------------------------------------------------+-------+
+
+Notes:
+
+(1)
+   The alternate form causes a leading zero (``'0'``) to be inserted between
+   left-hand padding and the formatting of the number if the leading character
+   of the result is not already a zero.
+
+(2)
+   The alternate form causes a leading ``'0x'`` or ``'0X'`` (depending on whether
+   the ``'x'`` or ``'X'`` format was used) to be inserted between left-hand padding
+   and the formatting of the number if the leading character of the result is not
+   already a zero.
+
+(3)
+   The alternate form causes the result to always contain a decimal point, even if
+   no digits follow it.
+
+   The precision determines the number of digits after the decimal point and
+   defaults to 6.
+
+(4)
+   The alternate form causes the result to always contain a decimal point, and
+   trailing zeroes are not removed as they would otherwise be.
+
+   The precision determines the number of significant digits before and after the
+   decimal point and defaults to 6.
+
+(5)
+   If precision is ``N``, the output is truncated to ``N`` characters.
+
+(6)
+   ``b'%s'`` is deprecated, but will not be removed during the 3.x series.
+
+(7)
+   See :pep:`237`.
+
+.. note::
+
+   The bytearray version of this method does *not* operate in place - it
+   always produces a new object, even if no changes were made.
+
+.. seealso:: :pep:`461`.
+.. versionadded:: 3.5
+
 .. _typememoryview:
 
 Memory Views
diff --git a/Include/bytesobject.h b/Include/bytesobject.h
index 0ee8d36..e379bac 100644
--- a/Include/bytesobject.h
+++ b/Include/bytesobject.h
@@ -62,6 +62,7 @@
 PyAPI_FUNC(void) PyBytes_ConcatAndDel(PyObject **, PyObject *);
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(int) _PyBytes_Resize(PyObject **, Py_ssize_t);
+PyAPI_FUNC(PyObject *) _PyBytes_Format(PyObject *, PyObject *);
 #endif
 PyAPI_FUNC(PyObject *) PyBytes_DecodeEscape(const char *, Py_ssize_t,
 						   const char *, Py_ssize_t,
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 729f584..d2ffabe 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -2245,6 +2245,8 @@
     Py_UNICODE c
     );
 
+PyAPI_FUNC(PyObject*) _PyUnicode_FormatLong(PyObject *, int, int, int);
+
 /* Create a copy of a unicode string ending with a nul character. Return NULL
    and raise a MemoryError exception on memory allocation failure, otherwise
    return a new allocated buffer (use PyMem_Free() to free the buffer). */
diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
index e8dde5d..995a908 100644
--- a/Lib/test/test_bytes.py
+++ b/Lib/test/test_bytes.py
@@ -461,6 +461,28 @@
         self.assertEqual(b.rindex(i, 3, 9), 7)
         self.assertRaises(ValueError, b.rindex, w, 1, 3)
 
+    def test_mod(self):
+        b = b'hello, %b!'
+        orig = b
+        b = b % b'world'
+        self.assertEqual(b, b'hello, world!')
+        self.assertEqual(orig, b'hello, %b!')
+        self.assertFalse(b is orig)
+        b = b'%s / 100 = %d%%'
+        a = b % (b'seventy-nine', 79)
+        self.assertEquals(a, b'seventy-nine / 100 = 79%')
+
+    def test_imod(self):
+        b = b'hello, %b!'
+        orig = b
+        b %= b'world'
+        self.assertEqual(b, b'hello, world!')
+        self.assertEqual(orig, b'hello, %b!')
+        self.assertFalse(b is orig)
+        b = b'%s / 100 = %d%%'
+        b %= (b'seventy-nine', 79)
+        self.assertEquals(b, b'seventy-nine / 100 = 79%')
+
     def test_replace(self):
         b = self.type2test(b'mississippi')
         self.assertEqual(b.replace(b'i', b'a'), b'massassappa')
@@ -990,6 +1012,28 @@
         b[8:] = b
         self.assertEqual(b, bytearray(list(range(8)) + list(range(256))))
 
+    def test_mod(self):
+        b = bytearray(b'hello, %b!')
+        orig = b
+        b = b % b'world'
+        self.assertEqual(b, b'hello, world!')
+        self.assertEqual(orig, bytearray(b'hello, %b!'))
+        self.assertFalse(b is orig)
+        b = bytearray(b'%s / 100 = %d%%')
+        a = b % (b'seventy-nine', 79)
+        self.assertEquals(a, bytearray(b'seventy-nine / 100 = 79%'))
+
+    def test_imod(self):
+        b = bytearray(b'hello, %b!')
+        orig = b
+        b %= b'world'
+        self.assertEqual(b, b'hello, world!')
+        self.assertEqual(orig, bytearray(b'hello, %b!'))
+        self.assertFalse(b is orig)
+        b = bytearray(b'%s / 100 = %d%%')
+        b %= (b'seventy-nine', 79)
+        self.assertEquals(b, bytearray(b'seventy-nine / 100 = 79%'))
+
     def test_iconcat(self):
         b = bytearray(b"abc")
         b1 = b
diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
index 631bf35..9d1f5d3 100644
--- a/Lib/test/test_format.py
+++ b/Lib/test/test_format.py
@@ -9,7 +9,7 @@
 # test string formatting operator (I am not sure if this is being tested
 # elsewhere but, surely, some of the given cases are *not* tested because
 # they crash python)
-# test on unicode strings as well
+# test on bytes object as well
 
 def testformat(formatstr, args, output=None, limit=None, overflowok=False):
     if verbose:
@@ -46,181 +46,209 @@
             if verbose:
                 print('yes')
 
+def testcommon(formatstr, args, output=None, limit=None, overflowok=False):
+    # if formatstr is a str, test str, bytes, and bytearray;
+    # otherwise, test bytes and bytearry
+    if isinstance(formatstr, str):
+        testformat(formatstr, args, output, limit, overflowok)
+        b_format = formatstr.encode('ascii')
+    else:
+        b_format = formatstr
+    ba_format = bytearray(b_format)
+    b_args = []
+    if not isinstance(args, tuple):
+        args = (args, )
+    b_args = tuple(args)
+    if output is None:
+        b_output = ba_output = None
+    else:
+        if isinstance(output, str):
+            b_output = output.encode('ascii')
+        else:
+            b_output = output
+        ba_output = bytearray(b_output)
+    testformat(b_format, b_args, b_output, limit, overflowok)
+    testformat(ba_format, b_args, ba_output, limit, overflowok)
+
 
 class FormatTest(unittest.TestCase):
-    def test_format(self):
-        testformat("%.1d", (1,), "1")
-        testformat("%.*d", (sys.maxsize,1), overflowok=True)  # expect overflow
-        testformat("%.100d", (1,), '00000000000000000000000000000000000000'
+
+    def test_common_format(self):
+        # test the format identifiers that work the same across
+        # str, bytes, and bytearrays (integer, float, oct, hex)
+        testcommon("%.1d", (1,), "1")
+        testcommon("%.*d", (sys.maxsize,1), overflowok=True)  # expect overflow
+        testcommon("%.100d", (1,), '00000000000000000000000000000000000000'
                  '000000000000000000000000000000000000000000000000000000'
                  '00000001', overflowok=True)
-        testformat("%#.117x", (1,), '0x00000000000000000000000000000000000'
+        testcommon("%#.117x", (1,), '0x00000000000000000000000000000000000'
                  '000000000000000000000000000000000000000000000000000000'
                  '0000000000000000000000000001',
                  overflowok=True)
-        testformat("%#.118x", (1,), '0x00000000000000000000000000000000000'
+        testcommon("%#.118x", (1,), '0x00000000000000000000000000000000000'
                  '000000000000000000000000000000000000000000000000000000'
                  '00000000000000000000000000001',
                  overflowok=True)
 
