Python Context Manager
External
Internal
Overview
An execution context is useful when the code uses resources that need closing, even if the code follows an unexpected execution path, like when an exception is raised.
An execution context is equivalent with the following construct:
r = Resource()
r.open()
try:
#
# use resource, while anywhere in this code block exception might be thrown
#
finally:
r.close()
The equivalent semantics using a context manager can be achieved with this syntax:
with <create-context> as ctx:
#
# use the context
#
#
# at the exit from the indented block, the context is automatically cleaned up
#
How it Works
When the interpreter encounters the with statement, it executes whatever follows with, expecting that the result of the execution is a context manager object: an object that implements the [[#The_Context_Manager_Protocol|context manager protocol].
The interpreter continues by invoking the __enter__() on the context manager instance. The __enter__() method may optionally return an object instance, which is assigned to the variable name following the as reserved keyword. The instance is accessible from the indented block that follows with.