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By the end of this chapter, you should be able to:
Lists in Python are simply collections of elements. They can be as long as you want, and the individual elements can have the same type or not:
number_list = [1, 2, 3, 4, 5] string_list = ["a", "b", "c", "d"] kitchen_sink_list = [4, "yo", None, False, True, ["another", "list"]]
To access an element in a list, we use bracket notation and pass in the index we're interested in. Lists in Python use a zero-based index:
my_list = ["a", 1, True] my_list # "a" my_list # True my_list # IndexError
Note that if you try to access an element with an invalid index, Python will give you an error.
We can also reassign values in lists using
my_list = ["a", 1, True] my_list = False my_list # ["a", 1, False]
In addition to getting and setting values in lists, there are a number of built-in methods you can use. Here are some more ones (in alphabetical order):
Adds a value to the end of a list:
my_list = [1,2,3] my_list.append(10) my_list # [1,2,3,10]
Removes all the values in a list:
l = [1,2,3] l.clear() l # 
Makes a copy of a list:
l =  l.append(2) l.append(3) l.append(4) l # [2, 3, 4] z = l.copy() z # [2, 3, 4] l # [2, 3, 4] z.remove(3) z # [2, 4] l # [2, 3, 4]
Counts how many times an item appears in a list:
l = [1,2,3,4,5,5,5,5] l.count(1) # 1 l.count(5) # 4
Appends another list and flattens the appended list (so there is not a list inside of a list):
l = [1,2,3] l.append() l # [1,2,3,] l.extend([5,6,7]) l # [1,2,3,,5,6,7]
Returns the index at which a value is found. If the value is not found,
index returns a
l = [4,8,6,4] l.index(8) # 1 l.index(4) # 0 - it only finds the index of the first matching element l.index(12) # ValueError
Takes in an index and a value, and inserts the value at the given index:
l = [4,6,8] l.insert(2,'awesome') # insert 'awesome' at index 2 l # [4,6,'awesome',8]
By default, pop removes the last element from a list. If pop is given an index as an argument, the element at that index is removed. This method returns the element removed:
l = [3,4] l.pop(0) # 3 l.pop() # 4 l.pop() # IndexError
pop throws an error if you call it on an empty list!
Removes the first occurrence of a value:
l = [1,2,3,1] l.remove(1) l # [2,3,1] l.remove(5) # ValueError
remove throws an error if you try to remove something not in the list.
Reverses a list in place (i.e. it mutates the original list):
l = [1,2,3,4] l.reverse() l # [4,3,2,1]
Sorts the list in place (i.e. it mutates the original list):
l = [1,4,3,2] l.sort() l # [1,2,3,4]
Slices return portions of a list or string. While this seems like a pretty minor concept, there's actually quite a bit you can do with slices that you might not expect.
The standard syntax for a slice is
list[start:end:step]. We can also do
list[:] to make a copy of a list, or even
list[::-1] to make a copy of a reversed list.
Let's see some examples :
first_list = [1,2,3,4,5,6] first_list[0:1] #  # if a value for end isn't provided, you'll slice to the end of the list first_list[1:] # [2, 3, 4, 5, 6] # if a value for start isn't provided, you'll slice from the start of the list first_list[:3] # [1,2,3] # get the last element in the list first_list[-1] # 6 # start from the second to last element in the list first_list[-2:] # [5, 6] # There is always more than one way of doing something... first_list[4:] == first_list[-2:] # True # step in the opposite direction first_list[::-1] # [6, 5, 4, 3, 2, 1] # step in the opposite direction by two elements first_list[::-2] # [6, 4, 2]
When you're ready, move on to List Iteration and Comprehension