Re-implement the 'warnings' module in C. This allows for usage of the
'warnings' code in places where it was previously not possible (e.g., the
parser). It could also potentially lead to a speed-up in interpreter start-up
if the C version of the code (_warnings) is imported over the use of the
Python version in key places.

Closes issue #1631171.
diff --git a/Lib/warnings.py b/Lib/warnings.py
index 970d0ff..3c7357b 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -3,127 +3,13 @@
 # Note: function level imports should *not* be used
 # in this module as it may cause import lock deadlock.
 # See bug 683658.
-import sys, types
 import linecache
+import sys
+import types
 
 __all__ = ["warn", "showwarning", "formatwarning", "filterwarnings",
            "resetwarnings"]
 
-# filters contains a sequence of filter 5-tuples
-# The components of the 5-tuple are:
-# - an action: error, ignore, always, default, module, or once
-# - a compiled regex that must match the warning message
-# - a class representing the warning category
-# - a compiled regex that must match the module that is being warned
-# - a line number for the line being warning, or 0 to mean any line
-# If either if the compiled regexs are None, match anything.
-filters = []
-defaultaction = "default"
-onceregistry = {}
-
-def warn(message, category=None, stacklevel=1):
-    """Issue a warning, or maybe ignore it or raise an exception."""
-    # Check if message is already a Warning object
-    if isinstance(message, Warning):
-        category = message.__class__
-    # Check category argument
-    if category is None:
-        category = UserWarning
-    assert issubclass(category, Warning)
-    # Get context information
-    try:
-        caller = sys._getframe(stacklevel)
-    except ValueError:
-        globals = sys.__dict__
-        lineno = 1
-    else:
-        globals = caller.f_globals
-        lineno = caller.f_lineno
-    if '__name__' in globals:
-        module = globals['__name__']
-    else:
-        module = "<string>"
-    filename = globals.get('__file__')
-    if filename:
-        fnl = filename.lower()
-        if fnl.endswith((".pyc", ".pyo")):
-            filename = filename[:-1]
-    else:
-        if module == "__main__":
-            try:
-                filename = sys.argv[0]
-            except AttributeError:
-                # embedded interpreters don't have sys.argv, see bug #839151
-                filename = '__main__'
-        if not filename:
-            filename = module
-    registry = globals.setdefault("__warningregistry__", {})
-    warn_explicit(message, category, filename, lineno, module, registry,
-                  globals)
-
-def warn_explicit(message, category, filename, lineno,
-                  module=None, registry=None, module_globals=None):
-    if module is None:
-        module = filename or "<unknown>"
-        if module[-3:].lower() == ".py":
-            module = module[:-3] # XXX What about leading pathname?
-    if registry is None:
-        registry = {}
-    if isinstance(message, Warning):
-        text = str(message)
-        category = message.__class__
-    else:
-        text = message
-        message = category(message)
-    key = (text, category, lineno)
-    # Quick test for common case
-    if registry.get(key):
-        return
-    # Search the filters
-    for item in filters:
-        action, msg, cat, mod, ln = item
-        if ((msg is None or msg.match(text)) and
-            issubclass(category, cat) and
-            (mod is None or mod.match(module)) and
-            (ln == 0 or lineno == ln)):
-            break
-    else:
-        action = defaultaction
-    # Early exit actions
-    if action == "ignore":
-        registry[key] = 1
-        return
-
-    # Prime the linecache for formatting, in case the
-    # "file" is actually in a zipfile or something.
-    linecache.getlines(filename, module_globals)
-
-    if action == "error":
-        raise message
-    # Other actions
-    if action == "once":
-        registry[key] = 1
-        oncekey = (text, category)
-        if onceregistry.get(oncekey):
-            return
-        onceregistry[oncekey] = 1
-    elif action == "always":
-        pass
-    elif action == "module":
-        registry[key] = 1
-        altkey = (text, category, 0)
-        if registry.get(altkey):
-            return
-        registry[altkey] = 1
-    elif action == "default":
-        registry[key] = 1
-    else:
-        # Unrecognized actions are errors
-        raise RuntimeError(
-              "Unrecognized action (%r) in warnings.filters:\n %s" %
-              (action, item))
-    # Print message and context
-    showwarning(message, category, filename, lineno)
 
 def warnpy3k(message, category=None, stacklevel=1):
     """Issue a deprecation warning for Python 3.x related changes.
@@ -135,21 +21,22 @@
             category = DeprecationWarning
         warn(message, category, stacklevel+1)
 
-def showwarning(message, category, filename, lineno, file=None):
+def showwarning(message, category, filename, lineno, file=None, line=None):
     """Hook to write a warning to a file; replace if you like."""
     if file is None:
         file = sys.stderr
     try:
-        file.write(formatwarning(message, category, filename, lineno))
+        file.write(formatwarning(message, category, filename, lineno, line))
     except IOError:
         pass # the file (probably stderr) is invalid - this warning gets lost.
 
