List comprehensions provide a concise way to create lists. They are generally faster and more readable than standard loops. Notice how the inner scope is evaluated:
squares = [x**2 for x in range(5)]
print(squares)A list comprehension can include an if clause to filter elements.
evens = [x for x in range(10) if x % 2 == 0]
print(evens)[0, 2, 4, 6, 8]
You can nest for clauses in a list comprehension, and the order is the same as in nested for loops.
matrix = [[1, 2], [3, 4]]
flat = [num for row in matrix for num in row]
print(flat)[1, 2, 3, 4]
If the expression in a list comprehension evaluates to a tuple, it must be parenthesized.
res = [(x, x**2) for x in range(3)]
print(res)[(0, 0), (1, 1), (2, 4)]
Python also supports set comprehensions using curly braces.
unique_chars = {c for c in 'abracadabra' if c not in 'abc'}
print(sorted(list(unique_chars)))['d', 'r']
Dict comprehensions allow creating dictionaries from arbitrary key and value expressions:
squares_dict = {x: x**2 for x in (2, 4, 6)}
print(squares_dict){2: 4, 4: 16, 6: 36} [x, x**2 for x in range(3)].[x for x in []] returns [].What does [n * n for n in range(4)] produce?
[0, 1, 4, 9] — range(4) gives 0, 1, 2, 3, and each value is squared: 0, 1, 4, 9.
Which comprehension keeps only the even numbers from nums?
[n for n in nums if n % 2 == 0] — A filtering if goes at the END of the comprehension: [n for n in nums if n % 2 == 0].
In Python 3, does the loop variable x still exist after [x for x in nums] runs?
No — a comprehension has its own scope, so x does not exist outside it. — Unlike a plain for-loop, a comprehension has its own scope, so its loop variable never leaks out.
What does (x for x in nums) create?
A generator expression — wrap it in tuple(...) if you want a tuple. — Parentheses make a lazy generator expression, not a tuple. Use tuple(x for x in nums) to get a tuple.
Use a list comprehension to create a list named `odds` containing only the odd numbers from the provided list `numbers`.
Review the syntax: [expression for item in iterable if condition]
numbers = [12, 5, 8, 9, 21, 4]
odds = [x for x in numbers if x % 2 != 0]
print(odds)Use a dictionary comprehension to create a dictionary named `word_lengths` that maps each word in the list `words` to its length.
Review the syntax: [expression for item in iterable if condition]
words = ['hello', 'world', 'python']
word_lengths = {w: len(w) for w in words}
print(word_lengths)