diff --git a/Lib/idlelib/FormatParagraph.py b/Lib/idlelib/FormatParagraph.py
index e3ca7b9..ae4e6e7 100644
--- a/Lib/idlelib/FormatParagraph.py
+++ b/Lib/idlelib/FormatParagraph.py
@@ -1,18 +1,19 @@
-# Extension to format a paragraph
+"""Extension to format a paragraph or selection to a max width.
 
-# Does basic, standard text formatting, and also understands Python
-# comment blocks.  Thus, for editing Python source code, this
-# extension is really only suitable for reformatting these comment
-# blocks or triple-quoted strings.
+Does basic, standard text formatting, and also understands Python
+comment blocks. Thus, for editing Python source code, this
+extension is really only suitable for reformatting these comment
+blocks or triple-quoted strings.
 
-# Known problems with comment reformatting:
-# * If there is a selection marked, and the first line of the
-#   selection is not complete, the block will probably not be detected
-#   as comments, and will have the normal "text formatting" rules
-#   applied.
-# * If a comment block has leading whitespace that mixes tabs and
-#   spaces, they will not be considered part of the same block.
-# * Fancy comments, like this bulleted list, arent handled :-)
+Known problems with comment reformatting:
+* If there is a selection marked, and the first line of the
+  selection is not complete, the block will probably not be detected
+  as comments, and will have the normal "text formatting" rules
+  applied.
+* If a comment block has leading whitespace that mixes tabs and
+  spaces, they will not be considered part of the same block.
+* Fancy comments, like this bulleted list, aren't handled :-)
+"""
 
 import re
 from idlelib.configHandler import idleConf
@@ -32,42 +33,31 @@
         self.editwin = None
 
     def format_paragraph_event(self, event):
-        maxformatwidth = int(idleConf.GetOption('main', 'FormatParagraph',
-                                                'paragraph', type='int'))
+        """Formats paragraph to a max width specified in idleConf.
+
+        If text is selected, format_paragraph_event will start breaking lines
+        at the max width, starting from the beginning selection.
+
+        If no text is selected, format_paragraph_event uses the current
+        cursor location to determine the paragraph (lines of text surrounded
+        by blank lines) and formats it.
+        """
+        maxformatwidth = idleConf.GetOption(
+                'main', 'FormatParagraph', 'paragraph', type='int')
         text = self.editwin.text
         first, last = self.editwin.get_selection_indices()
         if first and last:
             data = text.get(first, last)
-            comment_header = ''
+            comment_header = get_comment_header(data)
         else:
             first, last, comment_header, data = \
                     find_paragraph(text, text.index("insert"))
         if comment_header:
-            # Reformat the comment lines - convert to text sans header.
-            lines = data.split("\n")
-            lines = map(lambda st, l=len(comment_header): st[l:], lines)
-            data = "\n".join(lines)
-            # Reformat to maxformatwidth chars or a 20 char width,
-            # whichever is greater.
-            format_width = max(maxformatwidth - len(comment_header), 20)
-            newdata = reformat_paragraph(data, format_width)
-            # re-split and re-insert the comment header.
-            newdata = newdata.split("\n")
-            # If the block ends in a \n, we dont want the comment
-            # prefix inserted after it. (Im not sure it makes sense to
-            # reformat a comment block that isnt made of complete
-            # lines, but whatever!)  Can't think of a clean solution,
-            # so we hack away
-            block_suffix = ""
-            if not newdata[-1]:
-                block_suffix = "\n"
-                newdata = newdata[:-1]
-            builder = lambda item, prefix=comment_header: prefix+item
-            newdata = '\n'.join(map(builder, newdata)) + block_suffix
+            newdata = reformat_comment(data, maxformatwidth, comment_header)
         else:
-            # Just a normal text format
             newdata = reformat_paragraph(data, maxformatwidth)
         text.tag_remove("sel", "1.0", "end")
+
         if newdata != data:
             text.mark_set("insert", first)
             text.undo_block_start()
@@ -80,31 +70,44 @@
         return "break"
 
 def find_paragraph(text, mark):
+    """Returns the start/stop indices enclosing the paragraph that mark is in.
