# Comprehensions in Python

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One of Haskell’s features I really liked was list comprehensions. I created a comparison between Haskell and Julia list comprehensions almost exactly a year ago. It appears that Python list comprehensions are as nice as either Haskell’s or Julia’s! Although, having to use Python’s `lambda` form to simulate the more concise Haskell/Julia function definition is less than ideal.

Here is a comparison between the Haskell list comprehensions in “Chapter 5: List Comprehensions” in Graham Hutton’s “Programming Haskell” book and equivalent Python comprehensions.

`[ x^2 | x <- [1..5] ]` `[ x**2 for x in range(1,6) ]`
`[ (x,y) | x <- [1, 2, 3], y <- [4, 5] ]` `[ (x,y) for x in range(1,4) for y in range(4,6) ]`
`[ (x,y) | x <- [1..3], y <- [x..3 ]]` `[ (x,y) for x in range(1,4) for y in range(x,4) ]`
`concat xss = [ x | xs <- xss, x <- xs ]` `concat = lambda xss: [ x for xs in xss for x in xs ]`
`firsts ps = [ x | (x,_) <- ps ]` `firsts = lambda ps: [ x for (x,_) in ps ]`
`length xs = sum[ 1 | _ <- xs ]` `length = lambda xs: sum(1 for _ in xs)`
`factors n = [ x | x <- [1..n], n `mod` x == 0 ]` `factors = lambda n: [ x for x in range(1,n+1) if n % x == 0 ]`
`prime n = factors n == [1, n]` `prime = lambda n: factors(n) == [1, n]`
`primes n = [ x | x <- [2..n], prime x ]` `primes = lambda n: [ x for x in range(2, n+1) if prime(x) ]`
`find k t = [ v | (k', v) <- t, k == k' ]` `find = lambda k, t: [ v for (k1,v) in t if k == k1 ]`
`pairs xs = zip xs (tail xs)` `pairs = lambda xs: zip(xs, xs[1:])`
`sorted xs = and [x <= y | (x,y) <- pairs xs ]` `is_sorted = lambda xs: all(x >= y for (x,y) in pairs(xs))`
```positions x xs = [i | (x',i) <- zip xs [0..n], x == x']      where n = length xs - 1``` ```positions = lambda x, xs: [ i for (x1,i) in      zip(xs, range(len(xs))) if x == x1 ]```
`lowers xs = length [x | x <- xs, isLower x ]` `lowers = lambda xs: len([ x for x in xs if x.islower() ])`
`count x xs = length [x' | x' <- xs, x == x']` `count = lambda x, xs: len([ x1 for x1 in xs if x == x1])`