Working with Lists¶
This section is based on chapter 4 in Sweigart’s Automate the Boring Stuff with Python (second edition).
A new list can be made like this:
my_list = ['cat', 'bat', 'rat', 'elephant']
>>> my_list = ['cat', 'bat', 'rat', 'elephant'] >>> print(my_list) bat >>> print(my_list) 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 ) 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
>>> 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 >>>
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
>>> 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 >>>
item are both new variables. The words index and item are not special. You could just as well use
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
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
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
Methods, and finding things in lists¶
In web scraping, we use a BeautifulSoup method on Tag objects:
A method is a function (e.g.
get_text()), but it must be called on a value. In the
object contains one or more HTML elements and (probably) text. Calling
object returns the text alone, without any HTML tags.
For use with Python lists, Sweigart shows us the method
spam is a Python list and that list contains an item with the value
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
except combo. When you are scraping, that can be very useful indeed.
Other list methods include
>>> my_list = ['cat', 'bat', 'rat', 'elephant'] >>> my_list.append('rhino') >>> print(my_list) ['cat', 'bat', 'rat', 'elephant', 'rhino'] >>>
append() method is used often in web scraping.
Earlier in the chapter, we saw
del spam — this deletes the item with index 2 from the list
spam. Note how that is different from the
>>> my_list = ['cat', 'bat', 'rat', 'elephant', 'rhino'] >>> my_list.remove('bat') >>> print(my_list) ['cat', 'rat', 'elephant', 'rhino'] >>>
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,
'lake'. Now it is
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 >>>
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
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¶
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
len()to get the number of items in a list
del()to delete an item from a list using its index
Loop through a list in different ways
not inwith lists
Increment a value using
Use the following methods correctly:
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)