Lists

There is a data type very similar to tuples in Python. These are lists (list). Unlike tuples, which are constants, lists are structures that can change: you can add new elements to them, delete or change them.

Lists with specific items are created similar to tuples, however, instead of parentheses, square brackets are used:

list = ["first", 2, 2.5, 3-1j]

As with tuples, the list items don’t have to be of the same type. All the previous operations shown for tuples, such as iterating over them in a for loop, indexing elements, slices, and so on, work exactly the same for lists.

However, the list items can be changed. Please enter the following commands in the interactive console

list = [1, 2, 3] 

list[0] = 10

print(list)     # [ 10 , 2, 3]

If I did such an assignment for a tuple, it would cause a TypeError.

List items can also be removed with a command del. E.g:

list = ['A', 'B', 'C', 'D'] 

del list[1]     # delete the second element

print(list)     # ['A', 'C', 'D']

It is also possible to insert and add items to the list. They are used to this method with names insert and append. These methods are specific operations related to a variable of a given type. They are similar to a function, but affect only one specific object. They are called as follows object.method(arguments...), where object is usually a variable name, and the method name is separated from the object by a period. Adding an item to the list will look like this:

list = ['A', 'B', 'C']

list.append('D')

The method append puts the element given as its argument to the end of the list. So in the example above, the list would be ['A', 'B', 'C', 'D']. In contrast, the method insert takes two arguments: the place where the element should be inserted (i.e. the index of the inserted element) and its value:

list = ['A', 'B', 'C'] 

list.insert(1, 'X')

print(list)     # ['A', 'X', 'B', 'C']

Below is an example of creating a names list. Please run it and analyze it:

names = []     # this creates an empty list 

# The following loop will run infinitely 
# it can only be ended with break
while True:
    name = input("Enter name or write a dot to end:")
    if name == '.':
        break
    names.append(name) 

print(names)

Converting lists and other types

You can convert lists to tuples and vice versa without any restrictions. This way, you can work around the restriction not allowing tuples to be modified:

my_tuple = (1, 2, 3)
my_list = list(my_tuple)
my_list[0] = 10 
my_tuple = tuple(my_list)
print(my_tuple)

However, keep in mind that tuples are more efficient and run faster, so avoid this conversion unless absolutely necessary.

Similarly, you can convert strings into lists, but not the other way around: check what the following command will do str(['a', 'b', 'c']). However, there are two very useful methods defined for strings: split and join. The method split is used to split a text string into words, and join to concatenate list items. Please try to split the text yourself in the interactive console:

text = "one two three"

list = text.split()

print(list)     # ['one', 'two', 'three']

There is also an alternate way of calling the method split that allows you to split text not into words (that is, at spaces and newlines), but selected strings. For example, let’s see how to split a text containing words separated by a comma followed by a space:

text = "one, two, three"

list = text.split(', ')

print(list)     # ['one', 'two', 'three']

The method join (which works on text strings) is used to concatenate the elements of a list (or a tuple) given as its arguments. The hyphen is the text that this method works on. This is best illustrated by an example:

list = ['Kacper', 'Melchior', 'Balthazar'] 

hyphen = "+"

text = hyphen.join(list)

print(text)     # "Kacper+Melchior+Baltazar

The methods also work directly for text values:

list = ['Kacper', 'Melchior', 'Balthazar'] 

text = "+".join(list)

print(text)     # "Kacper+Melchior+Baltazar

Elegant list creation

The method append is used to add items to the list. If it can be avoided, it is better not to use it to build entire lists, repeating it inside a for loop. Python offers an elegant method of creating lists called list comprehension. Above is an example of creating a list by listing its elements separated by a comma and placed in square brackets [...]. Instead of listing the items manually, you can use a for loop with the following syntax:

list = [expression for counter in sequence]

This expression comes before the for command and there is no colon anywhere. This expression can use a loop counter to compute successive values to insert into the list. It is best to illustrate this with an example. Suppose we want to create a list containing squares of numbers in the range 1-10:

squares = [i**2 for i in range(1, 11)]

print(squares)

Please note that this is very similar to the natural language: “let the list of squares consist of the values i² for i in the range 1 to 10”.

In a more advanced version, we can add an additional condition that the loop counter must satisfy in order for the corresponding expression to be included in the list:

list = [expression for counter in sequence if condition]

For example, if we do not want to include the squares of 3 and 7 in our list of squares, we can write:

squares = [i**2 for i in range(1, 11) if i not in (3, 7)]

Exercise

Please write the shortest possible program, which will create a list containing the lengths of individual words in the sentence “To be or not to be” (for the sake of simplicity, punctuation marks have been omitted).

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