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Filename : contextdecorator.py
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# Copyright (C) 2007-2010 Michael Foord
# E-mail: michael AT voidspace DOT org DOT uk
# http://pypi.python.org/pypi/contextdecorator
'''
Create objects that act as both context managers *and* as decorators, and behave
the same in both cases.

Context managers inheriting from ``ContextDecorator`` have to implement 
``__enter__`` and ``__exit__`` as normal. ``__exit__`` retains its optional
exception handling even when used as a decorator.

Example::

   from contextlib import ContextDecorator

   class mycontext(ContextDecorator):
      def __enter__(self):
         print 'Starting'
         return self

      def __exit__(self, *exc):
         print 'Finishing'
         return False

   @mycontext()
   def function():
      print 'The bit in the middle'
   
   with mycontext():
      print 'The bit in the middle'

Existing context managers that already have a base class can be extended by
using ``ContextDecorator`` as a mixin class::

   from contextlib import ContextDecorator

   class mycontext(ContextBaseClass, ContextDecorator):
      def __enter__(self):
         return self

      def __exit__(self, *exc):
         return False

'''

import sys

try:
    from functools import wraps
except ImportError:
    # Python 2.4 compatibility
    def wraps(original):
        def inner(f):
            f.__name__ = original.__name__
            return f
        return inner

# horrible reraise code for compatibility
# with Python 2 & 3
if sys.version_info >= (3,0):
    exec ("""
def _reraise(cls, val, tb):
    raise val
""")
else:
    exec ("""
def _reraise(cls, val, tb):
    raise cls, val, tb
""")

try:
    next
except NameError:
    # Python 2.4 / 2.5
    def next(gen):
        return gen.next()

__all__ = ['__version__', 'ContextDecorator', 'contextmanager']

__version__ = '0.10.0'


_NO_EXCEPTION = (None, None, None)

class ContextDecorator(object):
    "A base class or mixin that enables context managers to work as decorators."

    def __call__(self, f):
        @wraps(f)
        def inner(*args, **kw):
            self.__enter__()
            
            exc = _NO_EXCEPTION
            try:
                result = f(*args, **kw)
            except Exception:
                exc = sys.exc_info()
            
            catch = self.__exit__(*exc)
            
            if not catch and exc is not _NO_EXCEPTION:
                _reraise(*exc)
            return result
        return inner



class GeneratorContextManager(ContextDecorator):
    """Helper for @contextmanager decorator."""

    def __init__(self, gen):
        self.gen = gen

    def __enter__(self):
        try:
            return next(self.gen)
        except StopIteration:
            raise RuntimeError("generator didn't yield")

    def __exit__(self, type, value, traceback):
        if type is None:
            try:
                next(self.gen)
            except StopIteration:
                return
            else:
                raise RuntimeError("generator didn't stop")
        else:
            if value is None:
                # Need to force instantiation so we can reliably
                # tell if we get the same exception back
                value = type()
            try:
                self.gen.throw(type, value, traceback)
                raise RuntimeError("generator didn't stop after throw()")
            except StopIteration:
                # Suppress the exception *unless* it's the same exception that
                # was passed to throw().  This prevents a StopIteration
                # raised inside the "with" statement from being suppressed
                exc = sys.exc_info()[1]
                return exc is not value
            except:
                # only re-raise if it's *not* the exception that was
                # passed to throw(), because __exit__() must not raise
                # an exception unless __exit__() itself failed.  But throw()
                # has to raise the exception to signal propagation, so this
                # fixes the impedance mismatch between the throw() protocol
                # and the __exit__() protocol.
                #
                if sys.exc_info()[1] is not value:
                    raise


def contextmanager(func):
    """@contextmanager decorator.

    Typical usage:

        @contextmanager
        def some_generator(<arguments>):
            <setup>
            try:
                yield <value>
            finally:
                <cleanup>

    This makes this:

        with some_generator(<arguments>) as <variable>:
            <body>

    equivalent to this:

        <setup>
        try:
            <variable> = <value>
            <body>
        finally:
            <cleanup>

    """
    @wraps(func)
    def helper(*args, **kwds):
        return GeneratorContextManager(func(*args, **kwds))
    return helper