List methods and supported operators
Starting with a given list a :
a = [1, 2, 3, 4, 5]
–> append(value) – appends a new element to the end of the list.
# Append values 6, 7, and 7 to the list a.append(6) a.append(7) a.append(7) # a: [1, 2, 3, 4, 5, 6, 7, 7] # Append another list b = [8, 9] a.append(b) # a: [1, 2, 3, 4, 5, 6, 7, 7, [8, 9]] # Append an element of a different type, as list elements do not need to have the same type my_string = "hello world" a.append(my_string) # a: [1, 2, 3, 4, 5, 6, 7, 7, [8, 9], "hello world"]
Note that the append() method only appends one new element to the end of the list. If you append a list to
another list, the list that you append becomes a single element at the end of the first list.
# Appending a list to another list a = [1, 2, 3, 4, 5, 6, 7, 7] b = [8, 9] a.append(b) # a: [1, 2, 3, 4, 5, 6, 7, 7, [8, 9]] a[8] # Returns: [8,9]
–> extend(enumerable) – extends the list by appending elements from another enumerable.
a = [1, 2, 3, 4, 5, 6, 7, 7] b = [8, 9, 10] # Extend list by appending all elements from b a.extend(b) # a: [1, 2, 3, 4, 5, 6, 7, 7, 8, 9, 10] # Extend list with elements from a non-list enumerable: a.extend(range(3)) # a: [1, 2, 3, 4, 5, 6, 7, 7, 8, 9, 10, 0, 1, 2]
Lists can also be concatenated with the + operator. Note that this does not modify any of the original lists:
a = [1, 2, 3, 4, 5, 6] + [7, 7] + b # a: [1, 2, 3, 4, 5, 6, 7, 7, 8, 9, 10]
–> index(value, [start Index]) – gets the index of the first occurrence of the input value. If the input value is not in the list a Value Error exception is raised. If a second argument is provided, the search is started at that specified index.
a.index(7) # Returns: 6 a.index(49) # ValueError, because 49 is not in a. a.index(7, 7) # Returns: 7 a.index(7, 8) # ValueError, because there is no 7 starting at index 8
–> insert(index, value) – inserts value just before the specified index . Thus after the insertion the new element occupies position index .
a.insert(0, 0) # insert 0 at position 0 a.insert(2, 5) # insert 5 at position 2 # a: [0, 1, 5, 2, 3, 4, 5, 6, 7, 7, 8, 9, 10]
–> pop([index]) – removes and returns the item at index . With no argument it removes and returns the last element of the list.
a.pop(2) # Returns: 5 # a: [0, 1, 2, 3, 4, 5, 6, 7, 7, 8, 9, 10] a.pop(8) # Returns: 7 # a: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] # With no argument: a.pop() # Returns: 10 # a: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]a.pop(2) # Returns: 5 # a: [0, 1, 2, 3, 4, 5, 6, 7, 7, 8, 9, 10] a.pop(8) # Returns: 7 # a: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] # With no argument: a.pop() # Returns: 10 # a: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
–> remove(value) – removes the first occurrence of the specified value. If the provided value cannot be found, a ValueError is raised.
a.remove(0) a.remove(9) # a: [1, 2, 3, 4, 5, 6, 7, 8] a.remove(10) # ValueError, because 10 is not in a
–> reverse() – reverses the list in-place and returns None .
a.reverse() # a: [8, 7, 6, 5, 4, 3, 2, 1]
–> count(value) – counts the number of occurrences of some value in the list.
a.count(7) # Returns: 2
–> sort() – sorts the list in numerical and lexicographical order and returns None .
a.sort() # a = [1, 2, 3, 4, 5, 6, 7, 8] # Sorts the list in numerical order a.sort(reverse=True) # a = [8, 7, 6, 5, 4, 3, 2, 1]
Lists can also be reversed when sorted using the reverse=True flag in the sort() method.
–> clear() – removes all items from the list
a = [1, 2, 3, 4, 5, 6, 7, 8] a.clear() # a = []
Replication – multiplying an existing list by an integer will produce a larger list consisting of that many copies of the original. This can be useful for example for list initialization:
b = ["blah"] * 3 # b = ["blah", "blah", "blah"] b = [1, 3, 5] * 5 # [1, 3, 5, 1, 3, 5, 1, 3, 5, 1, 3, 5, 1, 3, 5]
Take care doing this if your list contains references to objects (eg a list of lists).
Element deletion – it is possible to delete multiple elements in the list using the del keyword and slice notation:
a = list(range(10)) del a[::2] # a = [1,3,5,7,9] del a[-1] # a = [1,3,5,7] del a[:] # a = []
Copying:
The default assignment “=” assigns a reference of the original list to the new name. That is, the original name and new name are both pointing to the same list object. Changes made through any of them will be reflected in another. This is often not what you intended.
b = a a.append(6) # b: [1, 2, 3, 4, 5, 6]
If you want to create a copy of the list you have below options. You can slice it:
new_list = old_list[:]
You can use the built in list() function:
new_list = list(old_list) #You can use generic copy.copy(): import copy new_list = copy.copy(old_list) #inserts references to the objects found in the original.
This is a little slower than list() because it has to find out the datatype of old_list first. If the list contains objects and you want to copy them as well, use generic copy.deepcopy():
import copy new_list = copy.deepcopy(old_list) #inserts copies of the objects found in the original.
Obviously the slowest and most memory-needing method, but sometimes unavoidable.
copy() – Returns a shallow copy of the list.
aa = a.copy() # aa = [1, 2, 3, 4, 5]