Python list is ordered collection which can have duplicates.
Declaring list
even_numbers = [2, 4, 6]
print(even_numbers) [2, 4, 6]
Declaring list of mixed types
items = [2, "three", 2.3]
print(items) [2, 'three', 2.3]
Using type() on list
even_numbers = [2, 4, 6]
print(type(even_numbers)) <class 'list'>
Accessing list elements
Lists can be accessed via zero-based indexing:
even_numbers = [2, 4, 6]
print(even_numbers[0])
print(even_numbers[1])
print(even_numbers[2]) 2
4
6
Accessing list from right side
Negative indexing can be used to access the list from right side:
even_numbers = [2, 4, 6]
print(even_numbers[-1])
print(even_numbers[-2])
print(even_numbers[-3]) 6
4
2
Finding index of an element
To get zero-based index we can use list.index(x[, start[, end]])
alpha_list = ['a', 'b', 'c', 'd', 'e']
i = alpha_list.index('c')
print(i)2
Finding index in sub-list
alpha_list = ['a', 'b', 'a', 'c', 'a']
i = alpha_list.index('a', 2) # from index 2
print(i)
i = alpha_list.index('a', 3, 5) # from index 3 to index 5
print(i)2
4
ValueError
ValueError is raised if element does not exist when we use index() method:
alpha_list = ['a', 'b', 'c', 'd', 'e']
i = alpha_list.index('z')
print(i)Traceback (most recent call last):
i = alpha_list.index('z')
ValueError: 'z' is not in list
Getting length of a list
The built-in function len() can be used to find the length of a list:
even_numbers = [2, 4, 6]
length = len(even_numbers)
print(length)
3
Replacing list element
To change value of an element at specified index we can just assign a new value to that specific index container:
even_numbers = [2, 4, 6]
even_numbers[0] = 10
even_numbers[1] = 12
even_numbers[2] = 14
print(even_numbers) [10, 12, 14]
IndexError
If we try to access a list out side of it's index range:
even_numbers = [2, 4, 6]
print(even_numbers[3]) Traceback (most recent call last):
print(even_numbers[3])
IndexError: list index out of range
if we try to add a new element by using index outside of it's index range:
even_numbers = [2, 4, 6]
even_numbers[3]=8
print(even_numbers) Traceback (most recent call last):
even_numbers[3]=8
IndexError: list assignment index out of range
Appending new elements
We can use append() method of list object to add new element at the end:
even_numbers = [2, 4, 6]
even_numbers.append(8)
print(even_numbers) [2, 4, 6, 8]
Inserting new elements
We can use list.insert(i, x) method of list object to insert new element at the specified index:
even_numbers = [2, 4, 6]
even_numbers.insert(0, 10)
print(even_numbers) [10, 2, 4, 6]
Inserting from right side
even_numbers = [2, 4, 6]
even_numbers.insert(-1, 10)
print(even_numbers) [2, 4, 10, 6]
Appending elements from other list
The method list.extend(iterable) can be used to add another list to this list:
even_numbers = [2, 4, 6]
even_numbers2 = [12, 14, 16]
even_numbers.extend(even_numbers2)
print(even_numbers) [2, 4, 6, 12, 14, 16]
Appending elements using concatenation
The concatenation of two list with + returns a new instance of the list:
even_numbers = [2, 4, 6]
even_numbers2 = [12, 14, 16]
new_numbers = even_numbers + even_numbers2
print(new_numbers) [2, 4, 6, 12, 14, 16]
Removing elements by index
The method list.pop([i]) removes the item at the specified index and returns it. If we don't specify any index, element will be removed at the end
even_numbers = [2, 4, 6]
item = even_numbers.pop(0)