+
+    Also returns the comment format string, if any, and paragraph of text
+    between the start/stop indices.
+    """
     lineno, col = map(int, mark.split("."))
-    line = text.get("%d.0" % lineno, "%d.0 lineend" % lineno)
+    line = text.get("%d.0" % lineno, "%d.end" % lineno)
+
+    # Look for start of next paragraph if the index passed in is a blank line
     while text.compare("%d.0" % lineno, "<", "end") and is_all_white(line):
         lineno = lineno + 1
-        line = text.get("%d.0" % lineno, "%d.0 lineend" % lineno)
+        line = text.get("%d.0" % lineno, "%d.end" % lineno)
     first_lineno = lineno
     comment_header = get_comment_header(line)
     comment_header_len = len(comment_header)
+
+    # Once start line found, search for end of paragraph (a blank line)
     while get_comment_header(line)==comment_header and \
               not is_all_white(line[comment_header_len:]):
         lineno = lineno + 1
-        line = text.get("%d.0" % lineno, "%d.0 lineend" % lineno)
+        line = text.get("%d.0" % lineno, "%d.end" % lineno)
     last = "%d.0" % lineno
-    # Search back to beginning of paragraph
+
+    # Search back to beginning of paragraph (first blank line before)
     lineno = first_lineno - 1
-    line = text.get("%d.0" % lineno, "%d.0 lineend" % lineno)
+    line = text.get("%d.0" % lineno, "%d.end" % lineno)
     while lineno > 0 and \
               get_comment_header(line)==comment_header and \
               not is_all_white(line[comment_header_len:]):
         lineno = lineno - 1
-        line = text.get("%d.0" % lineno, "%d.0 lineend" % lineno)
+        line = text.get("%d.0" % lineno, "%d.end" % lineno)
     first = "%d.0" % (lineno+1)
+
     return first, last, comment_header, text.get(first, last)
 
+# This should perhaps be replaced with textwrap.wrap
 def reformat_paragraph(data, limit):
+    """Return data reformatted to specified width (limit)."""
     lines = data.split("\n")
     i = 0
     n = len(lines)
@@ -127,7 +130,7 @@
             if not word:
                 continue # Can happen when line ends in whitespace
             if len((partial + word).expandtabs()) > limit and \
-               partial != indent1:
+                   partial != indent1:
                 new.append(partial.rstrip())
                 partial = indent2
             partial = partial + word + " "
@@ -139,13 +142,50 @@
     new.extend(lines[i:])
     return "\n".join(new)
 
+def reformat_comment(data, limit, comment_header):
+    """Return data reformatted to specified width with comment header."""
+
+    # Remove header from the comment lines
+    lc = len(comment_header)
+    data = "\n".join(line[lc:] for line in data.split("\n"))
+    # Reformat to maxformatwidth chars or a 20 char width,
+    # whichever is greater.
+    format_width = max(limit - len(comment_header), 20)
+    newdata = reformat_paragraph(data, format_width)
+    # re-split and re-insert the comment header.
+    newdata = newdata.split("\n")
+    # If the block ends in a \n, we dont want the comment prefix
+    # inserted after it. (Im not sure it makes sense to reformat a
+    # comment block that is not made of complete lines, but whatever!)
+    # Can't think of a clean solution, so we hack away
+    block_suffix = ""
+    if not newdata[-1]:
+        block_suffix = "\n"
+        newdata = newdata[:-1]
+    return '\n'.join(comment_header+line for line in newdata) + block_suffix
+
 def is_all_white(line):
+    """Return True if line is empty or all whitespace."""
+
     return re.match(r"^\s*$", line) is not None
 
 def get_indent(line):
-    return re.match(r"^(\s*)", line).group()
+    """Return the initial space or tab indent of line."""
