用Python理解列表？

Question

I have this list :

a = [1,2,3,4,5]

I want to delete each element from the list. This is how I'm doing.

for i in range(0,len(a)):
    del a[0] 

Can I use list comprehension in this? such as del a[0] for i in range(0,len(a))

Answer 1

List comprehensions are for creating lists, not destroying them. They are not the right tool for this job.

If you want to clear the contents of a list, the quickest way is

del a[:]

This is a slice deletion. The omitted beginning and ending points default to the start and end of the list.

Note that frequently, it's better to just replace the list with a new one:

a = []

This works even if a didn't already exist, and modifications to the new list won't interfere with any other parts of the code that needed the old list. These attributes are often, though not always, desirable.

Answer 2

You wouldn't usually do this with a loop at all. To clear an existing list, use a[:] = [] .

Answer 3

No, you cannot del components in a list comprehension. Why would you want to do that in the first place? A list comprehension is for creating a list, not for modifying.

You can replace the list with a different one created with a list comprehension.

The reason is that a list comprehension needs an expression. But del is a statement.

Answer 4

List comprehension (coming from functional programming, where nothing is ever mutated) is for building lists. Since it appears you want to clear out the elements of an existing list (and not creating a new list with only some of the elements etc.), list comprehension is not the right tool.

That said, list comprehension can be abused to do it:

[a.pop() for i in range(len(a))]

NOTE: This is a bad idea (since it uses list comprehenstion for something completely different than what it is intended for) and I would strongly recommend you do not use it.