Python: Comprehensions
Comprehensions (and list comprehensions in particular) are probably the feature I love the most coming from JavaScript to Python. It enables you to write concise code for something you do very often: Iterating over a list, modifying the data a bit (“data massaging”) and returning a list with the new entries.
One thing to note: List comprehension load the entire output into memory so this should only be used for small to medium-sized lists. In cases of big lists, use generators instead.
Basic Syntax (for lists):
[expression for item in iterable if conditional]If you want to try the instructions below you can fire up the Python REPL. If
you have Python installed run python in your terminal. Any Python code that
starts with >>> symbols indicated that it was typed into a REPL.
To try out the JavaScript snippets you could use Quokka.js (opens in a new tab) in your editor.
#Examples
Iterating over the array and giving out the length of the word itself into a new list:
const names = ["James", "Naomi", "Amos", "Alex", "Bobbie", "Clarissa"]const len_names = names.map((name) => name.length)console.log(len_names)// [ 5, 5, 4, 4, 6, 8 ]>>> names = ["James", "Naomi", "Amos", "Alex", "Bobbie", "Clarissa"]>>> len_names = [len(name) for name in names]>>> print(len_names)[5, 5, 4, 4, 6, 8]Skipping the first item & last item (by index) and creating a list of dictionaries:
const names = ["James", "Naomi", "Amos", "Alex", "Bobbie", "Clarissa"]const names_list = names .filter((_, id) => id !== 0 && id !== names.length - 1) .map((name) => ({ name, length: name.length }))console.log(names_list)// [ { name: 'Naomi', length: 5 }, { name: 'Amos', length: 4 }, { name: 'Alex', length: 4 }, { name: 'Bobbie', length: 6 } ]>>> names = ["James", "Naomi", "Amos", "Alex", "Bobbie", "Clarissa"]>>> names_list = [{ "name": name, "length": len(name) } for idx, name in enumerate(names) if idx != 0 and idx != len(names)-1]>>> print(names_list)[{'name': 'Naomi', 'length': 5}, {'name': 'Amos', 'length': 4}, {'name': 'Alex', 'length': 4}, {'name': 'Bobbie', 'length': 6}]Tuple unpacking:
const students = [ ["James", 16], ["Naomi", 20],]const nameWithAge = students.map((s) => [s[0].length + s[1], ...s])console.log(nameWithAge)// [ [ 21, 'James', 16 ], [ 25, 'Naomi', 20 ] ]>>> students = [("James", 16), ("Naomi", 20)]>>> nameWithAge = [[len(s[0])+s[1], *s] for s in students]>>> print(nameWithAge)[[21, 'James', 16], [25, 'Naomi', 20]]Creating a list of characters from a string:
const myName = "LekoArts"const arr = [...myName]console.log(arr)// [ 'L', 'e', 'k', 'o', 'A', 'r', 't', 's' ]>>> my_name = 'LekoArts'>>> arr = [l for l in my_name]>>> print(arr)['L', 'e', 'k', 'o', 'A', 'r', 't', 's']Reversing strings in a tuple and returning them as a list:
const words = ["kayak", "LekoArts", "radar", "python"]const reversed = words.map((word) => word.split("").reverse().join(""))console.log(reversed)// [ 'kayak', 'strAokeL', 'radar', 'nohtyp' ]>>> words = ("kayak", "LekoArts", "radar", "python")>>> reversed = [word[::-1] for word in words]>>> print(reversed)['kayak', 'strAokeL', 'radar', 'nohtyp']You can also nest comprehensions. Here’s a 3 by 3 identity matrix:
>>> matrix = [[1 if item_idx == row_idx else 0 for item_idx in range(0, 3)] for row_idx in range(0, 3)]>>> print(matrix)[[1, 0, 0], [0, 1, 0], [0, 0, 1]]