4 ms·

Yes the regex-parsing FSM class in the article looks cleaner with a single coroutine: class FSM: def init(self): self.match = False

by glxxyz 7y ago

Yes the regex-parsing FSM class in the article looks cleaner with a single coroutine: class FSM: def __init__(self): self.match = False self.current_state = self.create_state_machine() next(self.current_state) def does_match(self): return self.match def send(self, char): try: self.current_state.send(char) except StopIteration: self.match = False pass def create_state_machine(self): char = yield if char == 'a': while True: char = yield if char == 'b': pass elif char == 'c': self.match = True yield break else: break

OJFord 7y ago
That makes more sense. And we could still break out states into functions without being quite so explicit as in the article (and without the __init__ problem I described): def create_state_machine(self): if yield == 'a': yield from self.q2() def q2(self): while (char := yield) == 'b': pass if char == 'c': self.match = True Or something like that anyway, untested of course. (NB that actually fixes a bug in yours in that it would continue processing 'b's after the 'c' - of course that's still a match for the regex, but the FSM should have stopped.)
- glxxyz 7y ago
  Thanks for pointing that out, I think I fixed it. Not well tested ;) I'm not sure if the way that you're using yield within the if and while statements works correctly. I like the idea of replacing state transitions with method calls but you'll run into recursion depth problems with a state machine that alternates between two states.
  - OJFord 7y ago
    > I'm not sure if the way that you're using yield within the if and while statements works correctly. You're right... Perhaps I'm not so surprised that `if yield == val:` is a SyntaxError, but that `yield` can't be used as an rvalue to a walrus operator (that the `while` example essentially boils down to) surprises me: >>> char = yield File "<stdin>", line 1 SyntaxError: 'yield' outside function >>> >>> # i.e. OK, that's the error we want if it works >>> >>> got_b = (char := yield) == 'b' File "<stdin>", line 1 got_b = (char := yield) == 'b' ^ SyntaxError: invalid syntax

Yes the regex-parsing FSM class in the article looks cleaner with a single coroutine: class FSM: def __init__(self): self.match = False

by glxxyz 7y ago

OJFord 7y ago

That makes more sense. And we could still break out states into functions without being quite so explicit as in the article (and without the __init__ problem I described): def create_state_machine(self): if yield == 'a': yield from self.q2() def q2(self): while (char := yield) == 'b': pass if char == 'c': self.match = True Or something like that anyway, untested of course. (NB that actually fixes a bug in yours in that it would continue processing 'b's after the 'c' - of course that's still a match for the regex, but the FSM should have stopped.)

glxxyz 7y ago
Thanks for pointing that out, I think I fixed it. Not well tested ;) I'm not sure if the way that you're using yield within the if and while statements works correctly. I like the idea of replacing state transitions with method calls but you'll run into recursion depth problems with a state machine that alternates between two states.
- OJFord 7y ago
  > I'm not sure if the way that you're using yield within the if and while statements works correctly. You're right... Perhaps I'm not so surprised that `if yield == val:` is a SyntaxError, but that `yield` can't be used as an rvalue to a walrus operator (that the `while` example essentially boils down to) surprises me: >>> char = yield File "<stdin>", line 1 SyntaxError: 'yield' outside function >>> >>> # i.e. OK, that's the error we want if it works >>> >>> got_b = (char := yield) == 'b' File "<stdin>", line 1 got_b = (char := yield) == 'b' ^ SyntaxError: invalid syntax