交换 Python 中的变量和列表元素意外工作

Question

I have just came up with an example of code:

ls = [2222, 1111]

a = ls[0]
print(a is ls[0])

a, ls[1] = ls[1], a
print(ls)

which prints:

True
[2222, 2222]

My question is, why isn't a the same object as ls[0] in the above case? The a is ls[0] check is True , therefore it must be the same as:

ls[0], ls[1] = ls[1], ls[0]

but it isn't. The latter one produces [1111, 2222] . What is this mYsTeRy?

Answer 1

Simple assignments have no affect on the previous value of the target.

After

ls = [2222, 1111]

a = ls[0]

both a and the first element of ls are references to the integer 2222 .

The assignment

a, ls[1] = ls[1], a

is effectively the same as

t = ls[1], a  # The tuple (1111, 2222)
a = t[0]  # Set a to t[0] == 1111
ls[1] = t[1]  # Set ls[1] to t[1] == 2222

At no point have you modified the first element of ls ; you've only changed what a refers to and what the second element of ls refers to. You can see that a is now refers to 1111 , since that's what the value of ls[1] was before ls[1] was modified.

>>> print(a)
1111

Answer 2

You are not setting ls[0]!

Fixing it:

ls = [2222, 1111]

# Assigns ls[0] to a, but not changes ls[0]!
a = ls[0]
print(a is ls[0])

# Assigns ls[1] to a and a to ls[1]. At this point ls[1] is a!
a, ls[1] = ls[1], a
print(ls)

# Now ls[0] is assigned!
ls[0] = a
print(ls)

Outputs:

True
[2222, 2222]
[1111, 2222]

BTW: You can do the reverse listing with:

ls = [2222, 1111]
print(ls[::-1])

Outputs:

[1111, 2222]

Answer 3

Taken literally, neither a nor ls[1] are objects: They are an identifier and subscription , respectively. While they evaluate to objects when used in an expression, that does not make them identical to these objects – and the identity of "their" objects does not extend to them, either.

• The expression a is ls[0] means "evaluate a and ls[0] and compare the result for identity". It does not check whether the identifier a is identical to the subscription ls[0] .

Notably, when used in an assignment statement , both a and ls[1] do not represent "an expression to look up an object" but rather "a target to assign an object to". The features of the former do not impact the latter.

---

The statement a, ls[1] = ls[1], a uses both a and ls[1] as both expressions and targets:

• The right hand side is an expression. It represents "a tuple of the results of evaluating ls[1] and a ".

• The left hand side is an assignment. It represents "assign to the identifier a and the subscription ls[1] ".

Assignment to an identifier always is immediate: it does not matter what value the identifier previously referred to. Specifically, it does not matter that a previously referred to an object that was also referred to by ls[0] , and it will not matter that its new referent was also referred to by ls[1] .

---

To understand why this distinction is important, consider the case that an identifier "is identical to" a literal:

>>> # integers < 256 are interned
>>> a = 12
>>> # identifier "is identical to" literal
>>> a is 12
True

If identity in expressions would equate to identity in assignments, then setting a = 13 would now reassign both a and 12 to have the value 13 .

Answer 4

This code would not work as in this case you have a reference to the object at ls[0] and not a reference to the memory position of ls[0] .