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

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`

  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.

  A simple example (this is not recommended as a real way of generating HTML!):

      from contextlib import contextmanager

      @contextmanager
      def tag(name):
          print("<%s>" % name)
          yield
          print("</%s>" % name)

      >>> with tag("h1"):
      ...    print("foo")
      ...
      <h1>
      foo
      </h1>

  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.

  `contextmanager()` uses `ContextDecorator` so the context managers it creates can be used as decorators as well as in `with` statements. When used as a decorator, a new generator instance is implicitly created on each function call (this allows the otherwise “one-shot” context managers created by `contextmanager()` to meet the requirement that context managers support multiple invocations in order to be used as decorators).

  Changed in version 3.2: Use of `ContextDecorator`.

`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
      from urllib.request import urlopen

      with closing(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.

class `contextlib.``ContextDecorator`

  A base class that enables a context manager to also be used as a decorator.

  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.

  `ContextDecorator` is used by `contextmanager()`, so you get this functionality automatically.

  Example of `ContextDecorator`:

      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')
      ...
      >>> function()
      Starting
      The bit in the middle
      Finishing

      >>> with mycontext():
      ...     print('The bit in the middle')
      ...
      Starting
      The bit in the middle
      Finishing

  This change is just syntactic sugar for any construct of the following form:

      def f():
          with cm():
              # Do stuff

  `ContextDecorator` lets you instead write:

      @cm()
      def f():
          # Do stuff

  It makes it clear that the `cm` applies to the whole function, rather than just a piece of it (and saving an indentation level is nice, too).

  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

  Note

  As the decorated function must be able to be called multiple times, the underlying context manager must support use in multiple `with` statements. If this is not the case, then the original construct with the explicit `with` statement inside the function should be used.

  New in version 3.2.

See also

PEP 0343 - The “with” statement
  The specification, background, and examples for the Python `with` statement.

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