+    return re.match(r"^([ \t]*)", line).group()
 
 def get_comment_header(line):
-    m = re.match(r"^(\s*#*)", line)
+    """Return string with leading whitespace and '#' from line or ''.
+
+    A null return indicates that the line is not a comment line. A non-
+    null return, such as '    #', will be used to find the other lines of
+    a comment block with the same  indent.
+    """
+    m = re.match(r"^([ \t]*#*)", line)
     if m is None: return ""
     return m.group(1)
+
+if __name__ == "__main__":
+    from test import support; support.use_resources = ['gui']
+    import unittest
+    unittest.main('idlelib.idle_test.test_formatparagraph',
+            verbosity=2, exit=False)
diff --git a/Lib/idlelib/idle_test/test_formatparagraph.py b/Lib/idlelib/idle_test/test_formatparagraph.py
new file mode 100644
index 0000000..2ec8232
--- /dev/null
+++ b/Lib/idlelib/idle_test/test_formatparagraph.py
@@ -0,0 +1,374 @@
+# Test the functions and main class method of FormatParagraph.py
+import unittest
+from idlelib import FormatParagraph as fp
+from idlelib.EditorWindow import EditorWindow
+from tkinter import Tk, Text, TclError
+from test.support import requires
+
+
+class Is_Get_Test(unittest.TestCase):
+    """Test the is_ and get_ functions"""
+    test_comment = '# This is a comment'
+    test_nocomment = 'This is not a comment'
+    trailingws_comment = '# This is a comment   '
+    leadingws_comment = '    # This is a comment'
+    leadingws_nocomment = '    This is not a comment'
+
+    def test_is_all_white(self):
+        self.assertTrue(fp.is_all_white(''))
+        self.assertTrue(fp.is_all_white('\t\n\r\f\v'))
+        self.assertFalse(fp.is_all_white(self.test_comment))
+
+    def test_get_indent(self):
+        Equal = self.assertEqual
+        Equal(fp.get_indent(self.test_comment), '')
+        Equal(fp.get_indent(self.trailingws_comment), '')
+        Equal(fp.get_indent(self.leadingws_comment), '    ')
+        Equal(fp.get_indent(self.leadingws_nocomment), '    ')
+
+    def test_get_comment_header(self):
+        Equal = self.assertEqual
+        # Test comment strings
+        Equal(fp.get_comment_header(self.test_comment), '#')
+        Equal(fp.get_comment_header(self.trailingws_comment), '#')
+        Equal(fp.get_comment_header(self.leadingws_comment), '    #')
+        # Test non-comment strings
+        Equal(fp.get_comment_header(self.leadingws_nocomment), '    ')
+        Equal(fp.get_comment_header(self.test_nocomment), '')
+
+
+class FindTest(unittest.TestCase):
+    """Test the find_paragraph function in FormatParagraph.
+
+    Using the runcase() function, find_paragraph() is called with 'mark' set at
+    multiple indexes before and inside the test paragraph.
+
+    It appears that code with the same indentation as a quoted string is grouped
+    as part of the same paragraph, which is probably incorrect behavior.
