Working with Lists¶

This section is based on chapter 4 in Sweigart’s Automate the Boring Stuff with Python (second edition).

Python lists are very similar to JavaScript arrays. However, in Python, an array is a different thing, and we will be using lists, not arrays. Read this if you’re curious about the difference.

A new list can be made like this:

my_list = ['cat', 'bat', 'rat', 'elephant']

As with JavaScript arrays, a Python list contains items that can be accessed via an index number. Each item in the list has a unique index, starting with 0:

>>> my_list = ['cat', 'bat', 'rat', 'elephant']
>>> print(my_list[1])
bat
>>> print(my_list[0])
cat
>>>

This next bit is pretty sophisticated, but you will probably use it: We can put lists *inside* of lists. When we do, we can access the list items with index numbers as usual, but you’ll need to use one index to access a list (in the list of lists) and a second index to access an item inside that list. See Sweigart page 79 for this. It’s like a double-decker list.

>>> new_list = [ [0, 1, 3], ['a', 'b', 'c'], ['red', 'white', 'blue'] ]
>>> print( new_list[2][0] )
red
>>>

Above is an example of a list (new_list) that contains three items. Each of the three items is a different list. To access the first item in the third list, we use the index of the third list, 2, followed by the index of the first item, 0.

Slices provide a way to get several consecutive items from a list all at once; see Sweigart pages 80–81.

The length of a list (just like the length of a string) can be found with len():

>>> my_list = ['cat', 'bat', 'rat', 'elephant']
>>> len(my_list)
4
>>> word = "fantastic"
>>> len(word)
9
>>>

Looping over a list¶

Sweigart covers a variety of things we can do to or with Python lists, including adding new items to them and deleting items from them. Perhaps the most common thing we do with lists is loop through them to print or inspect their contents:

>>> my_list = ['cat', 'bat', 'rat', 'elephant']
>>> for thing in my_list:
...   print(thing)
...
cat
bat
rat
elephant
>>>

Attention

Above, thing is a new variable. Its value is the value of the current list item. Each time the loop runs, the value of thing is the next item in the list.

If we need to know the index number while we are looping through a list, we can get it with enumerate():

>>> my_list = ['cat', 'bat', 'rat', 'elephant']
>>> for index, item in enumerate(my_list):
...   print('The index: ' + str(index) + ' The item: ' + item)
...
The index: 0 The item: cat
The index: 1 The item: bat
The index: 2 The item: rat
The index: 3 The item: elephant
>>>

Attention

Above, index and item are both new variables. The words index and item are not special. You could just as well use a and b, or i and thing.

You don’t need to loop through a list to find out if a particular item exists there. See Sweigart pages 84–85 for details (“The in and not in Operators”). He also shows you how to assign list items to individual variables (pages 85–86, “The Multiple Assignment Trick”).

Augmented assignment operators¶

I have no clue why this is in the middle of the lists chapter, but you should know that in Python we cannot increment a value with ++ as we can in JavaScript.

We can, however, use a shortened form instead of x = x + 1 to increment a value:

>>> x = 0
>>> x += 1
>>> print(x)
1
>>> x += 100
>>> print(x)
101
>>>

The same technique works with -, *, \, and % (modulus).

Methods, and finding things in lists¶

In web scraping, we use a BeautifulSoup method on Tag objects: object.get_text()

A method is a function (e.g. get_text()), but it must be called on a value. In the object.get_text() example, object contains one or more HTML elements and (probably) text. Calling get_text() on object returns the text alone, without any HTML tags.

For use with Python lists, Sweigart shows us the method index(): If spam is a Python list and that list contains an item with the value "hello", then spam.index('hello') will return the index number of that item.

It’s useful to know that if the list does not contain that value, then the method index() will return a ValueError. This is useful because (like any error) ValueError could be used in a try/except combo. When you are scraping, that can be very useful indeed.

Other list methods include append() and insert().

>>> my_list = ['cat', 'bat', 'rat', 'elephant']
>>> my_list.append('rhino')
>>> print(my_list)
['cat', 'bat', 'rat', 'elephant', 'rhino']
>>>

Note

The append() method is used often in web scraping.

