Slicing syntax question

[mpd]

Your syntax is wrong; there are no commas in the [].

Slices are really convenient ways to create a sublist (or tuple or array) from an existing one.

The syntax is [start:end:step]. The step field is frequently omitted and defaults to 1. If you omit start and end, they default to... the start and end.

>>> l = [1,2,3,4]
>>> l[:]     # create a new list from the old one
[1, 2, 3, 4]

The slice creates a new list:

>>> x = l[:]  
>>> x   # x has the same elements as l
[1, 2, 3, 4]
>>> x.append(5)  # add a new one to x
>>> l        # l doesn't get the new element
[1, 2, 3, 4]    
>>> x           # but x does
[1, 2, 3, 4, 5]

When would you use slices? I use them when parsing formatted text if I know, for example that two fields are the first and last name:

>>> x = "Matthew Doe New York"
>>> y = x.split(' ')
>>> y[:2]
['Matthew', 'Doe']

You can easily reverse a list with the step field:

>>> l
[1, 2, 3, 4]
>>> l[::-1]
[4, 3, 2, 1]

So that's a quick rundown of how to use slices. I wouldn't say that I use them a lot but they are an extremely useful tool to be familiar with.