Python: List Comprehension tricks

What is List Comprehension?

Konstantinos Patronas

Oct 1, 2020·2 min read

List comprehension is used to create new lists from existing iterables, usually the list comprehension code is an one liner that is more readable than a long function. The generic syntax of a list comprehension is

[ expression context condition ]

Syntax Explanation

• []: The surrounding brackets mean that the result is a new list
• context: elements of an object that is iterable.
• expression: defines how each element of the object to modified before added to the new list
• condition: its optional, defines which element of the context to be modified

Example: Multiply each element of a list by 2

new_list = [ i * 2 for i in [1,2,3,"b",4,5] ]
print(new_list)

Result:

[2, 4, 6, 'bb', 8, 10]

Notice that if we multiply a character by 2, the result will be the character twice

Example: Multiply each integer element of a list by 2, skip characters

new_list = [ i * 2 for i in [1,2,3,"b",4,5] if isinstance(i,int) ]
print(new_list)

Result:

[2, 4, 6, 8, 10]

Using the if isinstance(i,int)condition we filtered out the non integer elements of the list.

Example: Use functions within the list comprehension

The context, expression and condition parts of a list comprehension can be user functions if you need to write something that requires more complex code.

This code does the following:

1. generate_numbers function yields numbers from 1 to 6
2. filter_odd filters for modification only odd numbers
3. mult, multiplies each number that filter_odd allows with m, where m is equal to 2, then adds the result to a new list

def mult(i,m):
    return i*mdef generate_numbers(start,stop):
    for j in range(start,stop):
        yield jdef filter_odd(i):
    if i%2 > 0:
        return True
    else:
        return Falseif __name__ == '__main__':new_list = [ mult(i,m=2)  for i in generate_numbers(start=1,stop=6) if filter_odd(i=i) ]
    print(new_list)

Result:

[2, 6, 10]

Example: List comprehension with dictionaries

List comprehension can iterate dictionaries as well, the bellow code will create a new dictionary where values of an existing dictionary are greater than 100

pairs = {}
pairs['a'] = 10
pairs['b'] = 50
pairs['c'] = 133
pairs['d'] = 200if __name__ == '__main__':new_dict = dict([ (k,v)  for k,v in pairs.items() if v > 100 ])
    print(new_dict)

This code does the following:

1. for each k(ey) and v(alue) of pairs
2. filter only the pair that v(alue) is greater than 100
3. create a tuple with the filtered pair and add it to a list
4. convert the list to a dictionary

Result:

{'c': 133, 'd': 200}

I hope you found this article interesting and help you write great one-liners ;)