-        testformat("%f", (1.0,), "1.000000")
+        testcommon("%f", (1.0,), "1.000000")
         # these are trying to test the limits of the internal magic-number-length
         # formatting buffer, if that number changes then these tests are less
         # effective
-        testformat("%#.*g", (109, -1.e+49/3.))
-        testformat("%#.*g", (110, -1.e+49/3.))
-        testformat("%#.*g", (110, -1.e+100/3.))
+        testcommon("%#.*g", (109, -1.e+49/3.))
+        testcommon("%#.*g", (110, -1.e+49/3.))
+        testcommon("%#.*g", (110, -1.e+100/3.))
         # test some ridiculously large precision, expect overflow
-        testformat('%12.*f', (123456, 1.0))
+        testcommon('%12.*f', (123456, 1.0))
 
         # check for internal overflow validation on length of precision
         # these tests should no longer cause overflow in Python
         # 2.7/3.1 and later.
-        testformat("%#.*g", (110, -1.e+100/3.))
-        testformat("%#.*G", (110, -1.e+100/3.))
-        testformat("%#.*f", (110, -1.e+100/3.))
-        testformat("%#.*F", (110, -1.e+100/3.))
+        testcommon("%#.*g", (110, -1.e+100/3.))
+        testcommon("%#.*G", (110, -1.e+100/3.))
+        testcommon("%#.*f", (110, -1.e+100/3.))
+        testcommon("%#.*F", (110, -1.e+100/3.))
         # Formatting of integers. Overflow is not ok
-        testformat("%x", 10, "a")
-        testformat("%x", 100000000000, "174876e800")
-        testformat("%o", 10, "12")
-        testformat("%o", 100000000000, "1351035564000")
-        testformat("%d", 10, "10")
-        testformat("%d", 100000000000, "100000000000")
+        testcommon("%x", 10, "a")
+        testcommon("%x", 100000000000, "174876e800")
+        testcommon("%o", 10, "12")
+        testcommon("%o", 100000000000, "1351035564000")
+        testcommon("%d", 10, "10")
+        testcommon("%d", 100000000000, "100000000000")
         big = 123456789012345678901234567890
-        testformat("%d", big, "123456789012345678901234567890")
-        testformat("%d", -big, "-123456789012345678901234567890")
-        testformat("%5d", -big, "-123456789012345678901234567890")
-        testformat("%31d", -big, "-123456789012345678901234567890")
-        testformat("%32d", -big, " -123456789012345678901234567890")
-        testformat("%-32d", -big, "-123456789012345678901234567890 ")
-        testformat("%032d", -big, "-0123456789012345678901234567890")
-        testformat("%-032d", -big, "-123456789012345678901234567890 ")
-        testformat("%034d", -big, "-000123456789012345678901234567890")
-        testformat("%034d", big, "0000123456789012345678901234567890")
-        testformat("%0+34d", big, "+000123456789012345678901234567890")
-        testformat("%+34d", big, "   +123456789012345678901234567890")
-        testformat("%34d", big, "    123456789012345678901234567890")
-        testformat("%.2d", big, "123456789012345678901234567890")
-        testformat("%.30d", big, "123456789012345678901234567890")
-        testformat("%.31d", big, "0123456789012345678901234567890")
-        testformat("%32.31d", big, " 0123456789012345678901234567890")
-        testformat("%d", float(big), "123456________________________", 6)
+        testcommon("%d", big, "123456789012345678901234567890")
+        testcommon("%d", -big, "-123456789012345678901234567890")
+        testcommon("%5d", -big, "-123456789012345678901234567890")
+        testcommon("%31d", -big, "-123456789012345678901234567890")
+        testcommon("%32d", -big, " -123456789012345678901234567890")
+        testcommon("%-32d", -big, "-123456789012345678901234567890 ")
+        testcommon("%032d", -big, "-0123456789012345678901234567890")
+        testcommon("%-032d", -big, "-123456789012345678901234567890 ")
+        testcommon("%034d", -big, "-000123456789012345678901234567890")
+        testcommon("%034d", big, "0000123456789012345678901234567890")
+        testcommon("%0+34d", big, "+000123456789012345678901234567890")
+        testcommon("%+34d", big, "   +123456789012345678901234567890")
+        testcommon("%34d", big, "    123456789012345678901234567890")
+        testcommon("%.2d", big, "123456789012345678901234567890")
+        testcommon("%.30d", big, "123456789012345678901234567890")
+        testcommon("%.31d", big, "0123456789012345678901234567890")
+        testcommon("%32.31d", big, " 0123456789012345678901234567890")
+        testcommon("%d", float(big), "123456________________________", 6)
         big = 0x1234567890abcdef12345  # 21 hex digits
-        testformat("%x", big, "1234567890abcdef12345")
-        testformat("%x", -big, "-1234567890abcdef12345")
-        testformat("%5x", -big, "-1234567890abcdef12345")
-        testformat("%22x", -big, "-1234567890abcdef12345")
-        testformat("%23x", -big, " -1234567890abcdef12345")
-        testformat("%-23x", -big, "-1234567890abcdef12345 ")
-        testformat("%023x", -big, "-01234567890abcdef12345")
-        testformat("%-023x", -big, "-1234567890abcdef12345 ")
-        testformat("%025x", -big, "-0001234567890abcdef12345")
-        testformat("%025x", big, "00001234567890abcdef12345")
-        testformat("%0+25x", big, "+0001234567890abcdef12345")
-        testformat("%+25x", big, "   +1234567890abcdef12345")
-        testformat("%25x", big, "    1234567890abcdef12345")
-        testformat("%.2x", big, "1234567890abcdef12345")
-        testformat("%.21x", big, "1234567890abcdef12345")
-        testformat("%.22x", big, "01234567890abcdef12345")
-        testformat("%23.22x", big, " 01234567890abcdef12345")
-        testformat("%-23.22x", big, "01234567890abcdef12345 ")
-        testformat("%X", big, "1234567890ABCDEF12345")
-        testformat("%#X", big, "0X1234567890ABCDEF12345")
-        testformat("%#x", big, "0x1234567890abcdef12345")
-        testformat("%#x", -big, "-0x1234567890abcdef12345")
-        testformat("%#.23x", -big, "-0x001234567890abcdef12345")
-        testformat("%#+.23x", big, "+0x001234567890abcdef12345")
-        testformat("%# .23x", big, " 0x001234567890abcdef12345")
-        testformat("%#+.23X", big, "+0X001234567890ABCDEF12345")
-        testformat("%#-+.23X", big, "+0X001234567890ABCDEF12345")
-        testformat("%#-+26.23X", big, "+0X001234567890ABCDEF12345")
-        testformat("%#-+27.23X", big, "+0X001234567890ABCDEF12345 ")
-        testformat("%#+27.23X", big, " +0X001234567890ABCDEF12345")
+        testcommon("%x", big, "1234567890abcdef12345")
+        testcommon("%x", -big, "-1234567890abcdef12345")
+        testcommon("%5x", -big, "-1234567890abcdef12345")
+        testcommon("%22x", -big, "-1234567890abcdef12345")
+        testcommon("%23x", -big, " -1234567890abcdef12345")
+        testcommon("%-23x", -big, "-1234567890abcdef12345 ")
+        testcommon("%023x", -big, "-01234567890abcdef12345")
+        testcommon("%-023x", -big, "-1234567890abcdef12345 ")
+        testcommon("%025x", -big, "-0001234567890abcdef12345")
+        testcommon("%025x", big, "00001234567890abcdef12345")
+        testcommon("%0+25x", big, "+0001234567890abcdef12345")
+        testcommon("%+25x", big, "   +1234567890abcdef12345")
+        testcommon("%25x", big, "    1234567890abcdef12345")
+        testcommon("%.2x", big, "1234567890abcdef12345")
+        testcommon("%.21x", big, "1234567890abcdef12345")
+        testcommon("%.22x", big, "01234567890abcdef12345")
+        testcommon("%23.22x", big, " 01234567890abcdef12345")
+        testcommon("%-23.22x", big, "01234567890abcdef12345 ")
+        testcommon("%X", big, "1234567890ABCDEF12345")
+        testcommon("%#X", big, "0X1234567890ABCDEF12345")
+        testcommon("%#x", big, "0x1234567890abcdef12345")
+        testcommon("%#x", -big, "-0x1234567890abcdef12345")
+        testcommon("%#.23x", -big, "-0x001234567890abcdef12345")
+        testcommon("%#+.23x", big, "+0x001234567890abcdef12345")
+        testcommon("%# .23x", big, " 0x001234567890abcdef12345")
+        testcommon("%#+.23X", big, "+0X001234567890ABCDEF12345")
+        testcommon("%#-+.23X", big, "+0X001234567890ABCDEF12345")
+        testcommon("%#-+26.23X", big, "+0X001234567890ABCDEF12345")
+        testcommon("%#-+27.23X", big, "+0X001234567890ABCDEF12345 ")
+        testcommon("%#+27.23X", big, " +0X001234567890ABCDEF12345")
