You have already learnt about lists in a previous tutorial. In this tutorial, we’re going to dive deeper into Python lists.
Python has a great built-in list type named
list. List literals are written within square brackets
[ ]. Lists work similarly to strings — use the
len() function and square brackets
[ ] to access data, with the first element at index 0.
colors = ['red', 'blue', 'green'] print(colors) ## red print(colors) ## green print(len(colors)) ## 3
Assignment with an
= on lists does not make a copy. Instead, assignment makes the two variables point to the one list in memory.
colors = ['red', 'blue', 'green'] b = colors ## Does NOT copy the list colors = 'yellow' print(colors) # ['red', 'yellow', 'green'] print(b) # ['red', 'yellow', 'green']
The “empty list” is just an empty pair of brackets
[ ]. The
+ works to append two lists, so
[1, 2] + [3, 4] yields
[1, 2, 3, 4] (this is just like
+ with strings).
in constructs are extremely useful, and you have already seen its use with lists. The
for construct —
for var in list — is an easy way to look at each element in a list (or other collections). Do not add or remove from the list during iteration.
squares = [1, 4, 9, 16] total = 0 for num in squares: total += num print(total) ## 30
If you know what sort of thing is in the list, use a variable name in the loop that captures that information such as
url. Since python code does not have other syntax to remind you of types, your variable names are a key way for you to keep straight what is going on.
in construct on its own is an easy way to test if an element appears in a list (or other collection) —
value in collection — tests if the value is in the collection, returning
list = ['larry', 'curly', 'moe'] if 'curly' in list: print('yay')
in constructs are very commonly used in Python code and work on data types other than list, so you should just memorize their syntax. You may have habits from other languages where you start manually iterating over a collection, where in Python you should just use
You can also use
in to work on a string. The string acts like a list of its chars, so
for ch in s: print(ch) prints all the chars in a string.
Here are some other common list methods.
list.append(elem) — adds a single element to the end of the list. Common error: does not return the new list, just modifies the original.
list = ['larry', 'curly', 'moe'] list.append('shemp') ## append elem at end print(list)
list.insert(index, elem) — inserts the element at the given index, shifting elements to the right.
list = ['larry', 'curly', 'moe'] list.insert(0, 'xxx') ## insert elem at index 0 print(list)
list.extend(list2) adds the elements in
list2 to the end of the
+= on a list is similar to using
list = ['larry', 'curly', 'moe'] list.extend(['yyy', 'zzz']) ## add list of elems at end print(list)
list.index(elem) — searches for the given element from the start of the list and returns its index. Throws a
ValueError if the element does not appear (use
in to check without a
list = ['larry', 'curly', 'moe'] print(list.index('curly')) ## 1
list.remove(elem) — searches for the first instance of the given element and removes it (throws
ValueError if not present)
list = ['larry', 'curly', 'moe'] list.remove('curly') ## search and remove that element print(list)
list.pop(index) — removes and returns the element at the given index. Returns the rightmost element if index is omitted (roughly the opposite of
list = ['larry', 'curly', 'moe'] list.pop(1) ## removes and returns 'curly' print(list)
list.sort() — sorts the list in place (does not return it). (The
sorted() function shown below is preferred.)
list = ['larry', 'curly', 'moe'] list.sort() ## sorts the list print(list)
list.reverse() — reverses the list in place (does not return it)
list = ['larry', 'curly', 'moe'] list.reverse() ## reverses the list print(list)
Notice that these are methods on a list object, while
len() is a function that takes the list (or string or whatever) as an argument.
Common error: note that the above methods do not return the modified list, they just modify the original list.
list = [1, 2, 3] print(list.append(4)) ## NO, does not work, append() returns None ## Correct pattern: list = [1, 2, 3] list.append(4) print(list) ## [1, 2, 3, 4]
List comprehensions are a more advanced feature which is nice for some cases but is not needed for the exercises and is not something you need to learn at first (i.e. you can skip this section). A list comprehension is a compact way to write an expression that expands to a whole list. Suppose we have a list
[1, 2, 3, 4], here is the list comprehension to compute a list of their squares
[1, 4, 9, 16]:
nums = [1, 2, 3, 4] squares = [ n * n for n in nums ] ## [1, 4, 9, 16] print(squares)
The syntax is
[ expr for var in list ] — the
for var in list looks like a regular
for-loop, but without the colon
expr to its left is evaluated once for each element to give the values for the new list. Here is an example with strings, where each string is changed to upper case with
strs = ['hello', 'and', 'goodbye'] shouting = [ s.upper() + '!!!' for s in strs ] print(shouting) ## ['HELLO!!!', 'AND!!!', 'GOODBYE!!!']
You can add an
if test to the right of the
for-loop to narrow the result. The
if test is evaluated for each element, including only the elements where the test is
## Select values <= 2 nums = [2, 8, 1, 6] small = [ n for n in nums if n <= 2 ] print(small) ## [2, 1] ## Select fruits containing 'a', change to upper case fruits = ['apple', 'cherry', 'banana', 'lemon'] afruits = [ s.upper() for s in fruits if 'a' in s ] print(afruits) ## ['APPLE', 'BANANA']
In this tutorial, we covered a number of advanced topics in lists:
- Review of lists
- List Methods
- List Comprehensions