Python Tutorial

Python Tutorial, Release 2.6.2
>>> freshfruit = [’ banana’, ’ loganberry ’, ’passion fruit ’]
>>> [weapon.strip() for weapon in freshfruit]
[’banana’, ’loganberry’, ’passion fruit’]
>>> vec = [2, 4, 6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [3*x for x in vec if x > 3]
[12, 18]
>>> [3*x for x in vec if x < 2]
[]
>>> [[x,x**2] for x in vec]
[[2, 4], [4, 16], [6, 36]]
>>> [x, x**2 for x in vec] # error - parens required for tuples
File "", line 1, in
[x, x**2 for x in vec]
^
SyntaxError: invalid syntax
>>> [(x, x**2) for x in vec]
[(2, 4), (4, 16), (6, 36)]
>>> vec1 = [2, 4, 6]
>>> vec2 = [4, 3, -9]
>>> [x*y for x in vec1 for y in vec2]
[8, 6, -18, 16, 12, -36, 24, 18, -54]
>>> [x+y for x in vec1 for y in vec2]
[6, 5, -7, 8, 7, -5, 10, 9, -3]
>>> [vec1[i]*vec2[i] for i in range(len(vec1))]
[8, 12, -54]
List comprehensions are much more flexible than map() and can be applied to complex expressions and nested
functions:
>>> [str(round(355/113.0, i)) for i in range(1,6)]
[’3.1’, ’3.14’, ’3.142’, ’3.1416’, ’3.14159’]
5.1.5 Nested List Comprehensions
If you’ve got the stomach for it, list comprehensions can be nested. They are a powerful tool but – like all powerful
tools – they need to be used carefully, if at all.
Consider the following example of a 3x3 matrix held as a list containing three lists, one list per row:
>>> mat = [
... [1, 2, 3],
... [4, 5, 6],
... [7, 8, 9],
... ]
Now, if you wanted to swap rows and columns, you could use a list comprehension:
>>> print [[row[i] for row in mat] for i in [0, 1, 2]]
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
Special care has to be taken for the nested list comprehension:
To avoid apprehension when nesting list comprehensions, read from right to left.
A more verbose version of this snippet shows the flow explicitly:
32 Chapter 5. Data Structures