         # next one gets two leading zeroes from precision, and another from the
         # 0 flag and the width
-        testformat("%#+027.23X", big, "+0X0001234567890ABCDEF12345")
+        testcommon("%#+027.23X", big, "+0X0001234567890ABCDEF12345")
         # same, except no 0 flag
-        testformat("%#+27.23X", big, " +0X001234567890ABCDEF12345")
+        testcommon("%#+27.23X", big, " +0X001234567890ABCDEF12345")
         big = 0o12345670123456701234567012345670  # 32 octal digits
-        testformat("%o", big, "12345670123456701234567012345670")
-        testformat("%o", -big, "-12345670123456701234567012345670")
-        testformat("%5o", -big, "-12345670123456701234567012345670")
-        testformat("%33o", -big, "-12345670123456701234567012345670")
-        testformat("%34o", -big, " -12345670123456701234567012345670")
-        testformat("%-34o", -big, "-12345670123456701234567012345670 ")
-        testformat("%034o", -big, "-012345670123456701234567012345670")
-        testformat("%-034o", -big, "-12345670123456701234567012345670 ")
-        testformat("%036o", -big, "-00012345670123456701234567012345670")
-        testformat("%036o", big, "000012345670123456701234567012345670")
-        testformat("%0+36o", big, "+00012345670123456701234567012345670")
-        testformat("%+36o", big, "   +12345670123456701234567012345670")
-        testformat("%36o", big, "    12345670123456701234567012345670")
-        testformat("%.2o", big, "12345670123456701234567012345670")
-        testformat("%.32o", big, "12345670123456701234567012345670")
-        testformat("%.33o", big, "012345670123456701234567012345670")
-        testformat("%34.33o", big, " 012345670123456701234567012345670")
-        testformat("%-34.33o", big, "012345670123456701234567012345670 ")
-        testformat("%o", big, "12345670123456701234567012345670")
-        testformat("%#o", big, "0o12345670123456701234567012345670")
-        testformat("%#o", -big, "-0o12345670123456701234567012345670")
-        testformat("%#.34o", -big, "-0o0012345670123456701234567012345670")
-        testformat("%#+.34o", big, "+0o0012345670123456701234567012345670")
-        testformat("%# .34o", big, " 0o0012345670123456701234567012345670")
-        testformat("%#+.34o", big, "+0o0012345670123456701234567012345670")
-        testformat("%#-+.34o", big, "+0o0012345670123456701234567012345670")
-        testformat("%#-+37.34o", big, "+0o0012345670123456701234567012345670")
-        testformat("%#+37.34o", big, "+0o0012345670123456701234567012345670")
+        testcommon("%o", big, "12345670123456701234567012345670")
+        testcommon("%o", -big, "-12345670123456701234567012345670")
+        testcommon("%5o", -big, "-12345670123456701234567012345670")
+        testcommon("%33o", -big, "-12345670123456701234567012345670")
+        testcommon("%34o", -big, " -12345670123456701234567012345670")
+        testcommon("%-34o", -big, "-12345670123456701234567012345670 ")
+        testcommon("%034o", -big, "-012345670123456701234567012345670")
+        testcommon("%-034o", -big, "-12345670123456701234567012345670 ")
+        testcommon("%036o", -big, "-00012345670123456701234567012345670")
+        testcommon("%036o", big, "000012345670123456701234567012345670")
+        testcommon("%0+36o", big, "+00012345670123456701234567012345670")
+        testcommon("%+36o", big, "   +12345670123456701234567012345670")
+        testcommon("%36o", big, "    12345670123456701234567012345670")
+        testcommon("%.2o", big, "12345670123456701234567012345670")
+        testcommon("%.32o", big, "12345670123456701234567012345670")
+        testcommon("%.33o", big, "012345670123456701234567012345670")
+        testcommon("%34.33o", big, " 012345670123456701234567012345670")
+        testcommon("%-34.33o", big, "012345670123456701234567012345670 ")
+        testcommon("%o", big, "12345670123456701234567012345670")
+        testcommon("%#o", big, "0o12345670123456701234567012345670")
+        testcommon("%#o", -big, "-0o12345670123456701234567012345670")
+        testcommon("%#.34o", -big, "-0o0012345670123456701234567012345670")
+        testcommon("%#+.34o", big, "+0o0012345670123456701234567012345670")
+        testcommon("%# .34o", big, " 0o0012345670123456701234567012345670")
+        testcommon("%#+.34o", big, "+0o0012345670123456701234567012345670")
+        testcommon("%#-+.34o", big, "+0o0012345670123456701234567012345670")
+        testcommon("%#-+37.34o", big, "+0o0012345670123456701234567012345670")
+        testcommon("%#+37.34o", big, "+0o0012345670123456701234567012345670")
         # next one gets one leading zero from precision
-        testformat("%.33o", big, "012345670123456701234567012345670")
+        testcommon("%.33o", big, "012345670123456701234567012345670")
         # base marker shouldn't change that, since "0" is redundant
-        testformat("%#.33o", big, "0o012345670123456701234567012345670")
+        testcommon("%#.33o", big, "0o012345670123456701234567012345670")
         # but reduce precision, and base marker should add a zero
-        testformat("%#.32o", big, "0o12345670123456701234567012345670")
+        testcommon("%#.32o", big, "0o12345670123456701234567012345670")
         # one leading zero from precision, and another from "0" flag & width
-        testformat("%034.33o", big, "0012345670123456701234567012345670")
+        testcommon("%034.33o", big, "0012345670123456701234567012345670")
         # base marker shouldn't change that
-        testformat("%0#34.33o", big, "0o012345670123456701234567012345670")
+        testcommon("%0#34.33o", big, "0o012345670123456701234567012345670")
         # Some small ints, in both Python int and flavors).
-        testformat("%d", 42, "42")
-        testformat("%d", -42, "-42")
-        testformat("%d", 42, "42")
-        testformat("%d", -42, "-42")
-        testformat("%d", 42.0, "42")
-        testformat("%#x", 1, "0x1")
-        testformat("%#x", 1, "0x1")
-        testformat("%#X", 1, "0X1")
-        testformat("%#X", 1, "0X1")
-        testformat("%#o", 1, "0o1")
-        testformat("%#o", 1, "0o1")
-        testformat("%#o", 0, "0o0")
-        testformat("%#o", 0, "0o0")
-        testformat("%o", 0, "0")
-        testformat("%o", 0, "0")
-        testformat("%d", 0, "0")
-        testformat("%d", 0, "0")
-        testformat("%#x", 0, "0x0")
-        testformat("%#x", 0, "0x0")
-        testformat("%#X", 0, "0X0")
-        testformat("%#X", 0, "0X0")
-        testformat("%x", 0x42, "42")
-        testformat("%x", -0x42, "-42")
-        testformat("%x", 0x42, "42")
-        testformat("%x", -0x42, "-42")
-        testformat("%o", 0o42, "42")
-        testformat("%o", -0o42, "-42")
-        testformat("%o", 0o42, "42")
-        testformat("%o", -0o42, "-42")
+        testcommon("%d", 42, "42")
+        testcommon("%d", -42, "-42")
+        testcommon("%d", 42, "42")
+        testcommon("%d", -42, "-42")
+        testcommon("%d", 42.0, "42")
+        testcommon("%#x", 1, "0x1")
+        testcommon("%#x", 1, "0x1")
+        testcommon("%#X", 1, "0X1")
+        testcommon("%#X", 1, "0X1")
+        testcommon("%#o", 1, "0o1")
+        testcommon("%#o", 1, "0o1")
+        testcommon("%#o", 0, "0o0")
+        testcommon("%#o", 0, "0o0")
+        testcommon("%o", 0, "0")
+        testcommon("%o", 0, "0")
+        testcommon("%d", 0, "0")
+        testcommon("%d", 0, "0")
+        testcommon("%#x", 0, "0x0")
+        testcommon("%#x", 0, "0x0")
+        testcommon("%#X", 0, "0X0")
+        testcommon("%#X", 0, "0X0")
+        testcommon("%x", 0x42, "42")
+        testcommon("%x", -0x42, "-42")
+        testcommon("%x", 0x42, "42")
+        testcommon("%x", -0x42, "-42")
+        testcommon("%o", 0o42, "42")
+        testcommon("%o", -0o42, "-42")
+        testcommon("%o", 0o42, "42")
+        testcommon("%o", -0o42, "-42")
+        # alternate float formatting
+        testcommon('%g', 1.1, '1.1')
+        testcommon('%#g', 1.1, '1.10000')
+
+    def test_str_format(self):
         testformat("%r", "\u0378", "'\\u0378'")  # non printable
         testformat("%a", "\u0378", "'\\u0378'")  # non printable
         testformat("%r", "\u0374", "'\u0374'")   # printable
         testformat("%a", "\u0374", "'\\u0374'")  # printable
 
