What is the meaning of list[:] in this code? [duplicate]

Question

This code is from Python's Documentation. I'm a little confused.

words = ['cat', 'window', 'defenestrate'] for w in words[:]:     if len(w) > 6:         words.insert(0, w) print(words) 

And the following is what I thought at first:

words = ['cat', 'window', 'defenestrate'] for w in words:     if len(w) > 6:         words.insert(0, w) print(words) 

Why does this code create a infinite loop and the first one doesn't?

Answer 1

This is one of the gotchas! of python, that can escape beginners.

The words[:] is the magic sauce here.

Observe:

>>> words =  ['cat', 'window', 'defenestrate'] >>> words2 = words[:] >>> words2.insert(0, 'hello') >>> words2 ['hello', 'cat', 'window', 'defenestrate'] >>> words ['cat', 'window', 'defenestrate'] 

And now without the [:]:

>>> words =  ['cat', 'window', 'defenestrate'] >>> words2 = words >>> words2.insert(0, 'hello') >>> words2 ['hello', 'cat', 'window', 'defenestrate'] >>> words ['hello', 'cat', 'window', 'defenestrate'] 

The main thing to note here is that words[:] returns a copy of the existing list, so you are iterating over a copy, which is not modified.

You can check whether you are referring to the same lists using id():

In the first case:

>>> words2 = words[:] >>> id(words2) 4360026736 >>> id(words) 4360188992 >>> words2 is words False 

In the second case:

>>> id(words2) 4360188992 >>> id(words) 4360188992 >>> words2 is words True 

It is worth noting that [i:j] is called the slicing operator, and what it does is it returns a fresh copy of the list starting from index i, upto (but not including) index j.

So, words[0:2] gives you

>>> words[0:2] ['hello', 'cat'] 

Omitting the starting index means it defaults to 0, while omitting the last index means it defaults to len(words), and the end result is that you receive a copy of the entire list.

---

If you want to make your code a little more readable, I recommend the copy module.

from copy import copy   words = ['cat', 'window', 'defenestrate'] for w in copy(words):     if len(w) > 6:         words.insert(0, w) print(words) 

This basically does the same thing as your first code snippet, and is much more readable.

Alternatively (as mentioned by DSM in the comments) and on python >=3, you may also use words.copy() which does the same thing.