print(even_numbers)
print(item)
# without any index
odd_numbers = [1, 3, 5]
item = odd_numbers.pop()
print(odd_numbers)
print(item) [4, 6]
2
[1, 3]
5
Removing elements by index from right
We can do that by using negative index with pop() method:
even_numbers = [2, 4, 6]
even_numbers.pop(-1)
print(even_numbers) [2, 4]
Removing elements by item values
The method list.remove(x) can be used to remove a specified item:
even_numbers = [2, 4, 6]
even_numbers.remove(4)
print(even_numbers)
# in case we have multiple items with same values
alpha_list = ['a', 'b', 'c', 'b']
alpha_list.remove('b')
print(alpha_list)
alpha_list.remove('b')
print(alpha_list)[2, 6]
['a', 'c', 'b']
['a', 'c']
Removing elements by del keyword
The del keyword can also be used to removed a specified item:
alpha_list = ['a', 'b', 'c', 'e']
# removing at index 0
del alpha_list[0]
print(alpha_list)
# removing at index from right
del alpha_list[-1]
print(alpha_list) ['b', 'c', 'e']
['b', 'c']
Removing whole list
alpha_list = ['a', 'b', 'c', 'd', 'e']
#removing complete list (just like other objects via del)
del alpha_list
print(alpha_list) Traceback (most recent call last):
print(alpha_list)
NameError: name 'alpha_list' is not defined
Clearing all elements
list.clear() can be used to remove all elements from the list:
alpha_list = ['a', 'b', 'c', 'd', 'e']
alpha_list.clear()
print(alpha_list) []
Looping list
even_numbers = [2, 4, 6]
for x in even_numbers:
print(x)
2
4
6
looping by index
even_numbers = [2, 4, 6]
length = len(even_numbers)
for x in range(length):
print(even_numbers[x])2
4
6
Checking if an element exists in a list
alpha_list = ['a', 'b', 'c', 'd', 'e']
result = 'c' in alpha_list
print(result) True
Finding frequency of an element
The occurrence or count or frequency can be found by using count() method:
alpha_list = ['a', 'b', 'a', 'd', 'a']
i = alpha_list.count('a')
print(i)
# if element does not exist
i = alpha_list.count('z')
print(i)3
0
Finding substrings
[a:b] can be used to find sub-subsequence (substring) of a list:
alpha_list = ['a', 'b', 'c', 'd', 'e']
print(alpha_list[1:3])
print(alpha_list[:2])
print(alpha_list[2:])
# using del
del alpha_list[2:4]
print(alpha_list)
['b', 'c']
['a', 'b']
['c', 'd', 'e']
['a', 'b', 'e']
Sorting
The method list.sort(key=None, reverse=False) can be used to sort:
num = [9, 1, 5, 3, 8]
num.sort()
print(num) [1, 3, 5, 8, 9]
Sorting in reverse order
Use list.sort(reverse=True) to sort in reversed order:
num = [9, 1, 5, 3, 8]
num.sort(reverse=True)
print(num) [9, 8, 5, 3, 1]
Custom sorting by key
Following example shows how to use key to customize sorting:
# Custom sorting by key
alpha = ['a', 'B', 'b', 'A', 'c']
alpha.sort(key=str.lower)
print(alpha)
# without key=str.lower
alpha2 = ['a', 'B', 'b', 'A', 'c']
alpha2.sort()
print(alpha2)
# custom key function
def even(i):
return i % 2 == 0
num = [2, 3, 5, 6]
num.sort(key=even)
print(num)
['a', 'A', 'B', 'b', 'c']
['A', 'B', 'a', 'b', 'c']
[3, 5, 2, 6]
Reversing order
We can use list method list.reverse() to reverse the order:
num = [9, 1, 5, 3, 8]
num.reverse()
print(num) [8, 3, 5, 1, 9]
Copying list
The method list.copy() returns the shallow copy of this list:
alpha = ['a', 'b', 'c']
alpha2 =alpha.copy()
print(alpha2)
#checking ids
print(id(alpha))
print(id(alpha2)) ['a', 'b', 'c']
1929543915264
1929544170112
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