+    """
+
+    @classmethod
+    def setUpClass(cls):
+        from idlelib.idle_test.mock_tk import Text
+        cls.text = Text()
+
+    def runcase(self, inserttext, stopline, expected):
+        # Check that find_paragraph returns the expected paragraph when
+        # the mark index is set to beginning, middle, end of each line
+        # up to but not including the stop line
+        text = self.text
+        text.insert('1.0', inserttext)
+        for line in range(1, stopline):
+            linelength = int(text.index("%d.end" % line).split('.')[1])
+            for col in (0, linelength//2, linelength):
+                tempindex = "%d.%d" % (line, col)
+                self.assertEqual(fp.find_paragraph(text, tempindex), expected)
+        text.delete('1.0', 'end')
+
+    def test_find_comment(self):
+        comment = (
+            "# Comment block with no blank lines before\n"
+            "# Comment line\n"
+            "\n")
+        self.runcase(comment, 3, ('1.0', '3.0', '#', comment[0:58]))
+
+        comment = (
+            "\n"
+            "# Comment block with whitespace line before and after\n"
+            "# Comment line\n"
+            "\n")
+        self.runcase(comment, 4, ('2.0', '4.0', '#', comment[1:70]))
+
+        comment = (
+            "\n"
+            "    # Indented comment block with whitespace before and after\n"
+            "    # Comment line\n"
+            "\n")
+        self.runcase(comment, 4, ('2.0', '4.0', '    #', comment[1:82]))
+
+        comment = (
+            "\n"
+            "# Single line comment\n"
+            "\n")
+        self.runcase(comment, 3, ('2.0', '3.0', '#', comment[1:23]))
+
+        comment = (
+            "\n"
+            "    # Single line comment with leading whitespace\n"
+            "\n")
+        self.runcase(comment, 3, ('2.0', '3.0', '    #', comment[1:51]))
+
+        comment = (
+            "\n"
+            "# Comment immediately followed by code\n"
+            "x = 42\n"
+            "\n")
+        self.runcase(comment, 3, ('2.0', '3.0', '#', comment[1:40]))
+
+        comment = (
+            "\n"
+            "    # Indented comment immediately followed by code\n"
+            "x = 42\n"
+            "\n")
+        self.runcase(comment, 3, ('2.0', '3.0', '    #', comment[1:53]))
+
+        comment = (
+            "\n"
+            "# Comment immediately followed by indented code\n"
+            "    x = 42\n"
+            "\n")
+        self.runcase(comment, 3, ('2.0', '3.0', '#', comment[1:49]))
+
+    def test_find_paragraph(self):
+        teststring = (
+            '"""String with no blank lines before\n'
+            'String line\n'
+            '"""\n'
+            '\n')
+        self.runcase(teststring, 4, ('1.0', '4.0', '', teststring[0:53]))
+
+        teststring = (
+            "\n"
+            '"""String with whitespace line before and after\n'
+            'String line.\n'
+            '"""\n'
+            '\n')
+        self.runcase(teststring, 5, ('2.0', '5.0', '', teststring[1:66]))
+
+        teststring = (
+            '\n'
+            '    """Indented string with whitespace before and after\n'
+            '    Comment string.\n'
+            '    """\n'
+            '\n')
+        self.runcase(teststring, 5, ('2.0', '5.0', '    ', teststring[1:85]))
+
+        teststring = (
+            '\n'
+            '"""Single line string."""\n'
+            '\n')
+        self.runcase(teststring, 3, ('2.0', '3.0', '', teststring[1:27]))
+
+        teststring = (
+            '\n'
+            '    """Single line string with leading whitespace."""\n'
+            '\n')
+        self.runcase(teststring, 3, ('2.0', '3.0', '    ', teststring[1:55]))
+
+
+class ReformatFunctionTest(unittest.TestCase):
+    """Test the reformat_paragraph function without the editor window."""
+
+    def test_reformat_paragrah(self):
+        Equal = self.assertEqual
+        reform = fp.reformat_paragraph
+        hw = "O hello world"
+        Equal(reform(' ', 1), ' ')
+        Equal(reform("Hello    world", 20), "Hello  world")
+
+        # Test without leading newline
+        Equal(reform(hw, 1), "O\nhello\nworld")
+        Equal(reform(hw, 6), "O\nhello\nworld")
+        Equal(reform(hw, 7), "O hello\nworld")
+        Equal(reform(hw, 12), "O hello\nworld")
+        Equal(reform(hw, 13), "O hello world")
+
+        # Test with leading newline
+        hw = "\nO hello world"
+        Equal(reform(hw, 1), "\nO\nhello\nworld")
+        Equal(reform(hw, 6), "\nO\nhello\nworld")
+        Equal(reform(hw, 7), "\nO hello\nworld")
+        Equal(reform(hw, 12), "\nO hello\nworld")
+        Equal(reform(hw, 13), "\nO hello world")
+
+
+class ReformatCommentTest(unittest.TestCase):
+    """Test the reformat_comment function without the editor window."""
