Python Context Managers Exercises With Answers

Which two methods make an object a context manager?

1. __init__ and __repr__
2. __enter__ and __exit__correct
3. __iter__ and __next__
4. __get__ and __set__

Answer: __enter__ and __exit__

Why: The with statement calls __enter__ before the body and __exit__ after it.

When is __exit__ called?

1. When the with block exitscorrect
2. Only when the file is empty
3. Only when __enter__ fails
4. Before __enter__

Answer: When the with block exits

Why: __exit__ runs whether the body succeeds or raises an exception.

What does the program output?

class C:
    def __enter__(self):
        return "ok"
    def __exit__(self, *args):
        pass
with C() as value:
    print(value)

1. C
2. okcorrect
3. None
4. True

Answer: ok

Why: The value returned by __enter__ is bound by the as clause.

What does contextlib.contextmanager require in the function?

1. One return
2. One yieldcorrect
3. One class
4. One print

Answer: One yield

Why: The code before yield is setup, and the code after yield is cleanup.

Complete the statement that opens a managed block.

____ open("note.txt", "w") as f:
    f.write("hi")

Answer: with

Why: with starts a managed block and guarantees cleanup.

Complete the setup method.

class C:
    def ____(self):
        return self

Answer: __enter__

Why: __enter__ prepares the resource and returns the value bound by as.

Complete the cleanup method.

class C:
    def ____(self, exc_type, exc, tb):
        pass

Answer: __exit__

Why: __exit__ runs when the with block ends.

Complete the keyword that separates setup from cleanup.

from contextlib import contextmanager
@contextmanager
def c():
    ____ 1

Answer: yield

Why: contextmanager uses yield as the boundary between setup and cleanup.

What appears on the screen?

class C:
    def __enter__(self):
        return "ok"
    def __exit__(self, *args):
        return False
with C() as value:
    print(value)

Answer: ok

Why: The value from __enter__ is ok.

What is printed?

from contextlib import contextmanager
@contextmanager
def value():
    yield 7
with value() as number:
    print(number + 1)

Answer: 8

Why: The manager yields seven, and seven plus one is eight.

What is the output?

class C:
    def __enter__(self):
        return 2
    def __exit__(self, *args):
        pass
with C() as n:
    print(n * 3)

Answer: 6

Why: The manager yields two, and two times three is six.

What does this code print?

from contextlib import contextmanager
@contextmanager
def label():
    yield "x"
with label() as text:
    print(text.upper())

Answer: X

Why: The manager yields x, and upper makes it X.