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

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?

Solution

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.