-        # alternate float formatting
-        testformat('%g', 1.1, '1.1')
-        testformat('%#g', 1.1, '1.10000')
-
-        # Test exception for unknown format characters
+        # Test exception for unknown format characters, etc.
         if verbose:
             print('Testing exceptions')
         def test_exc(formatstr, args, exception, excmsg):
@@ -247,8 +275,83 @@
         test_exc('%g', '1', TypeError, "a float is required")
         test_exc('no format', '1', TypeError,
                  "not all arguments converted during string formatting")
-        test_exc('no format', '1', TypeError,
-                 "not all arguments converted during string formatting")
+
+        if maxsize == 2**31-1:
+            # crashes 2.2.1 and earlier:
+            try:
+                "%*d"%(maxsize, -127)
+            except MemoryError:
+                pass
+            else:
+                raise TestFailed('"%*d"%(maxsize, -127) should fail')
+
+    def test_bytes_and_bytearray_format(self):
+        # %c will insert a single byte, either from an int in range(256), or
+        # from a bytes argument of length 1, not from a str.
+        testcommon(b"%c", 7, b"\x07")
+        testcommon(b"%c", b"Z", b"Z")
+        testcommon(b"%c", bytearray(b"Z"), b"Z")
+        # %b will insert a series of bytes, either from a type that supports
+        # the Py_buffer protocol, or something that has a __bytes__ method
+        class FakeBytes(object):
+            def __bytes__(self):
+                return b'123'
+        fb = FakeBytes()
+        testcommon(b"%b", b"abc", b"abc")
+        testcommon(b"%b", bytearray(b"def"), b"def")
+        testcommon(b"%b", fb, b"123")
+        # # %s is an alias for %b -- should only be used for Py2/3 code
+        testcommon(b"%s", b"abc", b"abc")
+        testcommon(b"%s", bytearray(b"def"), b"def")
+        testcommon(b"%s", fb, b"123")
+        # %a will give the equivalent of
+        # repr(some_obj).encode('ascii', 'backslashreplace')
+        testcommon(b"%a", 3.14, b"3.14")
+        testcommon(b"%a", b"ghi", b"b'ghi'")
+        testcommon(b"%a", "jkl", b"'jkl'")
+        testcommon(b"%a", "\u0544", b"'\\u0544'")
+
+        # Test exception for unknown format characters, etc.
+        if verbose:
+            print('Testing exceptions')
+        def test_exc(formatstr, args, exception, excmsg):
+            try:
+                testformat(formatstr, args)
+            except exception as exc:
+                if str(exc) == excmsg:
+                    if verbose:
+                        print("yes")
+                else:
+                    if verbose: print('no')
+                    print('Unexpected ', exception, ':', repr(str(exc)))
+            except:
+                if verbose: print('no')
+                print('Unexpected exception')
+                raise
+            else:
+                raise TestFailed('did not get expected exception: %s' % excmsg)
+        test_exc(b'%d', '1', TypeError,
+                "%d format: a number is required, not str")
+        test_exc(b'%d', b'1', TypeError,
+                "%d format: a number is required, not bytes")
+        test_exc(b'%g', '1', TypeError, "float argument required, not str")
+        test_exc(b'%g', b'1', TypeError, "float argument required, not bytes")
+        test_exc(b'no format', 7, TypeError,
+                 "not all arguments converted during bytes formatting")
+        test_exc(b'no format', b'1', TypeError,
+                 "not all arguments converted during bytes formatting")
+        test_exc(b'no format', bytearray(b'1'), TypeError,
+                 "not all arguments converted during bytes formatting")
+        test_exc(b"%c", 256, TypeError,
+                "%c requires an integer in range(256) or a single byte")
+        test_exc(b"%c", b"Za", TypeError,
+                "%c requires an integer in range(256) or a single byte")
+        test_exc(b"%c", "Yb", TypeError,
+                "%c requires an integer in range(256) or a single byte")
+        test_exc(b"%b", "Xc", TypeError,
+                "%b requires bytes, or an object that implements __bytes__, not 'str'")
+        test_exc(b"%s", "Wd", TypeError,
+                "%b requires bytes, or an object that implements __bytes__, not 'str'")
 