+
+    def test_reformat_comment(self):
+        Equal = self.assertEqual
+
+        # reformat_comment formats to a minimum of 20 characters
+        test_string = (
+            "    \"\"\"this is a test of a reformat for a triple quoted string"
+            " will it reformat to less than 70 characters for me?\"\"\"")
+        result = fp.reformat_comment(test_string, 70, "    ")
+        expected = (
+            "    \"\"\"this is a test of a reformat for a triple quoted string will it\n"
+            "    reformat to less than 70 characters for me?\"\"\"")
+        Equal(result, expected)
+
+        test_comment = (
+            "# this is a test of a reformat for a triple quoted string will "
+            "it reformat to less than 70 characters for me?")
+        result = fp.reformat_comment(test_comment, 70, "#")
+        expected = (
+            "# this is a test of a reformat for a triple quoted string will it\n"
+            "# reformat to less than 70 characters for me?")
+        Equal(result, expected)
+
+
+class FormatClassTest(unittest.TestCase):
+    def test_init_close(self):
+        instance = fp.FormatParagraph('editor')
+        self.assertEqual(instance.editwin, 'editor')
+        instance.close()
+        self.assertEqual(instance.editwin, None)
+
+
+# For testing format_paragraph_event, Initialize FormatParagraph with
+# a mock Editor with .text and  .get_selection_indices.  The text must
+# be a Text wrapper that adds two methods
+
+# A real EditorWindow creates unneeded, time-consuming baggage and
+# sometimes emits shutdown warnings like this:
+# "warning: callback failed in WindowList <class '_tkinter.TclError'>
+# : invalid command name ".55131368.windows".
+# Calling EditorWindow._close in tearDownClass prevents this but causes
+# other problems (windows left open).
+
+class TextWrapper:
+    def __init__(self, master):
+        self.text = Text(master=master)
+    def __getattr__(self, name):
+        return getattr(self.text, name)
+    def undo_block_start(self): pass
+    def undo_block_stop(self): pass
+
+class Editor:
+    def __init__(self, root):
+        self.text = TextWrapper(root)
+    get_selection_indices = EditorWindow. get_selection_indices
+
+class FormatEventTest(unittest.TestCase):
+    """Test the formatting of text inside a Text widget.
+
+    This is done with FormatParagraph.format.paragraph_event,
+    which calls funtions in the module as appropriate.
+    """
+    test_string = (
+        "    '''this is a test of a reformat for a triple "
+        "quoted string will it reformat to less than 70 "
+        "characters for me?'''\n")
+    multiline_test_string = (
+        "    '''The first line is under the max width.\n"
+        "    The second line's length is way over the max width. It goes "
+        "on and on until it is over 100 characters long.\n"
+        "    Same thing with the third line. It is also way over the max "
+        "width, but FormatParagraph will fix it.\n"
+        "    '''\n")
+    multiline_test_comment = (
+        "# The first line is under the max width.\n"
+        "# The second line's length is way over the max width. It goes on "
+        "and on until it is over 100 characters long.\n"
+        "# Same thing with the third line. It is also way over the max "
+        "width, but FormatParagraph will fix it.\n"
+        "# The fourth line is short like the first line.")
+
+    @classmethod
+    def setUpClass(cls):
+        requires('gui')
+        cls.root = Tk()
+        editor = Editor(root=cls.root)
+        cls.text = editor.text.text  # Test code does not need the wrapper.
+        cls.formatter = fp.FormatParagraph(editor).format_paragraph_event
+        # Sets the insert mark just after the re-wrapped and inserted  text.
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.root.destroy()
+
+    def test_short_line(self):
+        self.text.insert('1.0', "Short line\n")
+        self.formatter("Dummy")
+        self.assertEqual(self.text.get('1.0', 'insert'), "Short line\n" )
+        self.text.delete('1.0', 'end')
+
+    def test_long_line(self):
+        text = self.text
+
+        # Set cursor ('insert' mark) to '1.0', within text.
