Shows how the with statement / context managers are semantically equivalent with a try-except-finally block.

contextmanagers.py

"""
Shows how the with statement / context managers are semantically equivalent
with a try-except-finally block.

https://docs.python.org/3/reference/compound_stmts.html#the-try-statement
https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement
https://docs.python.org/3/reference/compound_stmts.html#the-with-statement
https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers
https://docs.python.org/3.7/library/sys.html#sys.exc_info

"""

import sys


def using_try_except_finally():
    """What you did before context managers."""
    print('before')
    try:
        v = 'during'
        print(v)
        raise ValueError('error from try except')
    except Exception as e:
        print(e)
        raise
    finally:
        print('after')
 

class ContextManager:
    
    """Useful if at least one of the before, during or after parts
    are reusable.
    
    """
    
    def __enter__(self):
        print('before')
        return 'during'
        
    def __exit__(self, exc_type, exc_value, traceback):
        if exc_value:
            print(exc_value)
        print('after')
        

def using_context_managers():
    """What you do with context managers."""
    cm = ContextManager()
    with cm as v:
        print(v)
        raise ValueError('error from with')


def using_context_managers_manually():
    """What the interpreter does when you use context managers."""
    cm = ContextManager()
    exc_info = None
    v = cm.__enter__()
    try:
        print(v)
        raise ValueError('error from magic')
    except Exception as e:
        exc_info = sys.exc_info()
    finally:
        if not cm.__exit__(*exc_info):
            if exc_info:
                exc_type, exc_value, traceback = exc_info
                raise exc_value


functions = [
    using_try_except_finally,
    using_context_managers, 
    using_context_managers_manually,
]

for f in functions:
    try:
        print('# calling', f.__name__)
        f()
    except Exception as e:
        print('# caught exception:', e)
    print()