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# 28.7. `contextlib` — Utilities for `with`-statement contexts

2.5 版新加入.

Source code: Lib/contextlib.py

This module provides utilities for common tasks involving the `with` statement. For more information see also Context Manager Types and With Statement Context Managers.

Functions provided:

`contextlib.``contextmanager`(func)

  This function is a decorator that can be used to define a factory function for `with` statement context managers, without needing to create a class or separate `__enter__()` and `__exit__()` methods.

  While many objects natively support use in with statements, sometimes a resource needs to be managed that isn’t a context manager in its own right, and doesn’t implement a `close()` method for use with `contextlib.closing`

  An abstract example would be the following to ensure correct resource management:

      from contextlib import contextmanager

      @contextmanager
      def managed_resource(*args, **kwds):
          # Code to acquire resource, e.g.:
          resource = acquire_resource(*args, **kwds)
          try:
              yield resource
          finally:
              # Code to release resource, e.g.:
              release_resource(resource)

      >>> with managed_resource(timeout=3600) as resource:
      ...     # Resource is released at the end of this block,
      ...     # even if code in the block raises an exception

  The function being decorated must return a generator-iterator when called. This iterator must yield exactly one value, which will be bound to the targets in the `with` statement’s `as` clause, if any.

  At the point where the generator yields, the block nested in the `with` statement is executed. The generator is then resumed after the block is exited. If an unhandled exception occurs in the block, it is reraised inside the generator at the point where the yield occurred. Thus, you can use a `try`…`except`…`finally` statement to trap the error (if any), or ensure that some cleanup takes place. If an exception is trapped merely in order to log it or to perform some action (rather than to suppress it entirely), the generator must reraise that exception. Otherwise the generator context manager will indicate to the `with` statement that the exception has been handled, and execution will resume with the statement immediately following the `with` statement.

`contextlib.``nested`(mgr1[, mgr2[, ...]])

  Combine multiple context managers into a single nested context manager.

  This function has been deprecated in favour of the multiple manager form of the `with` statement.

  The one advantage of this function over the multiple manager form of the `with` statement is that argument unpacking allows it to be used with a variable number of context managers as follows:

      from contextlib import nested

      with nested(*managers):
          do_something()

  Note that if the `__exit__()` method of one of the nested context managers indicates an exception should be suppressed, no exception information will be passed to any remaining outer context managers. Similarly, if the `__exit__()` method of one of the nested managers raises an exception, any previous exception state will be lost; the new exception will be passed to the `__exit__()` methods of any remaining outer context managers. In general, `__exit__()` methods should avoid raising exceptions, and in particular they should not re-raise a passed-in exception.

  This function has two major quirks that have led to it being deprecated. Firstly, as the context managers are all constructed before the function is invoked, the `__new__()` and `__init__()` methods of the inner context managers are not actually covered by the scope of the outer context managers. That means, for example, that using `nested()` to open two files is a programming error as the first file will not be closed promptly if an exception is thrown when opening the second file.

  Secondly, if the `__enter__()` method of one of the inner context managers raises an exception that is caught and suppressed by the `__exit__()` method of one of the outer context managers, this construct will raise `RuntimeError` rather than skipping the body of the `with` statement.

  Developers that need to support nesting of a variable number of context managers can either use the `warnings` module to suppress the DeprecationWarning raised by this function or else use this function as a model for an application specific implementation.

  2.7 版後已棄用: The with-statement now supports this functionality directly (without the confusing error prone quirks).

`contextlib.``closing`(thing)

  Return a context manager that closes thing upon completion of the block. This is basically equivalent to:

      from contextlib import contextmanager

      @contextmanager
      def closing(thing):
          try:
              yield thing
          finally:
              thing.close()

  And lets you write code like this:

      from contextlib import closing
      import urllib

      with closing(urllib.urlopen('http://www.python.org')) as page:
          for line in page:
              print line

  without needing to explicitly close `page`. Even if an error occurs, `page.close()` will be called when the `with` block is exited.

也參考

PEP 343 - The 「with」 statement

  The specification, background, and examples for the Python `with` statement.