         if maxsize == 2**31-1:
             # crashes 2.2.1 and earlier:
diff --git a/Misc/NEWS b/Misc/NEWS
index 91643f3..127a747 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -14,6 +14,9 @@
   atomic memory access if available. Patch written by Vitor de Lima and Gustavo
   Temple.
 
+- Issue #20284: %-interpolation (aka printf) formatting added for bytes and
+  bytearray.
+
 - Issue #23048: Fix jumping out of an infinite while loop in the pdb.
 
 - Issue #20335: bytes constructor now raises TypeError when encoding or errors
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 323c985..9ab60f2 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -686,8 +686,9 @@
     Py_DECREF(meth);
 
     if (result && !PyUnicode_Check(result)) {
-        PyErr_SetString(PyExc_TypeError,
-                        "__format__ method did not return string");
+        PyErr_Format(PyExc_TypeError,
+             "__format__ must return a str, not %.200s",
+             Py_TYPE(result)->tp_name);
         Py_DECREF(result);
         result = NULL;
         goto done;
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 47d480f..b9a87d6 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -4,6 +4,7 @@
 #include "Python.h"
 #include "structmember.h"
 #include "bytes_methods.h"
+#include "bytesobject.h"
 
 /*[clinic input]
 class bytearray "PyByteArrayObject *" "&PyByteArray_Type"
@@ -294,6 +295,31 @@
     return (PyObject *)result;
 }
 
+static PyObject *
+bytearray_format(PyByteArrayObject *self, PyObject *args)
+{
+    PyObject *bytes_in, *bytes_out, *res;
+    char *bytestring;
+
+    if (self == NULL || !PyByteArray_Check(self) || args == NULL) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    bytestring = PyByteArray_AS_STRING(self);
+    bytes_in = PyBytes_FromString(bytestring);
+    if (bytes_in == NULL)
+        return NULL;
+    bytes_out = _PyBytes_Format(bytes_in, args);
+    Py_DECREF(bytes_in);
+    if (bytes_out == NULL)
+        return NULL;
+    res = PyByteArray_FromObject(bytes_out);
+    Py_DECREF(bytes_out);
+    if (res == NULL)
+        return NULL;
+    return res;
+}
+
 /* Functions stuffed into the type object */
 
 static Py_ssize_t
@@ -3723,6 +3749,21 @@
     {NULL}
 };
 
+static PyObject *
+bytearray_mod(PyObject *v, PyObject *w)
+{
+    if (!PyByteArray_Check(v))
+        Py_RETURN_NOTIMPLEMENTED;
+    return bytearray_format((PyByteArrayObject *)v, w);
+}
+
+static PyNumberMethods bytearray_as_number = {
+    0,              /*nb_add*/
+    0,              /*nb_subtract*/
+    0,              /*nb_multiply*/
+    bytearray_mod,  /*nb_remainder*/
+};
+
 PyDoc_STRVAR(bytearray_doc,
 "bytearray(iterable_of_ints) -> bytearray\n\
 bytearray(string, encoding[, errors]) -> bytearray\n\
@@ -3751,7 +3792,7 @@
     0,                                  /* tp_setattr */
     0,                                  /* tp_reserved */
     (reprfunc)bytearray_repr,           /* tp_repr */
-    0,                                  /* tp_as_number */
+    &bytearray_as_number,               /* tp_as_number */
     &bytearray_as_sequence,             /* tp_as_sequence */
     &bytearray_as_mapping,              /* tp_as_mapping */
     0,                                  /* tp_hash */
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index a5b9feb..bf919b5 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -400,6 +400,634 @@
     return ret;
 }
 
