learning python,5e中讲到.Python的函数参数传递机制是对象引用.

Arguments are passed by assignment (object reference). In Python, arguments

are passed to functions by assignment (which, as we’ve learned, means by object

reference). As you’ll see, in Python’s model the caller and function share objects

by references, but there is no name aliasing. Changing an argument name within

a function does not also change the corresponding name in the caller, but changing

passed-in mutable objects in place can change objects shared by the caller, and

serve as a function result.

 

SO上有一个很好的说明:

The problem comes from a misunderstanding of what variables are in Python. If you're used to most traditional languages, you have a mental model of what happens in the following sequence:

a =1a =2

You believe that a is a memory location that stores the value 1, then is updated to store the value 2. That's not how things work in Python. Rather, a starts as a reference to an object with the value 1, then gets reassigned as a reference to an object with the value 2. Those two objects may continue to coexist even though a doesn't refer to the first one anymore; in fact they may be shared by any number of other references within the program.

When you call a function with a parameter, a new reference is created that refers to the object passed in. This is separate from the reference that was used in the function call, so there's no way to update that reference and make it refer to a new object. In your example:

self.variable = 'Original'    self.Change(self.variable)def Change(self, var):    var = 'Changed'

self.variable is a reference to the string object 'Original'. When you call Change you create a second reference var to the object. Inside the function you reassign the reference var to a different string object 'Changed', but the reference self.variable is separate and does not change.

The only way around this is to pass a mutable object. Because both references refer to the same object, any changes to the object are reflected in both places.

self.variable = ['Original']    self.Change(self.variable)def Change(self, var):    var[0] = 'Changed'