enter and exit

Learn to write context managers with __enter__ and __exit__.

enter and exit

To make a class a context manager two methods are defined: __enter__ runs at the start of the block and returns the resource; __exit__ runs at the end and does cleanup:

class Kaynak:
    def __enter__(self):
        print("aç")
        return self
    def __exit__(self, *args):
        print("kapat")

These two methods make the with statement work. __exit__ takes three more arguments — they carry exception info.

This is the way to define a class-based context manager.

exit and exception info

The __exit__ method takes three extra arguments: exc_type, exc_val, exc_tb. They carry info when an error occurs inside the block. If no error they are all None:

def __exit__(self, exc_type, exc_val, exc_tb):
    if exc_type is not None:
        print("hata oluştu")
    print("kapat")

If __exit__ returns True the error is suppressed — it does not propagate outside the with block. If it returns False or None the error propagates normally.

This enables error management inside a context manager.

Exercise Write a class-based context manager

Define class Kaynak:. In __enter__ write print("aç") and return self. In __exit__ write print("kapat"). Then inside a with Kaynak(): block write print("içeride").

Show example solution

class Kaynak:
    def __enter__(self):
        print("aç")
        return self
    def __exit__(self, *a):
        print("kapat")

with Kaynak():
    print("içeride")

Test yourself

Complete the __enter__ method

class Kaynak:
    def ____(self):
        print("aç")
        return self

Show answer

class Kaynak:
    def __enter__(self):
        print("aç")
        return self

Match __enter__/__exit__

__enter__

Block entry
__exit__

Block exit
exc_type

Error type
return True

Suppress error
What happens if __exit__ returns True?
- The error is raised twice
- The program crashes
- The block reruns
- The error is suppressed
Show answer

The error is suppressed — If __exit__ returns True the error is suppressed and does not propagate outside with.

Next lesson: @contextmanager

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