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# 28.6. `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.

## 28.6.1. Utilities

Functions and classes 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.

class `contextlib.``ExitStack`

  A context manager that is designed to make it easy to programmatically combine other context managers and cleanup functions, especially those that are optional or otherwise driven by input data.

  For example, a set of files may easily be handled in a single with statement as follows:

      with ExitStack() as stack:
          files = [stack.enter_context(open(fname)) for fname in filenames]
          # All opened files will automatically be closed at the end of
          # the with statement, even if attempts to open files later
          # in the list throw an exception

  Each instance maintains a stack of registered callbacks that are called in reverse order when the instance is closed (either explicitly or implicitly at the end of a `with` statement). Note that callbacks are not invoked implicitly when the context stack instance is garbage collected.

  This stack model is used so that context managers that acquire their resources in their `__init__` method (such as file objects) can be handled correctly.

  Since registered callbacks are invoked in the reverse order of registration, this ends up behaving as if multiple nested `with` statements had been used with the registered set of callbacks. This even extends to exception handling - if an inner callback suppresses or replaces an exception, then outer callbacks will be passed arguments based on that updated state.

  This is a relatively low level API that takes care of the details of correctly unwinding the stack of exit callbacks. It provides a suitable foundation for higher level context managers that manipulate the exit stack in application specific ways.

  New in version 3.3.

  `enter_context`(cm)

    Enters a new context manager and adds its `__exit__()` method to the callback stack. The return value is the result of the context manager’s own `__enter__()` method.

    These context managers may suppress exceptions just as they normally would if used directly as part of a `with` statement.

  `push`(exit)

    Adds a context manager’s `__exit__()` method to the callback stack.

    As `__enter__` is not invoked, this method can be used to cover part of an `__enter__()` implementation with a context manager’s own `__exit__()` method.

    If passed an object that is not a context manager, this method assumes it is a callback with the same signature as a context manager’s `__exit__()` method and adds it directly to the callback stack.

    By returning true values, these callbacks can suppress exceptions the same way context manager `__exit__()` methods can.

    The passed in object is returned from the function, allowing this method to be used as a function decorator.

  `callback`(callback, *args, **kwds)

    Accepts an arbitrary callback function and arguments and adds it to the callback stack.

    Unlike the other methods, callbacks added this way cannot suppress exceptions (as they are never passed the exception details).

    The passed in callback is returned from the function, allowing this method to be used as a function decorator.

  `pop_all`()

    Transfers the callback stack to a fresh `ExitStack` instance and returns it. No callbacks are invoked by this operation - instead, they will now be invoked when the new stack is closed (either explicitly or implicitly at the end of a `with` statement).

    For example, a group of files can be opened as an “all or nothing” operation as follows:

        with ExitStack() as stack:
            files = [stack.enter_context(open(fname)) for fname in filenames]
            close_files = stack.pop_all().close
            # If opening any file fails, all previously opened files will be
            # closed automatically. If all files are opened successfully,
            # they will remain open even after the with statement ends.
            # close_files() can then be invoked explicitly to close them all

  `close`()

    Immediately unwinds the callback stack, invoking callbacks in the reverse order of registration. For any context managers and exit callbacks registered, the arguments passed in will indicate that no exception occurred.

## 28.6.2. Examples and Recipes

This section describes some examples and recipes for making effective use of the tools provided by `contextlib`.

### 28.6.2.1. Supporting a variable number of context managers

The primary use case for `ExitStack` is the one given in the class documentation: supporting a variable number of context managers and other cleanup operations in a single `with` statement. The variability may come from the number of context managers needed being driven by user input (such as opening a user specified collection of files), or from some of the context managers being optional:

    with ExitStack() as stack:
        for resource in resources:
            stack.enter_context(resource)
        if need_special resource:
            special = acquire_special_resource()
            stack.callback(release_special_resource, special)
        # Perform operations that use the acquired resources

As shown, `ExitStack` also makes it quite easy to use `with` statements to manage arbitrary resources that don’t natively support the context management protocol.

### 28.6.2.2. Simplifying support for single optional context managers

In the specific case of a single optional context manager, `ExitStack` instances can be used as a “do nothing” context manager, allowing a context manager to easily be omitted without affecting the overall structure of the source code:

    def debug_trace(details):
        if __debug__:
            return TraceContext(details)
        # Don't do anything special with the context in release mode
        return ExitStack()

    with debug_trace():
        # Suite is traced in debug mode, but runs normally otherwise

### 28.6.2.3. Catching exceptions from `__enter__` methods

It is occasionally desirable to catch exceptions from an `__enter__` method implementation, without inadvertently catching exceptions from the `with` statement body or the context manager’s `__exit__` method. By using `ExitStack` the steps in the context management protocol can be separated slightly in order to allow this:

    stack = ExitStack()
    try:
        x = stack.enter_context(cm)
    except Exception:
        # handle __enter__ exception
    else:
        with stack:
            # Handle normal case

Actually needing to do this is likely to indicate that the underlying API should be providing a direct resource management interface for use with `try`/`except`/`finally` statements, but not all APIs are well designed in that regard. When a context manager is the only resource management API provided, then `ExitStack` can make it easier to handle various situations that can’t be handled directly in a `with` statement.

