Python：为什么带有附加参数的方法的行为有所不同？

Question

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?

Answer 1

You don't explain how it's "broken", but the main difference is that

speed_x = -speed_x

which you have in your second version, is only changing the local variable (arguments are local variables!) speed_x , so that changed value does not persist.

In the first version,

self.move = -self.move 

does alter self (specifically one of its attriubtes) and the alteration "persists" in future method calls on the object which is here accessed as self .

Just one of the many key differences between bare names (like speed_x ) and qualified names (line self.move ), and, I suspect, what's biting you here (hard as you may make it to guess by not saying how the second version is failing your expectations).

Answer 2

您无需将偏移量存储回self.move中。

Answer 3

If you want to use the second version of your code, try adding this line:

    self.move = speed_x

At the bottom of your function.

Answer 4

As mentioned by others, you are not changing the value of self.move in your new code. I assume the reason you modified this function was so you could reuse this function for values other than self.move .

If you want to be able to pass different arguments into your function and modify them as well, you could pass the modified value of speed_x back as a return value:

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
    return speed_x

And call the function like this as:

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

Note: This answer assumes that self.move should not always be updated when calling walk . If this assumption is false and self.move should in fact be updated every time walk is run, then you should instead use Xavier Ho's answer.