+/* Helpers for formatstring */
+
+Py_LOCAL_INLINE(PyObject *)
+getnextarg(PyObject *args, Py_ssize_t arglen, Py_ssize_t *p_argidx)
+{
+    Py_ssize_t argidx = *p_argidx;
+    if (argidx < arglen) {
+        (*p_argidx)++;
+        if (arglen < 0)
+            return args;
+        else
+            return PyTuple_GetItem(args, argidx);
+    }
+    PyErr_SetString(PyExc_TypeError,
+                    "not enough arguments for format string");
+    return NULL;
+}
+
+/* Format codes
+ * F_LJUST      '-'
+ * F_SIGN       '+'
+ * F_BLANK      ' '
+ * F_ALT        '#'
+ * F_ZERO       '0'
+ */
+#define F_LJUST (1<<0)
+#define F_SIGN  (1<<1)
+#define F_BLANK (1<<2)
+#define F_ALT   (1<<3)
+#define F_ZERO  (1<<4)
+
+/* Returns a new reference to a PyBytes object, or NULL on failure. */
+
+static PyObject *
+formatfloat(PyObject *v, int flags, int prec, int type)
+{
+    char *p;
+    PyObject *result;
+    double x;
+
+    x = PyFloat_AsDouble(v);
+    if (x == -1.0 && PyErr_Occurred()) {
+        PyErr_Format(PyExc_TypeError, "float argument required, "
+                     "not %.200s", Py_TYPE(v)->tp_name);
+        return NULL;
+    }
+
+    if (prec < 0)
+        prec = 6;
+
+    p = PyOS_double_to_string(x, type, prec,
+                              (flags & F_ALT) ? Py_DTSF_ALT : 0, NULL);
+
+    if (p == NULL)
+        return NULL;
+    result = PyBytes_FromStringAndSize(p, strlen(p));
+    PyMem_Free(p);
+    return result;
+}
+
+/* format_long emulates the format codes d, u, o, x and X, and
+ * the F_ALT flag, for Python's long (unbounded) ints.  It's not used for
+ * Python's regular ints.
+ * Return value:  a new PyBytes*, or NULL if error.
+ *  .  *pbuf is set to point into it,
+ *     *plen set to the # of chars following that.
+ *     Caller must decref it when done using pbuf.
+ *     The string starting at *pbuf is of the form
+ *         "-"? ("0x" | "0X")? digit+
+ *     "0x"/"0X" are present only for x and X conversions, with F_ALT
+ *         set in flags.  The case of hex digits will be correct,
+ *     There will be at least prec digits, zero-filled on the left if
+ *         necessary to get that many.
+ * val          object to be converted
+ * flags        bitmask of format flags; only F_ALT is looked at
+ * prec         minimum number of digits; 0-fill on left if needed
+ * type         a character in [duoxX]; u acts the same as d
+ *
+ * CAUTION:  o, x and X conversions on regular ints can never
+ * produce a '-' sign, but can for Python's unbounded ints.
+ */
+
+static PyObject *
+format_long(PyObject *val, int flags, int prec, int type,
+	    char **pbuf, int *plen)
+{
+    PyObject *s;
+    PyObject *result = NULL;
+
+    s = _PyUnicode_FormatLong(val, flags & F_ALT, prec, type);
+    if (!s)
+        return NULL;
+    result = _PyUnicode_AsASCIIString(s, "strict");
+    Py_DECREF(s);
+    if (!result)
+        return NULL;
+    *pbuf = PyBytes_AS_STRING(result);
+    *plen = PyBytes_GET_SIZE(result);
+    return result;
+}
+
+Py_LOCAL_INLINE(int)
+formatchar(char *buf, size_t buflen, PyObject *v)
+{
+    PyObject *w = NULL;
+    /* convert bytearray to bytes */
+    if (PyByteArray_Check(v)) {
+        w = PyBytes_FromObject(v);
+        if (w == NULL)
+            goto error;
+        v = w;
+    }
+    /* presume that the buffer is at least 2 characters long */
+    if (PyBytes_Check(v)) {
+        if (!PyArg_Parse(v, "c;%c requires an integer in range(256) or a single byte", &buf[0]))
+            goto error;
+    }
+    else {
+        long ival = PyLong_AsLong(v);
+        if (ival == -1 && PyErr_Occurred()) {
+            PyErr_SetString(PyExc_TypeError,
+                "%c requires an integer in range(256) or a single byte");
+            goto error;
+        }
+        if (ival < 0 || ival > 255) {
+            PyErr_SetString(PyExc_TypeError,
+                "%c requires an integer in range(256) or a single byte");
+            goto error;
+        }
+        buf[0] = ival;
+    }
+    Py_XDECREF(w);
+    buf[1] = '\0';
+    return 1;
+
+ error:
+    Py_XDECREF(w);
+    return -1;
+}
+
+static PyObject *
+format_obj(PyObject *v)
+{
+    PyObject *result = NULL, *w = NULL;
+    PyObject *func;
+    _Py_IDENTIFIER(__bytes__);
+    /* convert bytearray to bytes */
+    if (PyByteArray_Check(v)) {
+        w = PyBytes_FromObject(v);
+        if (w == NULL)
+            return NULL;
+        v = w;
+    }
+    /* is it a bytes object? */
+    if (PyBytes_Check(v)) {
+        result = v;
+        Py_INCREF(v);
+        Py_XDECREF(w);
+        return result;
+    }
+    /* does it support __bytes__? */
+    func = _PyObject_LookupSpecial(v, &PyId___bytes__);
+    if (func != NULL) {
+        result = PyObject_CallFunctionObjArgs(func, NULL);
+        Py_DECREF(func);
+        if (result == NULL)
+            return NULL;
+        if (!PyBytes_Check(result)) {
+            PyErr_Format(PyExc_TypeError,
+                         "__bytes__ returned non-bytes (type %.200s)",
+                         Py_TYPE(result)->tp_name);
+            Py_DECREF(result);
+            return NULL;
+        }
+        return result;
+    }
+    PyErr_Format(PyExc_TypeError,
+                 "%%b requires bytes, or an object that implements __bytes__, not '%.100s'",
+                 Py_TYPE(v)->tp_name);
+    return NULL;
+}
+
+/* fmt%(v1,v2,...) is roughly equivalent to sprintf(fmt, v1, v2, ...)
+
+   FORMATBUFLEN is the length of the buffer in which the ints &
+   chars are formatted. XXX This is a magic number. Each formatting
+   routine does bounds checking to ensure no overflow, but a better
+   solution may be to malloc a buffer of appropriate size for each
+   format. For now, the current solution is sufficient.
+*/
+#define FORMATBUFLEN (size_t)120
+
+PyObject *
+_PyBytes_Format(PyObject *format, PyObject *args)
+{
+    char *fmt, *res;
+    Py_ssize_t arglen, argidx;
+    Py_ssize_t reslen, rescnt, fmtcnt;
+    int args_owned = 0;
+    PyObject *result;
+    PyObject *repr;
+    PyObject *dict = NULL;
+    if (format == NULL || !PyBytes_Check(format) || args == NULL) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    fmt = PyBytes_AS_STRING(format);
+    fmtcnt = PyBytes_GET_SIZE(format);
+    reslen = rescnt = fmtcnt + 100;
+    result = PyBytes_FromStringAndSize((char *)NULL, reslen);
+    if (result == NULL)
+        return NULL;
+    res = PyBytes_AsString(result);
+    if (PyTuple_Check(args)) {
+        arglen = PyTuple_GET_SIZE(args);
+        argidx = 0;
+    }
+    else {
+        arglen = -1;
+        argidx = -2;
+    }
+    if (Py_TYPE(args)->tp_as_mapping && Py_TYPE(args)->tp_as_mapping->mp_subscript &&
+        !PyTuple_Check(args) && !PyBytes_Check(args) && !PyUnicode_Check(args) &&
+        !PyByteArray_Check(args)) {
+            dict = args;
+    }
+    while (--fmtcnt >= 0) {
+        if (*fmt != '%') {
+            if (--rescnt < 0) {
+                rescnt = fmtcnt + 100;
+                reslen += rescnt;
+                if (_PyBytes_Resize(&result, reslen))
+                    return NULL;
+                res = PyBytes_AS_STRING(result)
+                    + reslen - rescnt;
+                --rescnt;
+            }
+            *res++ = *fmt++;
+        }
+        else {
+            /* Got a format specifier */
+            int flags = 0;
+            Py_ssize_t width = -1;
+            int prec = -1;
+            int c = '\0';
+            int fill;
+            int isnumok;
+            PyObject *v = NULL;
+            PyObject *temp = NULL;
+            Py_buffer buf;
+            char *pbuf;
+            int sign;
+            Py_ssize_t len;
+            char formatbuf[FORMATBUFLEN];
+                 /* For format{int,char}() */
+
+            buf.obj = NULL;
+            fmt++;
+            if (*fmt == '(') {
+                char *keystart;
+                Py_ssize_t keylen;
+                PyObject *key;
+                int pcount = 1;
+
+                if (dict == NULL) {
+                    PyErr_SetString(PyExc_TypeError,
+                             "format requires a mapping");
+                    goto error;
+                }
+                ++fmt;
+                --fmtcnt;
+                keystart = fmt;
+                /* Skip over balanced parentheses */
+                while (pcount > 0 && --fmtcnt >= 0) {
+                    if (*fmt == ')')
+                        --pcount;
+                    else if (*fmt == '(')
+                        ++pcount;
+                    fmt++;
+                }
+                keylen = fmt - keystart - 1;
+                if (fmtcnt < 0 || pcount > 0) {
+                    PyErr_SetString(PyExc_ValueError,
+                               "incomplete format key");
+                    goto error;
+                }
+                key = PyBytes_FromStringAndSize(keystart,
+                                                 keylen);
+                if (key == NULL)
+                    goto error;
+                if (args_owned) {
+                    Py_DECREF(args);
+                    args_owned = 0;
+                }
+                args = PyObject_GetItem(dict, key);
+                Py_DECREF(key);
+                if (args == NULL) {
+                    goto error;
+                }
+                args_owned = 1;
+                arglen = -1;
+                argidx = -2;
+            }
+            while (--fmtcnt >= 0) {
+                switch (c = *fmt++) {
+                case '-': flags |= F_LJUST; continue;
+                case '+': flags |= F_SIGN; continue;
+                case ' ': flags |= F_BLANK; continue;
+                case '#': flags |= F_ALT; continue;
+                case '0': flags |= F_ZERO; continue;
+                }
+                break;
+            }
+            if (c == '*') {
+                v = getnextarg(args, arglen, &argidx);
+                if (v == NULL)
+                    goto error;
+                if (!PyLong_Check(v)) {
+                    PyErr_SetString(PyExc_TypeError,
+                                    "* wants int");
+                    goto error;
+                }
+                width = PyLong_AsSsize_t(v);
+                if (width == -1 && PyErr_Occurred())
+                    goto error;
+                if (width < 0) {
+                    flags |= F_LJUST;
+                    width = -width;
