- 5.1 Introduction
- 5.2 Lists
- 5.3 Tuples
- 5.4 Unpacking Sequences
- 5.5 Sequence Slicing
- 5.6 del Statement
- 5.7 Passing Lists to Functions
- 5.8 Sorting Lists
- 5.9 Searching Sequences
- 5.10 Other List Methods
- 5.11 Simulating Stacks with Lists
- 5.12 List Comprehensions
- 5.13 Generator Expressions
- 5.14 Filter, Map and Reduce
- 5.15 Other Sequence Processing Functions
- 5.16 Two-Dimensional Lists
- 5.17 Intro to Data Science: Simulation and Static Visualizations
- 5.18 Wrap-Up
This chapter is from the book
This chapter is from the book
This chapter is from the book
5.10 Other List Methods
Lists also have methods that add and remove elements. Consider the list color_names:
In [1]: color_names = ['orange', 'yellow', 'green']
Inserting an Element at a Specific List Index
Method insert adds a new item at a specified index. The following inserts 'red' at index 0:
In [2]: color_names.insert(0, 'red') In [3]: color_names Out[3]: ['red', 'orange', 'yellow', 'green']
Adding an Element to the End of a List
You can add a new item to the end of a list with method append:
In [4]: color_names.append('blue')
In [5]: color_names
Out[5]: ['red', 'orange', 'yellow', 'green', 'blue']
Adding All the Elements of a Sequence to the End of a List
Use list method extend to add all the elements of another sequence to the end of a list:
In [6]: color_names.extend(['indigo', 'violet']) In [7]: color_names Out[7]: ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
This is the equivalent of using +=. The following code adds all the characters of a string then all the elements of a tuple to a list:
In [8]: sample_list = [] In [9]: s = 'abc' In [10]: sample_list.extend(s) In [11]: sample_list Out[11]: ['a', 'b', 'c'] In [12]: t = (1, 2, 3) In [13]: sample_list.extend(t) In [14]: sample_list Out[14]: ['a', 'b', 'c', 1, 2, 3]
Rather than creating a temporary variable, like t, to store a tuple before appending it to a list, you might want to pass a tuple directly to extend. In this case, the tuple’s parentheses are required, because extend expects one iterable argument:
In [15]: sample_list.extend((4, 5, 6)) # note the extra parentheses In [16]: sample_list Out[16]: ['a', 'b', 'c', 1, 2, 3, 4, 5, 6]
A TypeError occurs if you omit the required parentheses.
Removing the First Occurrence of an Element in a List
Method remove deletes the first element with a specified value—a ValueError occurs if remove’s argument is not in the list:
In [17]: color_names.remove('green')
In [18]: color_names
Out[18]: ['red', 'orange', 'yellow', 'blue', 'indigo', 'violet']
Emptying a List
To delete all the elements in a list, call method clear:
In [19]: color_names.clear() In [20]: color_names Out[20]: []
This is the equivalent of the previously shown slice assignment
color_names[:] = []
Counting the Number of Occurrences of an Item
List method count searches for its argument and returns the number of times it is found:
In [21]: responses = [1, 2, 5, 4, 3, 5, 2, 1, 3, 3,
...: 1, 4, 3, 3, 3, 2, 3, 3, 2, 2]
...:
In [22]: for i in range(1, 6):
...: print(f'{i} appears {responses.count(i)} times in responses')
...:
1 appears 3 times in responses
2 appears 5 times in responses
3 appears 8 times in responses
4 appears 2 times in responses
5 appears 2 times in responses
Reversing a List’s Elements
List method reverse reverses the contents of a list in place, rather than creating a reversed copy, as we did with a slice previously:
In [23]: color_names = ['red', 'orange', 'yellow', 'green', 'blue'] In [24]: color_names.reverse() In [25]: color_names Out[25]: ['blue', 'green', 'yellow', 'orange', 'red']
Copying a List
List method copy returns a new list containing a shallow copy of the original list:
In [26]: copied_list = color_names.copy() In [27]: copied_list Out[27]: ['blue', 'green', 'yellow', 'orange', 'red']
This is equivalent to the previously demonstrated slice operation:
copied_list = color_names[:]