在 Python 列表理解中可迭代

Question

If I have:

usernames = [user.get('username') for user in json.loads(users)]

Will the json.loads be called once as in normal for loop or many times on each iteration?

Answer 1

Don't worry: it is called only once, although the language reference isn't exactly clear on this point:

The comprehension consists of a single expression followed by at least one for clause and zero or more for or if clauses. In this case, the elements of the new container are those that would be produced by considering each of the for or if clauses a block, nesting from left to right, and evaluating the expression to produce an element each time the innermost block is reached.

Answer 2

It will be called only once as in a normal for-loop. For example:

x = [2,4,6,8,10]
y = [i/2 for i in x]

Output:

>>> y
[1.0, 2.0, 3.0, 4.0, 5.0]

Here each element of x is called once as i and divided by 2 as i/2 . Similarly, json.loads(users) will also be called once.