A list comprehension creates a new list by applying an expression to each element of an iterable. The most basic form is:
[ <expression> for <element> in <iterable> ]
There’s also an optional ‘if’ condition:
[ <expression> for <element> in <iterable> if <condition> ]
Each <element> in the <iterable> is plugged in to the <expression> if the (optional) <condition> evaluates to true . All results are returned at once in the new list. Generator expressions are evaluated lazily, but list comprehensions evaluate the entire iterator immediately - consuming memory proportional to the iterator’s length.
To create a list of squared integers:
squares = [x * x for x in (1, 2, 3, 4)]
# squares: [1, 4, 9, 16]
The for expression sets x to each value in turn from (1, 2, 3, 4). The result of the expression x * x is appended to an internal list. The internal list is assigned to the variable squares when completed.
Besides a speed increase (as explained here), a list comprehension is roughly equivalent to the following for-loop:
squares = []
for x in (1, 2, 3, 4):
squares.append(x * x)
# squares: [1, 4, 9, 16]
The expression applied to each element can be as complex as needed:
# Get a list of uppercase characters from a string
[s.upper() for s in "Hello World"]
# ['H', 'E', 'L', 'L', 'O', ' ', 'W', 'O', 'R', 'L', 'D']
# Strip off any commas from the end of strings in a list
[w.strip(',') for w in ['these,', 'words,,', 'mostly', 'have,commas,']]
# ['these', 'words', 'mostly', 'have,commas']
# Organize letters in words more reasonably - in an alphabetical order
sentence = "Beautiful is better than ugly"
["".join(sorted(word, key = lambda x: x.lower())) for word in sentence.split()]
# ['aBefiltuu', 'is', 'beertt', 'ahnt', 'gluy']
else can be used in List comprehension constructs, but be careful regarding the syntax. The if/else clauses should be used before for loop, not after:
# create a list of characters in apple, replacing non vowels with '*'
# Ex - 'apple' --> ['a', '*', '*', '*' ,'e']
[x for x in 'apple' if x in 'aeiou' else '*']
#SyntaxError: invalid syntax
# When using if/else together use them before the loop
[x if x in 'aeiou' else '*' for x in 'apple']
#['a', '*', '*', '*', 'e']
Note this uses a different language construct, a conditional expression, which itself is not part of the comprehension syntax. Whereas the if after the for…in is a part of list comprehensions and used to filter elements from the source iterable.
Order of double iteration [... for x in ... for y in ...] is either natural or counter-intuitive. The rule of thumb is to follow an equivalent for loop:
def foo(i):
return i, i + 0.5
for i in range(3):
for x in foo(i):
yield str(x)
This becomes: