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MLC123: ApplyFunction callback returns no mobject on some path

qual reports MLC123 when the callback passed to ApplyFunction provably fails to return a mobject on some normal return path. ApplyFunction.create_target() calls function(mobject.copy()) and uses the return value as the transform target (transform.py ApplyFunction.create_target); a callback that mutates its argument but returns None — a bare return or falling off the end — or returns a definite non-mobject breaks the play the moment it begins.

  • Default severity: error
  • Minimum confidence: high
  • Implementation phase: 2
  • Fix: none (adding the missing return would be an unsafe edit and is suggested in the message)

The rule resolves lambdas by their exact source span and named arguments by qualified project-function name, and fires only on a proven returns_mobject == No: a path returning an untracked value is Maybe and stays silent, and paths ending in raise never count as a missing return.

Wrong

def push_right(mob):
    mob.shift(RIGHT)  # falls off the end: returns None

class Demo(Scene):
    def construct(self):
        square = Square()
        self.add(square)
        self.play(ApplyFunction(push_right, square))

def push_right(mob):
    mob.shift(RIGHT)
    return mob

class Demo(Scene):
    def construct(self):
        square = Square()
        self.add(square)
        self.play(ApplyFunction(push_right, square))