A context to exit deeply nested loops

[Gribouillis]

The break statement doesn't allow to exit several nested loops. It is a simple exercise to develop a block() context to do so. Here is an attempt

from contextlib import contextmanager @contextmanager def block(): """Block of code context allowing to exit deeply nested loops Usage: with block() as b: ... # somewhere in a nested loop b.exit() ... """ tok = BlockToken() try: yield tok except EndOfBlock as exc: if exc.args[0] is not tok: raise class EndOfBlock(RuntimeError): """Helper exception type for the block() context""" class BlockToken: """Helper class for block() context""" def exit(self): raise EndOfBlock(self) def func(x): """Test function for the block() context""" result = '' with block() as outer: result += 'A' for i in range(4): result += 'B' if x > 0 and i > 2: outer.exit() result += 'D' with block() as inner: result += 'C' if x == 1: outer.exit() elif x == 2: inner.exit() result += 'E' result += 'F' result += 'G' result += 'H' return result if __name__ == '__main__': assert func(0) == "ABDCEFBDCEFBDCEFBDCEFGH" assert func(1) == "ABDCH" assert func(2) == "ABDCFBDCFBDCFBH" print('SUCCESS')

An interesting feature here is that the BlockToken instance can be used as argument for functions called within the block. For example you could write

def do_something(session): if input('Do you want to quit?').strip().lower() in ('y', 'yes'): session.exit() for i in range(5): print(i) with block() as user_session: do_something(user_session) print("If we're here, the user chose to stay!")

[Yoriz]

If the nested loop is in a function, return can be used instead of break to exit.

[Gribouillis]

if the nested loop is in a function, return can be used instead of break to exit.

Yes, but in this case, session.exit() exits more than the function. It exits from a block of code that contained the call to the function.

[Gribouillis]

In this second version, we avoid using the @contextmanager decorator. Instead, block is now a class. The advantage of this is that one can subclass block to create exitable contexts with more data, such as a user session with a user name, a user device, etc

__version__ = '2109.05.19' class block: """Block of code context allowing to exit deeply nested loops Usage: with block() as b: ... # somewhere in a nested loop b.exit() ... This type can be subclassed to create exitable contexts with useful data, for example class UserSession(block) def __init__(self, user_name): self.user_name = user_name with UserSession('Doe') as session: ... session.exit() ... """ def __enter__(self): return self def exit(self): raise EndOfBlock(self) def __exit__(self, exc_type, exc_value, ex_tb): if exc_type is EndOfBlock and exc_value.args[0] is self: # exception isn't propagated if __exit__ returns true value return True class EndOfBlock(RuntimeError): """Helper exception type for the block() context""" if __name__ == '__main__': # Example code for blocks class UserSession(block): def __init__(self, user_name): super().__init__() self.user_name = user_name def __enter__(self): print("User {}'s session starting now!".format(self.user_name)) return super().__enter__() def __exit__(self, *args): print('User {} is leaving.'.format(self.user_name)) return super().__exit__(*args) def func(x): """Test function for the block() context""" result = '' with UserSession('Doe') as outer: result += 'A' for i in range(4): result += 'B' if x > 0 and i > 2: outer.exit() result += 'D' with block() as inner: result += 'C' if x == 1: outer.exit() elif x == 2: inner.exit() result += 'E' result += 'F' result += 'G' result += 'H' return result assert func(0) == "ABDCEFBDCEFBDCEFBDCEFGH" assert func(1) == "ABDCH" assert func(2) == "ABDCFBDCFBDCFBH" print('SUCCESS')

Output:
User Doe's session starting now! User Doe is leaving. User Doe's session starting now! User Doe is leaving. User Doe's session starting now! User Doe is leaving. SUCCESS