### 28.6.2.4. Cleaning up in an `__enter__` implementation

As noted in the documentation of `ExitStack.push()`, this method can be useful in cleaning up an already allocated resource if later steps in the `__enter__()` implementation fail.

Here’s an example of doing this for a context manager that accepts resource acquisition and release functions, along with an optional validation function, and maps them to the context management protocol:

    from contextlib import contextmanager, ExitStack

    class ResourceManager(object):

        def __init__(self, acquire_resource, release_resource, check_resource_ok=None):
            self.acquire_resource = acquire_resource
            self.release_resource = release_resource
            if check_resource_ok is None:
                def check_resource_ok(resource):
                    return True
            self.check_resource_ok = check_resource_ok

        @contextmanager
        def _cleanup_on_error(self):
            with ExitStack() as stack:
                stack.push(self)
                yield
                # The validation check passed and didn't raise an exception
                # Accordingly, we want to keep the resource, and pass it
                # back to our caller
                stack.pop_all()

        def __enter__(self):
            resource = self.acquire_resource()
            with self._cleanup_on_error():
                if not self.check_resource_ok(resource):
                    msg = "Failed validation for {!r}"
                    raise RuntimeError(msg.format(resource))
            return resource

        def __exit__(self, *exc_details):
            # We don't need to duplicate any of our resource release logic
            self.release_resource()

### 28.6.2.5. Replacing any use of `try-finally` and flag variables

A pattern you will sometimes see is a `try-finally` statement with a flag variable to indicate whether or not the body of the `finally` clause should be executed. In its simplest form (that can’t already be handled just by using an `except` clause instead), it looks something like this:

    cleanup_needed = True
    try:
        result = perform_operation()
        if result:
            cleanup_needed = False
    finally:
        if cleanup_needed:
            cleanup_resources()

As with any `try` statement based code, this can cause problems for development and review, because the setup code and the cleanup code can end up being separated by arbitrarily long sections of code.

`ExitStack` makes it possible to instead register a callback for execution at the end of a `with` statement, and then later decide to skip executing that callback:

    from contextlib import ExitStack

    with ExitStack() as stack:
        stack.callback(cleanup_resources)
        result = perform_operation()
        if result:
            stack.pop_all()

This allows the intended cleanup up behaviour to be made explicit up front, rather than requiring a separate flag variable.

If a particular application uses this pattern a lot, it can be simplified even further by means of a small helper class:

    from contextlib import ExitStack

    class Callback(ExitStack):
        def __init__(self, callback, *args, **kwds):
            super(Callback, self).__init__()
            self.callback(callback, *args, **kwds)

        def cancel(self):
            self.pop_all()

    with Callback(cleanup_resources) as cb:
        result = perform_operation()
        if result:
            cb.cancel()

If the resource cleanup isn’t already neatly bundled into a standalone function, then it is still possible to use the decorator form of `ExitStack.callback()` to declare the resource cleanup in advance:

    from contextlib import ExitStack

    with ExitStack() as stack:
        @stack.callback
        def cleanup_resources():
            ...
        result = perform_operation()
        if result:
            stack.pop_all()

Due to the way the decorator protocol works, a callback function declared this way cannot take any parameters. Instead, any resources to be released must be accessed as closure variables

### 28.6.2.6. Using a context manager as a function decorator

`ContextDecorator` makes it possible to use a context manager in both an ordinary `with` statement and also as a function decorator.

For example, it is sometimes useful to wrap functions or groups of statements with a logger that can track the time of entry and time of exit. Rather than writing both a function decorator and a context manager for the task, inheriting from `ContextDecorator` provides both capabilities in a single definition:

    from contextlib import ContextDecorator
    import logging

    logging.basicConfig(level=logging.INFO)

    class track_entry_and_exit(ContextDecorator):
        def __init__(self, name):
            self.name = name

        def __enter__(self):
            logging.info('Entering: {}'.format(name))

        def __exit__(self, exc_type, exc, exc_tb):
            logging.info('Exiting: {}'.format(name))

Instances of this class can be used as both a context manager:

    with track_entry_and_exit('widget loader'):
        print('Some time consuming activity goes here')
        load_widget()

And also as a function decorator:

    @track_entry_and_exit('widget loader')
    def activity():
        print('Some time consuming activity goes here')
        load_widget()

Note that there is one additional limitation when using context managers as function decorators: there’s no way to access the return value of `__enter__()`. If that value is needed, then it is still necessary to use an explicit `with` statement.

See also

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

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