with Statement

Syntax

with EXPRESSION as TARGET:
    SUITE

as TARGET is optional.

Execution Steps

  1. The context expression (the expression given in the EXPRESSION) is evaluated to obtain a context manager.
  2. The context manager’s __enter__() is loaded for later use.
  3. The context manager’s __exit__() is loaded for later use.
  4. The context manager’s __enter__() method is invoked.
  5. If TARGET was included in the with statement, the return value from __enter__() is assigned to it.
  6. The SUITE is executed.
  7. The context manager’s __exit__() method is invoked.
  8. If an exception caused the SUITE to be exited, its type, value, and traceback are passed as arguments to __exit__(). Otherwise, three None arguments are supplied.
  9. If the SUITE was exited due to an exception, and the return value from the __exit__() method was False, the exception is reraised. If the return value was true, the exception is suppressed, and execution continues with the statement following the with statement.

semantically equivalent to:

manager = (EXPRESSION)
enter = type(manager).__enter__  # Not calling it yet
exit = type(manager).__exit__    # Not calling it yet
value = enter(manager)
hit_except = False

try:
    TARGET = value  # Only if "as VAR" is present
    SUITE
except:
    hit_except = True
    if not exit(manager, *sys.exc_info()):
        raise
finally:
    if not hit_except:
        exit(manager, None, None, None)

References