+                }
+                if (--fmtcnt >= 0)
+                    c = *fmt++;
+            }
+            else if (c >= 0 && isdigit(c)) {
+                width = c - '0';
+                while (--fmtcnt >= 0) {
+                    c = Py_CHARMASK(*fmt++);
+                    if (!isdigit(c))
+                        break;
+                    if (width > (PY_SSIZE_T_MAX - ((int)c - '0')) / 10) {
+                        PyErr_SetString(
+                            PyExc_ValueError,
+                            "width too big");
+                        goto error;
+                    }
+                    width = width*10 + (c - '0');
+                }
+            }
+            if (c == '.') {
+                prec = 0;
+                if (--fmtcnt >= 0)
+                    c = *fmt++;
+                if (c == '*') {
+                    v = getnextarg(args, arglen, &argidx);
+                    if (v == NULL)
+                        goto error;
+                    if (!PyLong_Check(v)) {
+                        PyErr_SetString(
+                            PyExc_TypeError,
+                            "* wants int");
+                        goto error;
+                    }
+                    prec = PyLong_AsSsize_t(v);
+                    if (prec == -1 && PyErr_Occurred())
+                        goto error;
+                    if (prec < 0)
+                        prec = 0;
+                    if (--fmtcnt >= 0)
+                        c = *fmt++;
+                }
+                else if (c >= 0 && isdigit(c)) {
+                    prec = c - '0';
+                    while (--fmtcnt >= 0) {
+                        c = Py_CHARMASK(*fmt++);
+                        if (!isdigit(c))
+                            break;
+                        if (prec > (INT_MAX - ((int)c - '0')) / 10) {
+                            PyErr_SetString(
+                                PyExc_ValueError,
+                                "prec too big");
+                            goto error;
+                        }
+                        prec = prec*10 + (c - '0');
+                    }
+                }
+            } /* prec */
+            if (fmtcnt >= 0) {
+                if (c == 'h' || c == 'l' || c == 'L') {
+                    if (--fmtcnt >= 0)
+                        c = *fmt++;
+                }
+            }
+            if (fmtcnt < 0) {
+                PyErr_SetString(PyExc_ValueError,
+                                "incomplete format");
+                goto error;
+            }
+            if (c != '%') {
+                v = getnextarg(args, arglen, &argidx);
+                if (v == NULL)
+                    goto error;
+            }
+            sign = 0;
+            fill = ' ';
+            switch (c) {
+            case '%':
+                pbuf = "%";
+                len = 1;
+                break;
+            case 'a':
+                temp = PyObject_Repr(v);
+                if (temp == NULL)
+                    goto error;
+                repr = PyUnicode_AsEncodedObject(temp, "ascii", "backslashreplace");
+                if (repr == NULL) {
+                    Py_DECREF(temp);
+                    goto error;
+                }
+                if (_getbuffer(repr, &buf) < 0) {
+                    temp = format_obj(repr);
+                    if (temp == NULL) {
+                        Py_DECREF(repr);
+                        goto error;
+                    }
+                    Py_DECREF(repr);
+                    repr = temp;
+                }
+                pbuf = PyBytes_AS_STRING(repr);
+                len = PyBytes_GET_SIZE(repr);
+                Py_DECREF(repr);
+                if (prec >= 0 && len > prec)
+                    len = prec;
+                break;
+            case 's':
+                // %s is only for 2/3 code; 3 only code should use %b
+            case 'b':
+                temp = format_obj(v);
+                if (temp == NULL)
+                    goto error;
+                pbuf = PyBytes_AS_STRING(temp);
+                len = PyBytes_GET_SIZE(temp);
+                if (prec >= 0 && len > prec)
+                    len = prec;
+                break;
+            case 'i':
+            case 'd':
+            case 'u':
+            case 'o':
+            case 'x':
+            case 'X':
+                if (c == 'i')
+                    c = 'd';
+                isnumok = 0;
+                if (PyNumber_Check(v)) {
+                    PyObject *iobj=NULL;
+
+                    if ((PyLong_Check(v))) {
+                        iobj = v;
+                        Py_INCREF(iobj);
+                    }
+                    else {
+                        iobj = PyNumber_Long(v);
+                    }
+                    if (iobj!=NULL) {
+                        if (PyLong_Check(iobj)) {
+                            int ilen;
+
+                            isnumok = 1;
+                            temp = format_long(iobj, flags, prec, c,
+                                    &pbuf, &ilen);
+                            Py_DECREF(iobj);
+                            len = ilen;
+                            if (!temp)
+                                goto error;
+                            sign = 1;
+                        }
+                        else {
+                            Py_DECREF(iobj);
+                        }
+                    }
+                }
+                if (!isnumok) {
+                    PyErr_Format(PyExc_TypeError,
+                        "%%%c format: a number is required, "
+                        "not %.200s", c, Py_TYPE(v)->tp_name);
+                    goto error;
+                }
+                if (flags & F_ZERO)
+                    fill = '0';
+                break;
+            case 'e':
+            case 'E':
+            case 'f':
+            case 'F':
+            case 'g':
+            case 'G':
+                temp = formatfloat(v, flags, prec, c);
+                if (temp == NULL)
+                    goto error;
+                pbuf = PyBytes_AS_STRING(temp);
+                len = PyBytes_GET_SIZE(temp);
+                sign = 1;
+                if (flags & F_ZERO)
+                    fill = '0';
+                break;
+            case 'c':
+                pbuf = formatbuf;
+                len = formatchar(pbuf, sizeof(formatbuf), v);
+                if (len < 0)
+                    goto error;
+                break;
+            default:
+                PyErr_Format(PyExc_ValueError,
+                  "unsupported format character '%c' (0x%x) "
+                  "at index %zd",
+                  c, c,
+                  (Py_ssize_t)(fmt - 1 -
+                               PyBytes_AsString(format)));
+                goto error;
+            }
+            if (sign) {
+                if (*pbuf == '-' || *pbuf == '+') {
+                    sign = *pbuf++;
+                    len--;
+                }
+                else if (flags & F_SIGN)
+                    sign = '+';
+                else if (flags & F_BLANK)
+                    sign = ' ';
+                else
+                    sign = 0;
+            }
+            if (width < len)
+                width = len;
+            if (rescnt - (sign != 0) < width) {
+                reslen -= rescnt;
+                rescnt = width + fmtcnt + 100;
+                reslen += rescnt;
+                if (reslen < 0) {
+                    Py_DECREF(result);
+                    PyBuffer_Release(&buf);
+                    Py_XDECREF(temp);
+                    return PyErr_NoMemory();
+                }
+                if (_PyBytes_Resize(&result, reslen)) {
+                    PyBuffer_Release(&buf);
+                    Py_XDECREF(temp);
+                    return NULL;
+                }
+                res = PyBytes_AS_STRING(result)
+                    + reslen - rescnt;
+            }
+            if (sign) {
+                if (fill != ' ')
+                    *res++ = sign;
+                rescnt--;
+                if (width > len)
+                    width--;
+            }
+            if ((flags & F_ALT) && (c == 'x' || c == 'X')) {
+                assert(pbuf[0] == '0');
+                assert(pbuf[1] == c);
+                if (fill != ' ') {
+                    *res++ = *pbuf++;
+                    *res++ = *pbuf++;
+                }
+                rescnt -= 2;
+                width -= 2;
+                if (width < 0)
+                    width = 0;
+                len -= 2;
+            }
+            if (width > len && !(flags & F_LJUST)) {
+                do {
+                    --rescnt;
+                    *res++ = fill;
+                } while (--width > len);
+            }
+            if (fill == ' ') {
+                if (sign)
+                    *res++ = sign;
+                if ((flags & F_ALT) &&
+                    (c == 'x' || c == 'X')) {
+                    assert(pbuf[0] == '0');
+                    assert(pbuf[1] == c);
+                    *res++ = *pbuf++;
+                    *res++ = *pbuf++;
+                }
+            }
+            Py_MEMCPY(res, pbuf, len);
+            res += len;
+            rescnt -= len;
+            while (--width >= len) {
+                --rescnt;
+                *res++ = ' ';
+            }
+            if (dict && (argidx < arglen) && c != '%') {
+                PyErr_SetString(PyExc_TypeError,
+                           "not all arguments converted during bytes formatting");
+                PyBuffer_Release(&buf);
+                Py_XDECREF(temp);
+                goto error;
+            }
+            PyBuffer_Release(&buf);
+            Py_XDECREF(temp);
+        } /* '%' */
+    } /* until end */
+    if (argidx < arglen && !dict) {
+        PyErr_SetString(PyExc_TypeError,
+                        "not all arguments converted during bytes formatting");
+        goto error;
+    }
+    if (args_owned) {
+        Py_DECREF(args);
+    }
+    if (_PyBytes_Resize(&result, reslen - rescnt))
+        return NULL;
+    return result;
+
+ error:
+    Py_DECREF(result);
+    if (args_owned) {
+        Py_DECREF(args);
+    }
+    return NULL;
+}
+
+/* =-= */
+
 static void
 bytes_dealloc(PyObject *op)
 {
@@ -2996,6 +3624,21 @@
 };
 
