Python dictionary "copy value"

Question

I was looking at the docutil source code (which is in python), when I saw this (redacted) :

def __init__(self, **attributes):
    for att, value in attributes.items():
        att = att.lower()
        if att in self.list_attributes:
            # mutable list; make a copy for this node
            self.attributes[att] = value[:]
        else:
            self.attributes[att] = value

The line I'm talking about is this one:

            self.attributes[att] = value[:]

What does the "[:]" do exactly ? The comment above it hint a copy of some kind but my google searchs were not that successful and I can't figure if it's a language feature or a trick/shortcut of some kind.

score 8 · Accepted Answer · edited May 23 '17 at 12:21

8

It makes a copy of the list (it's not a dictionary)

The notation is called "slicing". You can also specify where to start and end copying, if you don't specify anything - as in your code extract - it will copy from the first to the last element.

For instance, mylist[1:] will copy the entire list omitting the first element.

Have a look here for a comprehensive explanation.

edited May 23 '17 at 12:21

Community

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answered Jul 29 '12 at 17:16

phant0m

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Thank you, it answers my question perfectly. I'll mark it as an answer as soon as the site allows me too (in 8 minutes more or less) – Sebastien F. Jul 29 '12 at 17:22
Note that it is not a deep copy; it simply copies the references to each object and stores them in a new list. While this should not be your copy method of choice for everything, it is useful if you want to iterate over and modify a list at the same time without screwing up your iteration. – algorowara Jul 29 '12 at 22:27

Python dictionary "copy value"

1 Answers1