I have a method in a Pygame Sprite subclass, defined as such:

def walk(self):
    """move across screen"""
    displacement = self.rect.move((self.move, 0))
    if self.rect.left < self.area.left or self.rect.right > self.area.right:
        self.move = -self.move
        displacement = self.rect.move((self.move, 0))
    self.rect = displacement

I modified it, adding a parameter speed_x, and now the program is broken.

def walk(self, speed_x):
    """move across screen"""
    displacement = self.rect.move((speed_x, 0))
    if self.rect.left < self.area.left or self.rect.right > self.area.right:
        speed_x = -speed_x
        displacement = self.rect.move((speed_x, 0))
    self.rect = displacement

Before I called the method like this:

def update(self):
        self.walk()

Now I do:

def update(self):
    self.walk(self.move)

Why doesn't this work?