Earlier in the chapter, we saw del spam[2] — this deletes the item with index 2 from the list spam. Note how that is different from the remove() method:

>>> my_list = ['cat', 'bat', 'rat', 'elephant', 'rhino']
>>> my_list.remove('bat')
>>> print(my_list)
['cat', 'rat', 'elephant', 'rhino']
>>>

The sort() method will only work if your list items are all strings or all numbers. Also, strings that begin with an uppercase letter will be sorted separately from strings that begin with a lowercase letter.

>>> water_list = ['lake', 'Ontario', 'river', 'Hudson', 'ocean', 'Atlantic']
>>> water_list.sort()
>>> print(water_list)
['Atlantic', 'Hudson', 'Ontario', 'lake', 'ocean', 'river']
>>>

If you try to use .sort() on a list that contains both strings and integers, you’ll see this error:

TypeError: '<' not supported between instances of 'str' and 'int'

Note that by using the sort() method, the list is changed. The old order cannot be regained. The old indexes are destroyed. Originally, water_list[0] was 'lake'. Now it is 'Atlantic'.

There is a method Sweigart did not cover, and it’s very handy: pop(). We can put an index inside the parentheses, and then the item at that index will be removed, permanently, from the list. If no index is specified, pop() removes and returns the last item in the list. Note that leaving the parentheses empty is the most common way to use pop(). Here’s how it works:

>>> print(water_list)
['Atlantic', 'Hudson', 'Ontario', 'lake', 'ocean', 'river']
>>> water_list.pop()
'river'
>>> next_item = water_list.pop()
>>> another_item = water_list.pop()
>>> print(water_list)
['Atlantic', 'Hudson', 'Ontario']
>>> print(next_item)
ocean
>>> print(another_item)
lake
>>>

Here are all of the methods of Python lists.

Tuples and immutability¶

Sweigart explains the difference between mutable and immutable data types and then goes on to introduce tuples (pronounced too-puls). A tuple might look like a list at first glance, but it’s not — and it doesn’t behave like a list, either.

A tuple can contain one or more items, like a list, but the items cannot be changed. They cannot be sorted into order, and they cannot be deleted or removed. Perhaps most surprising, you cannot even add a new item to a tuple. Once it is made, a tuple is immutable.

Lists are mutable, and that means we can change and reorder their contents at any time.

Lists and references¶

Another important thing to know about Python lists is that you can’t simply duplicate one. You might think, “Oh, I’m going to change the contents of my_list, so I’ll make a copy of it as a backup.” This is not going to do what you probably expect:

>>> my_list = ['cat', 'bat', 'rat']
>>> foobar = my_list
>>> print(foobar)
['cat', 'bat', 'rat']
>>> # you think you have a copy of my_list in foobar - you are wrong
>>> my_list.append('aardvark')
>>> my_list.append('zebra')
>>> my_list.remove('rat')
>>> my_list.sort()
>>> print(my_list)
['aardvark', 'bat', 'cat', 'zebra']
>>> print(foobar)
['aardvark', 'bat', 'cat', 'zebra']
>>>

Sweigart explains this in chapter 4. Both my_list and foobar are simply references to the list, which exists elsewhere in memory. To make a real copy that is independent of the original, you have to use other means.

Chapter review: chapter 4¶

Key points¶

Create a new list
Get the value of one item in a list using its index
Make a double-decker list (lists inside a list) and access specific items in the inner lists.
Use slices to get multiple items from a list all at once
Use len() to get the number of items in a list
Use del() to delete an item from a list using its index
Loop through a list in different ways
Use of in and not in with lists
Increment a value using +=
Use the following methods correctly:

index()

append()

remove()

sort()

pop() not in Sweigart; see above

The differences between a Python list and a tuple
You can’t simply make a copy of a list in the way you might expect (know how to look up the correct way to make a copy if you need to do so)

Slides: chapters 9 and 4¶

Slide deck