+        text.insert('1.0', self.test_string)
+        text.mark_set('insert', '1.0')
+        self.formatter('ParameterDoesNothing')
+        result = text.get('1.0', 'insert')
+        # find function includes \n
+        expected = (
+"    '''this is a test of a reformat for a triple quoted string will it\n"
+"    reformat to less than 70 characters for me?'''\n")  # yes
+        self.assertEqual(result, expected)
+        text.delete('1.0', 'end')
+
+        # Select from 1.11 to line end.
+        text.insert('1.0', self.test_string)
+        text.tag_add('sel', '1.11', '1.end')
+        self.formatter('ParameterDoesNothing')
+        result = text.get('1.0', 'insert')
+        # selection excludes \n
+        expected = (
+"    '''this is a test of a reformat for a triple quoted string will it reformat\n"
+" to less than 70 characters for me?'''")  # no
+        self.assertEqual(result, expected)
+        text.delete('1.0', 'end')
+
+    def test_multiple_lines(self):
+        text = self.text
+        #  Select 2 long lines.
+        text.insert('1.0', self.multiline_test_string)
+        text.tag_add('sel', '2.0', '4.0')
+        self.formatter('ParameterDoesNothing')
+        result = text.get('2.0', 'insert')
+        expected = (
+"    The second line's length is way over the max width. It goes on and\n"
+"    on until it is over 100 characters long. Same thing with the third\n"
+"    line. It is also way over the max width, but FormatParagraph will\n"
+"    fix it.\n")
+        self.assertEqual(result, expected)
+        text.delete('1.0', 'end')
+
+    def test_comment_block(self):
+        text = self.text
+
+        # Set cursor ('insert') to '1.0', within block.
+        text.insert('1.0', self.multiline_test_comment)
+        self.formatter('ParameterDoesNothing')
+        result = text.get('1.0', 'insert')
+        expected = (
+"# The first line is under the max width. The second line's length is\n"
+"# way over the max width. It goes on and on until it is over 100\n"
+"# characters long. Same thing with the third line. It is also way over\n"
+"# the max width, but FormatParagraph will fix it. The fourth line is\n"
+"# short like the first line.\n")
+        self.assertEqual(result, expected)
+        text.delete('1.0', 'end')
+
+        # Select line 2, verify line 1 unaffected.
+        text.insert('1.0', self.multiline_test_comment)
+        text.tag_add('sel', '2.0', '3.0')
+        self.formatter('ParameterDoesNothing')
+        result = text.get('1.0', 'insert')
+        expected = (
+"# The first line is under the max width.\n"
+"# The second line's length is way over the max width. It goes on and\n"
+"# on until it is over 100 characters long.\n")
+        self.assertEqual(result, expected)
+        text.delete('1.0', 'end')
+
+# The following block worked with EditorWindow but fails with the mock.
+# Lines 2 and 3 get pasted together even though the previous block left
+# the previous line alone. More investigation is needed.
+##        # Select lines 3 and 4
+##        text.insert('1.0', self.multiline_test_comment)
+##        text.tag_add('sel', '3.0', '5.0')
+##        self.formatter('ParameterDoesNothing')
+##        result = text.get('3.0', 'insert')
+##        expected = (
+##"# Same thing with the third line. It is also way over the max width,\n"
+##"# but FormatParagraph will fix it. The fourth line is short like the\n"
+##"# first line.\n")
+##        self.assertEqual(result, expected)
+##        text.delete('1.0', 'end')
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2, exit=2)
diff --git a/Misc/NEWS b/Misc/NEWS
index ccbb57a..948d54f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -212,6 +212,9 @@
 IDLE
 ----
 
+- Issue #18226: Add docstrings and unittests for FormatParagraph.py.
+  Original patches by Todd Rovito and Phil Webster.
+
 - Issue #18279: Format - Strip trailing whitespace no longer marks a file as
   changed when it has not been changed. This fix followed the addition of a
   test file originally written by Phil Webster (the issue's main goal).