源码# -*- coding: utf-8 -*-"""    flask._compat    ~~~~~~~~~~~~~    Some py2/py3 compatibility support based on a stripped down    version of six so we don't have to depend on a specific version    of it.    :copyright: © 2010 by the Pallets team.    :license: BSD, see LICENSE for more details."""import sysPY2 = sys.version_info[0] == 2_identity = lambda x: xif not PY2:    text_type = str    string_types = (str,)    integer_types = (int,)    iterkeys = lambda d: iter(d.keys())    itervalues = lambda d: iter(d.values())    iteritems = lambda d: iter(d.items())    from inspect import getfullargspec as getargspec    from io import StringIO    def reraise(tp, value, tb=None):        if value.__traceback__ is not tb:            raise value.with_traceback(tb)        raise value    implements_to_string = _identityelse:    text_type = unicode    string_types = (str, unicode)    integer_types = (int, long)    iterkeys = lambda d: d.iterkeys()    itervalues = lambda d: d.itervalues()    iteritems = lambda d: d.iteritems()    from inspect import getargspec    from cStringIO import StringIO    exec('def reraise(tp, value, tb=None):\n raise tp, value, tb')    def implements_to_string(cls):        cls.__unicode__ = cls.__str__        cls.__str__ = lambda x: x.__unicode__().encode('utf-8')        return clsdef with_metaclass(meta, *bases):    """Create a base class with a metaclass."""    # This requires a bit of explanation: the basic idea is to make a    # dummy metaclass for one level of class instantiation that replaces    # itself with the actual metaclass.    class metaclass(type):        def __new__(cls, name, this_bases, d):            return meta(name, bases, d)    return type.__new__(metaclass, 'temporary_class', (), {})# Certain versions of pypy have a bug where clearing the exception stack# breaks the __exit__ function in a very peculiar way.  The second level of# exception blocks is necessary because pypy seems to forget to check if an# exception happened until the next bytecode instruction?## Relevant PyPy bugfix commit:# https://bitbucket.org/pypy/pypy/commits/77ecf91c635a287e88e60d8ddb0f4e9df4003301# According to ronan on #pypy IRC, it is released in PyPy2 2.3 and later# versions.## Ubuntu 14.04 has PyPy 2.2.1, which does exhibit this bug.BROKEN_PYPY_CTXMGR_EXIT = Falseif hasattr(sys, 'pypy_version_info'):    class _Mgr(object):        def __enter__(self):            return self        def __exit__(self, *args):            if hasattr(sys, 'exc_clear'):                # Python 3 (PyPy3) doesn't have exc_clear                sys.exc_clear()    try:        try:            with _Mgr():                raise AssertionError()        except:            raise    except TypeError:        BROKEN_PYPY_CTXMGR_EXIT = True    except AssertionError:        pass解读本模块主要是做了些兼容处理，主要从python2与其他版本进行比较。从这可以看出python2在编码等方面是有一定差异的，我们了解即可，以下一些关键语句就是对这种差异做兼容处理：

text_type = unicode   string_types = (str, unicode)

integer_types = (int, long)

cls.__str__ = lambda x: x.__unicode__().encode('utf-8')