Python Functional Programming | Map, Filter and Reduce

Table of Contents:

• The built-in map() function in Python
  • Using map() with the operator
  • Using map() with the lambda function
  • map() takes a function
  • Using map() to return a tuple
  • map() takes multiple iterators
• Using Python built-in filter() function
• Python reduce() function
• Conclusion

map(), filter() and reduce() are functions forcing a functional programming approach in Python.

The built-in map() function in Python

Python map() function:

• takes two parameters: a function and an iterable
• returns a map object
• applies a given function to each item of an iterable
• allows you to process the iterables without a loop

Let’s create a map() function example. To use the map() function we need to have:

• a function and
• an iterable (list, set, tuple, another map object, etc.)

Using map() with the operator

Example: map() with the Operator

import operator
it = range(10) # iterable
fun = operator.neg # operator
t2 = map(fun, it)
list(t2)

Out:

[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]

We can do another map() function on a map object:

Using map() with the lambda function

Example: map() with the lambda function

import operator
it = range(10) # iterable
fun = operator.neg # operator
t2 = map(fun, it)
t3 = map(lambda o: o+100, t2)
list(t3)

Out:

[100, 99, 98, 97, 96, 95, 94, 93, 92, 91]

Now it should be evident how it is possible to process the iterables without a loop. Of course, if we use the map() function.

We used the operator in the above example, and after that we used the inline “lambda” function. Here is the list of operators we can use with the map() function:

['__abs__', '__add__', '__all__', '__and__', '__builtins__', '__cached__', '__concat__', '__contains__', '__delitem__', '__doc__', '__eq__', '__file__', '__floordiv__', '__ge__', '__getitem__', '__gt__', '__iadd__', '__iand__', '__iconcat__', '__ifloordiv__', '__ilshift__', '__imatmul__', '__imod__', '__imul__', '__index__', '__inv__', '__invert__', '__ior__', '__ipow__', '__irshift__', '__isub__', '__itruediv__', '__ixor__', '__le__', '__loader__', '__lshift__', '__lt__', '__matmul__', '__mod__', '__mul__', '__name__', '__ne__', '__neg__', '__not__', '__or__', '__package__', '__pos__', '__pow__', '__rshift__', '__setitem__', '__spec__', '__sub__', '__truediv__', '__xor__', '_abs', 'abs', 'add', 'and_', 'attrgetter', 'concat', 'contains', 'countOf', 'delitem', 'eq', 'floordiv', 'ge', 'getitem', 'gt', 'iadd', 'iand', 'iconcat', 'ifloordiv', 'ilshift', 'imatmul', 'imod', 'imul', 'index', 'indexOf', 'inv', 'invert', 'ior', 'ipow', 'irshift', 'is_', 'is_not', 'isub', 'itemgetter', 'itruediv', 'ixor', 'le', 'length_hint', 'lshift', 'lt', 'matmul', 'methodcaller', 'mod', 'mul', 'ne', 'neg', 'not_', 'or_', 'pos', 'pow', 'rshift', 'setitem', 'sub', 'truediv', 'truth', 'xor']

map() takes a function

Now let’s create another example where we use just a function (without an operator, and without lambda function).

Example:

def fun(x):
   return x+100
it = range(10) # iterable
t2 = map(fun, it)
list(t2)

Out:

[100, 101, 102, 103, 104, 105, 106, 107, 108, 109]

Using map() to return a tuple

Next, we will define a function fun() and we will use that function to return more than one element.

Example: Return a tuple

def fun(o):
   return (o, o+1)
it = range(10) # iterable
t = map(fun, it)
list(t)

Out:

[(0, 1),
 (1, 2),
 (2, 3),
 (3, 4),
 (4, 5),
 (5, 6),
 (6, 7),
 (7, 8),
 (8, 9),
 (9, 10)]

map() takes multiple iterators

Now we will show map() can take more than one iterator.

Example: Two iterators

r1 = range(10)
r2 = range(-10,0)
t = map(lambda t1, t2: t1+t2, r1, r2)
list(t)

Out:

[-10, -8, -6, -4, -2, 0, 2, 4, 6, 8]

Using Python built-in filter() function

There is an extreme similarity:

• filter() and map() both take a function and an iterator (at least one iterator).

Example: filter object

r = range(-10, 10) # range
f = filter(lambda x: x > -1, r) # filter object
print(f)
print(type(f))
list(f)

Out:

<filter object at 0x0000018FCA1701C0>
<class 'filter'>
 
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

In this case filter() is a way to replace the for loop with the single condition ` x > -1`.

We could rewrite the above example:

Example: Using the for loop

r = range(-10, 10)
l = [] # empty list
for e in r:
    if e > -1:
        l.append(e)
print(l)

Out:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Python reduce() function

There is an extreme similarity:

• reduce() and map() both take a function and an iterator (at least one iterator).

Reduce will apply a rolling computation to an iterator we set at the input.

Example:

from functools import reduce
it = [1,2,3]
t = reduce((lambda x, y: x * y), it)
print(t)

Out:

6

The equivalent example in non functional programming would be:

it = [1,2,3]
t = 1
for e in it:
   t = t*e
print(t)

Conclusion

Built-in functions map(), filter() and functools.reduce() are useful for those who like the functional programming style in Python. If you use these functions you can avoid the Python for loops.