List Comprehensions in Python: A Comprehensive Guide, Study notes of Web Programming and Technologies

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M A R C H 2 4 T H^ , 2 0 1 4

List Comprehensions

Examples to Get Us Started

— [x**2 for x in range(10)] [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] — [str(x)+str(x) for x in range(10)] ['00', '11', '22', '33', '44', '55', '66', '77', '88', '99'] — [str(x)+str(x) for x in range(10) if x%2 == 0] ['00', '22', '44', '66', '88']

List Comprehension: Basic Syntax

[ expr for x in list] Notes: — for and in are Python keywords, used just as in for-loops. — x is a variable that takes on values of elements in list, in order. — expr is Python expression, typically involving the variable x. — The expression [ expr for x in list] evaluates to a list made up of the different values that expr takes on for different x. — This is similar to the “set builder” notation used in math: {x*y | x and y are even}.

List Comprehensions: Syntax with if-clause

[ expr for x in list if bool-expr ]

Notes:

— bool-expr is a boolean expression involving x.

— The overall expression evaluates to a list of values of

expr evaluated for all values of x in list satisfying the

bool-expr.

— Example: [str(x)+str(x) for x in range(10) if x%2 == 0]

evaluates to ['00', '22', '44', '66', '88']

Nested List Comprehensions

Example: [x*y for x in range(3) for y in range(3)] [0, 0, 0, 0, 1, 2, 0, 2, 4] Notes:

— As in nested loops, for every iteration of the first loop

(the for-x loop), all iterations of the second loop (the

for-y loop) are executed.

Example: Generating Perfect Squares

[x for x in range(100) for y in range(x) if y*y == x]

[4, 9, 16, 25, 36, 49, 64, 81]

Notes:

— Those x and y values (from their respective lists) that

satisfy the condition y

2

= x, are generated.

— Thus all x values generated in this manner are

perfect squares.

Example: Generating Prime Numbers

primes = [x for x in range(2, 100) if x not in composites] Notes: — Primes in the range 2..99 can be obtained by taking the complement of the generated composites.

Example: Flattening Lists

nestedList = [range(x) for x in range(1, 4)] nestedList [[0], [0, 1], [0, 1, 2]] [y for x in nestedList for y in x] [0, 0, 1, 0, 1, 2]

Warning!

— The danger with list comprehensions is that your

code may become hard to understand, especially

with nested list comprehensions.

— If by using a list comprehension, you are making

your code hard to understand, then it is time to

desist.