引用列表中的元素

Question

I am a little confused about how python deal with reference to an element in a list, considering these two examples:

First example:

import random
a = [[1,2],[3,4],[5,6],[7,8]]
b = [0.1,0.2]
c = random.choice(a)
c[:] = b
print(a)

Second example:

import random
a = [1, 2, 3, 4, 5, 6, 7, 8]
b = 0.1
c = random.choice(a)
c = b
print(a)

In first example, the content in list a is changed; while in the second example, the content of list a is not changed. Why is that?

Answer 1

Let's start with the second case. You write

c = random.choice(a)

so the name c gets bound to some element of a, then

c = b

so the name c gets bound to some other object (the one to which the name b is referring - the float 0.1).

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Now to the first case. You start with

c = random.choice(a)

So the name c gets bound to an object in a , which is a list itself. Then you write

c[:] = b

which means, replace all items in the list bound to by the name c , by some other list. In fact, this is called slice assignment , and is basically syntactic sugar for calling a method of the object to which c is bound.

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The difference, then, is that in the first case, it doesn't just bind a name first to one object, then to another. It binds a name to a list, then uses this name to indirectly call a method of the list.