Is a context manager exited if a generator is garbage collected?

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Suppose we have a simple generator like this:

def generator():
    with open('example.txt', 'r') as f:
        yield f.readline()
        yield f.readline()

and use it like this to get a single element/line:

line = next(generator())

Is the context manager closed once the temporary generator object becomes garbage collected?

contextmanager generator python
2 Answers

A with statement has the same semantics as a try statement, with some additional code execute aside from the body of the with statement itself. From https://docs.python.org/3/reference/compound_stmts.html#the-with-statement"

The following code:

with EXPRESSION as TARGET:
    SUITE

is semantically equivalent to:

manager = (EXPRESSION)
enter = type(manager).__enter__
exit = type(manager).__exit__
value = enter(manager)
hit_except = False

try:
    TARGET = value
    SUITE
except:
    hit_except = True
    if not exit(manager, *sys.exc_info()):
        raise
finally:
    if not hit_except:
        exit(manager, None, None, None)

So our yield expressions are inside the try statement implied by the with statement.

From https://docs.python.org/3/reference/expressions.html#yieldexpr

Yield expressions are allowed anywhere in a try construct. If the generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), the generator-iterator’s close() method will be called, allowing any pending finally clauses to execute.

The finally clause implied by the with statement explicitly calls the __exit__ method of the context manager, which closes the file.

user_11268410

We can test it. The answer is yes, it is closed.

import contextlib
@contextlib.contextmanager
def tester():
    try:
        print("enter")
        yield 42
    finally:
        print("exit")
        
       
def generator():
    with tester() as m:
        yield m
        
print(next(generator()))

Result:

enter
exit
42
user_93479590
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