-def formatwarning(message, category, filename, lineno):
+def formatwarning(message, category, filename, lineno, line=None):
     """Function to format a warning the standard way."""
     s =  "%s:%s: %s: %s\n" % (filename, lineno, category.__name__, message)
-    line = linecache.getline(filename, lineno).strip()
+    line = linecache.getline(filename, lineno) if line is None else line
     if line:
-        s = s + "  " + line + "\n"
+        line = line.strip()
+        s += "  %s\n" % line
     return s
 
 def filterwarnings(action, message="", category=Warning, module="", lineno=0,
@@ -268,7 +155,145 @@
         raise _OptionError("invalid warning category: %r" % (category,))
     return cat
 
+
+# Code typically replaced by _warnings
+def warn(message, category=None, stacklevel=1):
+    """Issue a warning, or maybe ignore it or raise an exception."""
+    # Check if message is already a Warning object
+    if isinstance(message, Warning):
+        category = message.__class__
+    # Check category argument
+    if category is None:
+        category = UserWarning
+    assert issubclass(category, Warning)
+    # Get context information
+    try:
+        caller = sys._getframe(stacklevel)
+    except ValueError:
+        globals = sys.__dict__
+        lineno = 1
+    else:
+        globals = caller.f_globals
+        lineno = caller.f_lineno
+    if '__name__' in globals:
+        module = globals['__name__']
+    else:
+        module = "<string>"
+    filename = globals.get('__file__')
+    if filename:
+        fnl = filename.lower()
+        if fnl.endswith((".pyc", ".pyo")):
+            filename = filename[:-1]
+    else:
+        if module == "__main__":
+            try:
+                filename = sys.argv[0]
+            except AttributeError:
+                # embedded interpreters don't have sys.argv, see bug #839151
+                filename = '__main__'
+        if not filename:
+            filename = module
+    registry = globals.setdefault("__warningregistry__", {})
+    warn_explicit(message, category, filename, lineno, module, registry,
+                  globals)
+
+def warn_explicit(message, category, filename, lineno,
+                  module=None, registry=None, module_globals=None):
+    if module is None:
+        module = filename or "<unknown>"
+        if module[-3:].lower() == ".py":
+            module = module[:-3] # XXX What about leading pathname?
+    if registry is None:
+        registry = {}
+    if isinstance(message, Warning):
+        text = str(message)
+        category = message.__class__
+    else:
+        text = message
+        message = category(message)
+    key = (text, category, lineno)
+    # Quick test for common case
+    if registry.get(key):
+        return
+    # Search the filters
+    for item in filters:
+        action, msg, cat, mod, ln = item
+        if ((msg is None or msg.match(text)) and
+            issubclass(category, cat) and
+            (mod is None or mod.match(module)) and
+            (ln == 0 or lineno == ln)):
+            break
+    else:
+        action = defaultaction
+    # Early exit actions
+    if action == "ignore":
+        registry[key] = 1
+        return
+
+    # Prime the linecache for formatting, in case the
+    # "file" is actually in a zipfile or something.
+    linecache.getlines(filename, module_globals)
+
+    if action == "error":
+        raise message
+    # Other actions
+    if action == "once":
+        registry[key] = 1
+        oncekey = (text, category)
+        if onceregistry.get(oncekey):
+            return
+        onceregistry[oncekey] = 1
+    elif action == "always":
+        pass
+    elif action == "module":
+        registry[key] = 1
+        altkey = (text, category, 0)
+        if registry.get(altkey):
+            return
+        registry[altkey] = 1
+    elif action == "default":
+        registry[key] = 1
+    else:
+        # Unrecognized actions are errors
+        raise RuntimeError(
+              "Unrecognized action (%r) in warnings.filters:\n %s" %
+              (action, item))
+    # Print message and context
+    showwarning(message, category, filename, lineno)
+
+
+# filters contains a sequence of filter 5-tuples
+# The components of the 5-tuple are:
+# - an action: error, ignore, always, default, module, or once
+# - a compiled regex that must match the warning message
+# - a class representing the warning category
+# - a compiled regex that must match the module that is being warned
+# - a line number for the line being warning, or 0 to mean any line
+# If either if the compiled regexs are None, match anything.
+_warnings_defaults = False
+try:
+    from _warnings import (filters, default_action, once_registry,
+                            warn, warn_explicit)
+    defaultaction = default_action
+    onceregistry = once_registry
+    _warnings_defaults = True
+except ImportError:
+    filters = []
+    defaultaction = "default"
+    onceregistry = {}
+
+
 # Module initialization
 _processoptions(sys.warnoptions)
-simplefilter("ignore", category=PendingDeprecationWarning, append=1)
-simplefilter("ignore", category=ImportWarning, append=1)
+if not _warnings_defaults:
+    simplefilter("ignore", category=PendingDeprecationWarning, append=1)
+    simplefilter("ignore", category=ImportWarning, append=1)
+    bytes_warning = sys.flags.bytes_warning
+    if bytes_warning > 1:
+        bytes_action = "error"
+    elif bytes_warning:
+        bytes_action = "default"
+    else:
+        bytes_action = "ignore"
+    simplefilter(bytes_action, category=BytesWarning, append=1)
+del _warnings_defaults