Context Managers (“with” Statement) Python 2.x2.5–2.7Python 3.x3.0–3.6

Examples

• 21

  Introduction to context managers and the with statement
  21

  A context manager is an object that is notified when a context (a block of code) starts and ends. You commonly use one with the with statement. It takes care of the notifying.

  For example, file objects are context managers. When a context ends, the file object is closed automatically:

  open_file = open(filename)
  with open_file:
      file_contents = open_file.read()
  
  # the open_file object has automatically been closed.
  

  The above example is usually simplified by using the as keyword:

  with open(filename) as open_file:
      file_contents = open_file.read()
  
  # the open_file object has automatically been closed.
  

  Anything that ends execution of the block causes the context manager's exit method to be called. This includes exceptions, and can be useful when an error causes you to prematurely exit from an open file or connection. Exiting a script without properly closing files/connections is a bad idea, that may cause data loss or other problems. By using a context manager you can ensure that precautions are always taken to prevent damage or loss in this way. This feature was added in Python 2.5.

  edited Nov 5 '16 at 13:23

  

  

  

  

  
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• 20

  Writing your own context manager
  20

  A context manager is any object that implements two magic methods __enter__() and __exit__() (although it can implement other methods as well):

  class AContextManager():
  
      def __enter__(self):
          print("Entered")
          # optionally return an object
          return "A-instance"
  
      def __exit__(self, exc_type, exc_value, traceback):
          print("Exited" + (" (with an exception)" if exc_type else ""))
          # return True if you want to suppress the exception
  

  If the context exits with an exception, the information about that exception will be passed as a triple exc_type, exc_value, traceback (these are the same variables as returned by the sys.exc_info() function). If the context exits normally, all three of these arguments will be None.

  If an exception occurs and is passed to the __exit__ method, the method can return True in order to suppress the exception, or the exception will be re-raised at the end of the __exit__ function.

  with AContextManager() as a:
      print("a is %r" % a)
  # Entered
  # a is 'A-instance'
  # Exited
  
  with AContextManager() as a:
      print("a is %d" % a)
  # Entered
  # Exited (with an exception)
  # Traceback (most recent call last):
  #   File "<stdin>", line 2, in <module>
  # TypeError: %d format: a number is required, not str
  

  Note that in the second example even though an exception occurs in the middle of the body of the with-statement, the __exit__ handler still gets executed, before the exception propagates to the outer scope.

  If you only need an __exit__ method, you can return the instance of the context manager:

  class MyContextManager:
      def __enter__(self):
          return self
  
      def __exit__(self):
          print('something')
  

  edited Dec 5 '16 at 19:46

  

  

  

  

  
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• 8

  Writing your own contextmanager using generator syntax
  8

  It is also possible to write a context manager using generator syntax thanks to the contextlib.contextmanager decorator:

  import contextlib
  
  @contextlib.contextmanager
  def context_manager(num):
      print('Enter')
      yield num + 1
      print('Exit')
  
  with context_manager(2) as cm:
      # the following instructions are run when the 'yield' point of the context
      # manager is reached.
      # 'cm' will have the value that was yielded
      print('Right in the middle with cm = {}'.format(cm))
  

  produces:

  Enter
  Right in the middle with cm = 3
  Exit
  

  The decorator simplifies the task of writing a context manager by converting a generator into one. Everything before the yield expression becomes the __enter__ method, the value yielded becomes the value returned by the generator (which can be bound to a variable in the with statement), and everything after the yield expression becomes the __exit__ method.

  If an exception needs to be handled by the context manager, a try..except..finally-block can be written in the generator and any exception raised in the with-block will be handled by this exception block.

  @contextlib.contextmanager
  def error_handling_context_manager(num):
      print("Enter")
      try:
          yield num + 1
      except ZeroDivisionError:
          print("Caught error")
      finally:
          print("Cleaning up")
      print("Exit")
  
  with error_handling_context_manager(-1) as cm:
      print("Dividing by cm = {}".format(cm))
      print(2 / cm)
  

  This produces:

  Enter
  Dividing by cm = 0
  Caught error
  Cleaning up
  Exit
  

  edited Jul 29 '16 at 14:51

  

  

  

  
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• 6

  Multiple context managers
  6

  You can open several content managers at the same time:

  with open(input_path) as input_file, open(output_path, 'w') as output_file:
  
      # do something with both files. 
      
      # e.g. copy the contents of input_file into output_file
      for line in input_file:
          output_file.write(line + '\n')
  

  It has the same effect as nesting context managers:

  with open(input_path) as input_file:
      with open(output_path, 'w') as output_file:
          for line in input_file:
              output_file.write(line + '\n')
  

  edited Jul 27 '16 at 13:18

  

  
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• 2

  Assigning to a target
  2

  Many context managers return an object when entered. You can assign that object to a new name in the with statement.

  For example, using a database connection in a with statement could give you a cursor object:

  with database_connection as cursor:
      cursor.execute(sql_query)
  

  File objects return themselves, this makes it possible to both open the file object and use it as a context manager in one expression:

  with open(filename) as open_file:
      file_contents = open_file.read()
  

  edited Mar 11 at 16:09

  

  

  
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• 2

  Manage Resources
  2

  class File():
      def __init__(self, filename, mode):
          self.filename = filename
          self.mode = mode
  
      def __enter__(self):
          self.open_file = open(self.filename, self.mode)
          return self.open_file
  
      def __exit__(self, *args):
          self.open_file.close()
  

  __init__() method sets up the object, in this case setting up the file name and mode to open file. __enter__() opens and returns the file and __exit__() just closes it.

  Using these magic methods (__enter__, __exit__) allows you to implement objects which can be used easily with the with statement.

  Use File class:

  for _ in range(10000):
      with File('foo.txt', 'w') as f:
          f.write('foo')
  

  created Jul 22 '16 at 9:20

  
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