Python 3: An interactive deep dive/

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Lists

Lists

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Lists are Python’s workhorse datatype. When I say “list,” you might be thinking “array whose size I have to declare in advance, that can only contain items of the same type, &c.” Don’t think that. Lists are much cooler than that.

A list in Python is like an array in Perl 5. In Perl 5, variables that store arrays always start with the @ character; in Python, variables can be named anything, and Python keeps track of the datatype internally.

A list in Python is much more than an array in Java (although it can be used as one if that’s really all you want out of life). A better analogy would be to the ArrayList class, which can hold arbitrary objects and can expand dynamically as new items are added.

Creating a list

Creating a list is easy: use square brackets to wrap a comma-separated list of values.

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a_list = ['a', 'b', 'mpilgrim', 'z', 'example']          #①
print (a_list)
#['a', 'b', 'mpilgrim', 'z', 'example']
print (a_list[0] )                                       #②
#a
print (a_list[4] )                                       #③
#example
print (a_list[-1])                                       #④
#example
print (a_list[-3])                                       #⑤
#mpilgrim

① First, you define a list of five items. Note that they retain their original order. This is not an accident. A list is an ordered set of items.

② A list can be used like a zero-based array. The first item of any non-empty list is always a_list[0].

③ The last item of this five-item list is a_list[4], because lists are always zero-based.

④ A negative index accesses items from the end of the list counting backwards. The last item of any non-empty list is always a_list[-1].

⑤ If the negative index is confusing to you, think of it this way: a_list[-n] == a_list[len(a_list) - n]. So in this list, a_list[-3] == a_list[5 - 3] == a_list[2].

Slicing a list

a_list[0] is the first item of a_list.

Once you’ve defined a list, you can get any part of it as a new list. This is called slicing the list.

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a_list = ['a', 'b', 'mpilgrim', 'z', 'example'] 
print (a_list)
#['a', 'b', 'mpilgrim', 'z', 'example']
print (a_list[1:3] )           #①
#['b', 'mpilgrim']
print (a_list[1:-1])           #②
#['b', 'mpilgrim', 'z']
print (a_list[0:3] )           #③
#['a', 'b', 'mpilgrim']
print (a_list[:3])             #④
#['a', 'b', 'mpilgrim']
print (a_list[3:] )            #⑤
#['z', 'example']
print (a_list[:] )             #⑥
#['a', 'b', 'mpilgrim', 'z', 'example']

① You can get ...

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