 static PyObject *
+bytes_mod(PyObject *v, PyObject *w)
+{
+    if (!PyBytes_Check(v))
+        Py_RETURN_NOTIMPLEMENTED;
+    return _PyBytes_Format(v, w);
+}
+
+static PyNumberMethods bytes_as_number = {
+    0,              /*nb_add*/
+    0,              /*nb_subtract*/
+    0,              /*nb_multiply*/
+    bytes_mod,      /*nb_remainder*/
+};
+
+static PyObject *
 str_subtype_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
 
 static PyObject *
@@ -3286,7 +3929,7 @@
     0,                                          /* tp_setattr */
     0,                                          /* tp_reserved */
     (reprfunc)bytes_repr,                       /* tp_repr */
-    0,                                          /* tp_as_number */
+    &bytes_as_number,                           /* tp_as_number */
     &bytes_as_sequence,                         /* tp_as_sequence */
     &bytes_as_mapping,                          /* tp_as_mapping */
     (hashfunc)bytes_hash,                       /* tp_hash */
@@ -3377,14 +4020,14 @@
 }
 
 
-/* The following function breaks the notion that strings are immutable:
-   it changes the size of a string.  We get away with this only if there
+/* The following function breaks the notion that bytes are immutable:
+   it changes the size of a bytes object.  We get away with this only if there
    is only one module referencing the object.  You can also think of it
-   as creating a new string object and destroying the old one, only
-   more efficiently.  In any case, don't use this if the string may
+   as creating a new bytes object and destroying the old one, only
+   more efficiently.  In any case, don't use this if the bytes object may
    already be known to some other part of the code...
-   Note that if there's not enough memory to resize the string, the original
-   string object at *pv is deallocated, *pv is set to NULL, an "out of
+   Note that if there's not enough memory to resize the bytes object, the
+   original bytes object at *pv is deallocated, *pv is set to NULL, an "out of
    memory" exception is set, and -1 is returned.  Else (on success) 0 is
    returned, and the value in *pv may or may not be the same as on input.
    As always, an extra byte is allocated for a trailing \0 byte (newsize
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 3d51f34..ee0c044 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -13893,8 +13893,8 @@
  * CAUTION:  o, x and X conversions on regular ints can never
  * produce a '-' sign, but can for Python's unbounded ints.
  */
-static PyObject*
-formatlong(PyObject *val, struct unicode_format_arg_t *arg)
+PyObject *
+_PyUnicode_FormatLong(PyObject *val, int alt, int prec, int type)
 {
     PyObject *result = NULL;
     char *buf;
@@ -13904,8 +13904,6 @@
     Py_ssize_t llen;
     int numdigits;      /* len == numnondigits + numdigits */
     int numnondigits = 0;
-    int prec = arg->prec;
-    int type = arg->ch;
 
     /* Avoid exceeding SSIZE_T_MAX */
     if (prec > INT_MAX-3) {
@@ -13954,7 +13952,7 @@
     if (llen > INT_MAX) {
         Py_DECREF(result);
         PyErr_SetString(PyExc_ValueError,
-                        "string too large in _PyBytes_FormatLong");
+                        "string too large in _PyUnicode_FormatLong");
         return NULL;
     }
     len = (int)llen;
@@ -13964,7 +13962,7 @@
     assert(numdigits > 0);
 
     /* Get rid of base marker unless F_ALT */
-    if (((arg->flags & F_ALT) == 0 &&
+    if (((alt) == 0 &&
         (type == 'o' || type == 'x' || type == 'X'))) {
         assert(buf[sign] == '0');
         assert(buf[sign+1] == 'x' || buf[sign+1] == 'X' ||
@@ -14099,7 +14097,7 @@
         return 1;
     }
 
-    res = formatlong(iobj, arg);
+    res = _PyUnicode_FormatLong(iobj, arg->flags & F_ALT, arg->prec, type);
     Py_DECREF(iobj);
     if (res == NULL)
         return -1;