Python List Comprehension Simplified

Hello,

This is going to be as simple introduction to Python List Comprehension.

A "list comprehension" in python provides a concise way to create lists. Basically, it nests 'expression', 'loops' and 'conditions' within a list on a single line. The basic structure of the python's list comprehension is like this: [*expression*  *for object in loop*  *if condition*] - without the "*" of-course.

Example

Assuming we have a list called "my_list" and we want to get the length of individual elements within the list, a 'for' loop can do it as follow:-

When you run above code it will output the result as: 4 10 2 5 1 2

Now lets use "list comprehension" to achieve the same result as above:-

The result returns a list of the lengths like so: [4, 10, 2, 5, 1, 2]

Note that we didn't add a condition to the list comprehension above. len(item): is the expression value and for item in my_list: is the loop part of the "list comprehension". We will add a condition in a moment.

List comprehension has two quick noticeable advantages over using the "for" loop namely:-

1) the code is shorter in length.

2) the result is in an object list instead of individual numbers as obtained using "for" loop.

To add a condition, we simply add an "if" statement to the end of the "list comprehension" as follow;-

Lets add a condition to display only length of items greater than four: if len(item) > 4. Add that syntax to the end of the "list comprehension".

The result returns a list of the lengths greater than four like so: [10, 5]

For more than one condition

Assuming we have to check for more than one conditions as follow; instead just having the lengths greater than four, we also want to consider the lengths less than four and equal to four. This makes it about three conditions to check as follow:-

Less than four = x

Greater than four = y

Equal to four = z

The syntax will be like this: [*expression* *if elif else condition*  *for object in loop* ]

In a normal forLoop it is like this....

for item in my_list:
    if len(item) < 4:
        print('x')
    elif len(item) > 4:
        print('y')
    else:
        print('z')

In list it will look like this....

print(['x' if len(item) < 4 else 'y' if len(item) > 4 else 'z' for item in my_list])

Note that 'elif' is not used in the list comprehension.

Conclusion

Even though "list comprehension" may sound very nice to use it has a little draw back in the sense that it can easily make your code very difficult to read in the nearest feature.

Imagine a situation where you compressed hundreds lines of code using "list comprehension" into a few lines. It become unreadable or difficult to read when time passes.

So use it with caution.

Thank you for following the blog post.