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134 CHAPTER 6 ■ ABSTRACTION
LAMBDA EXPRESSIONS
In the material that follows, I sometimes use something called lambda expressions. These are small, unnamed
functions that can only contain an expression, and that return its value. A lambda expression is written like this:
lambda x, y, z: x + y + z
The first word, lambda, is a reserved word (keyword). 1 It is followed by the parameters, a colon (:), and
finally the body (an expression).
Although lambdas can be useful at times, you are usually better off writing a full-fledged function, especially
because the function name will then say something about what your function does.
map
The map function “maps” one sequence into another (of the same length) by applying a function
to each of the elements. For example, you may have a list of numbers, and you want to create
another list in which all the numbers are doubled:
>>> numbers = [72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33]
>>> map(lambda n: 2*n, numbers)
[144, 202, 216, 216, 222, 88, 64, 238, 222, 228, 216, 200, 66]
You don’t have to use lambda expressions—it works just fine with named functions as well:
>>> map(chr, numbers)
['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd', '!']
The built-in function chr takes a number as its only parameter and returns the character
corresponding to that number (the so-called ordinal number, which is really its ASCII code).
The reverse of chr is ord:
>>> map(ord, 'Hello, world!')
[72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33]
Because strings are just sequences of characters, you can use map directly. Note that the
result is a list, not another string. See the following section for a note about map, filter, and list
comprehensions.
filter
The filter function returns a new list in which the elements that you don’t want have been
filtered out. Or, to put it another way, it returns exactly those you do want. You supply filter
with a function that returns a Boolean (truth) value for a given sequence element. If the function
returns true, the element is part of the returned sequence; if it returns false, the element is not
included in the returned sequence. (The original sequence is not modified.) For example, you
might want to retain only the even numbers from the list numbers:
1. The name “lambda” comes from the Greek letter λ, which is used in mathematics to indicate an
anonymous function.