Passing values in Python

 

This question already has an answer here:

  • How do I pass a variable by reference? 23 answers

    • Python passes references-to-objects by value.

    Python passes references-to-objects by value (like Java), and everything in Python is an object. This sounds simple, but then you will notice that some data types seem to exhibit pass-by-value characteristics, while others seem to act like pass-by-reference... what's the deal?

    It is important to understand mutable and immutable objects. Some objects, like strings, tuples, and numbers, are immutable. Altering them inside a function/method will create a new instance and the original instance outside the function/method is not changed. Other objects, like lists and dictionaries are mutable, which means you can change the object in-place. Therefore, altering an object inside a function/method will also change the original object outside.

    Thing is, the whole reference/value concept won't fit into python. Python has no "value" of a variable. Python has only objects and names that refer to objects.

    So when you call a function and put a "name" inside the parenthesis, like this: 

    def func(x): # defines a function that takes an argument
    ... # do something here

func(myname) # calling the function

    The actual object that myname is pointing is passed, not the name myname itself . Inside the function another name ( x ) is given to refer to the same object passed.

    You can modify the object inside the function if it is mutable, but you can't change what the outside name is pointing to . Just the same that happens when you do 

    anothername = myname

    Therefore I can answer your question with:

    it is "pass by value" but all values are just references to objects.

    Answers here have been helpful, but I find the need to exhibit this fine distinction which I haven't seen covered, which I've proven to myself with the subsequent CL experiment:

  • An immutable object ALONE CANNOT be changed within a function call. (answers so far have said that much...)
  • BUT, an immutable object CONTAINED WITHIN a mutable object CAN be re-assigned within a method call.

    • 'num' does not change here because it is an immutable Number object [supports my point 1.]: 

    def incr_num(num):
    num += 1

num = 0

num
0

incr_num(num)

num
0

    'list[0]' here is an immutable Number object also. 

    def incr_list(list):
    list[0] += 1

list = [0]

list[0]
0

incr_list(list)

list[0]
1

    So how did 'list[0]', being an immutable Number object, change (supports my point 2.) while the above example's Number object 'num' did not? The immutable Number object 'list[0]' is contained within the mutable list object 'list', while 'num' from the 1st example is just a non-contianed Number object.

    Although well-intended, I feel @Stephen Pape top-rated answer (quoted below), and some other similar ones, were not totally correct (and that motivated me to write this answer):

    Some objects, like strings, tuples, and numbers, are immutable. Altering them inside a function/method will create a new instance and the original instance outside the function/method is not changed.

    My 2nd code experiment above shows a Number object ('list[0]') being altered within a method, and then the original instance outside the function changed.

    链接地址: http://www.djcxy.com/p/3538.html

    上一篇: 为什么我应该在Java的方法参数中使用关键字“final”？

    下一篇: 在Python中传递值