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Context managers

Basic structure

class BasicContextManager:
    def __enter__(self):
        print("Entering Context Manager")

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Exiting Context Manager")

Usage:

with BasicContextManager():
    print('In context')

A context manager is an object

A context manager is a class and so, an object. It can have a __init__(self) method.

class BasicContextManager:
    def __init__(self):
        print("Initializing")
        self.my_id = "I'm the context"

    def __enter__(self):
        print("Entering Context Manager")

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Exiting Context Manager")

bc = BasicContextManager()
with bc:
    print(f'In context {bc.my_id}')

Idiome with as

The keyword as gets the return of the __enter__(self) method.

class BasicContextManager:
    def __init__(self):
        print("Initializing")
        self.my_id = "I'm the context"

    def __enter__(self):
        print("Entering Context Manager")
        return self

Let us write: 

with BasicContextManager() as bc:
    print(f'In context {bc.my_id}')

Exceptions management

__exit__() parameters

Exceptions thrown in the context can be managed in the __exit__() method.

The method parameters will be of use there. The entire signature is __exit__(self, exc_type, exc_value, exc_tb).

  • exc_type : is the exception type (class)
  • exc_value : is the exception instance (object)
  • exc_tb : is a traceback object

Warning, you shouldn't rise an exception in the exit method.

Exception propagation

If you have managed the exception and it shouldn't be propagated, the method must return a truthy value. If the method returns a falsy value, which is the default as it returns None, the exception is propagated up.