Functional tools

Python provides a few in-built commands such as map, filter, reduce as well lambda (to create anonymous functions) and list comprehension. These are typical commands from functional languages of which LISP is probably best known.

Functional programming can be extremely powerful and one of the strengths of Python is that it allows to program using (i) imperative/procedural programming style, (ii) object oriented style and (iii) functional style. It is the programmers choice which tools to select from which style and how to mix them to best address a given problem.

In this chapter, we provide some examples for usage of the commands listed above.

Anonymous functions

All functions we have seen in Python so far have been defined through the def keyword, for example:

In [1]:
def f(x):
     return x ** 2

This funtion has the name f. Once the function is defined (i.e. the Python interpreter has come across the def line), we can call the function using its name, for example

In [2]:
y = f(6)

Sometimes, we need to define a function that is only used once, or we want to create a function but don’t need a name for it (as for creating closures). In this case, this is called anonymous function as it does not have a name. In Python, the lambda keyword can create an anonymous function.

We create a (named) function first, check it’s type and behaviour:

In [3]:
def f(x):
    return x ** 2

<function __main__.f>
In [4]:
In [5]:

Now we do the same with an anonymous function:

In [6]:
lambda x: x ** 2
<function __main__.<lambda>>
In [7]:
type(lambda x: x ** 2)
In [8]:
(lambda x: x ** 2)(10)

This works exactly in the same way but – as the anonymous function does not have a name – we need to define the function (through the lambda expression) – every time we need it.

Anonymous functions can take more than one argument:

In [9]:
(lambda x, y: x + y)(10, 20)
In [10]:
(lambda x, y, z: (x + y) * z )(10, 20, 2)

We will see some examples using lambda which will clarify typical use cases.


The map function lst2 = map(f, s ) applies a function f to all elements in a sequence s. The result of map can be turned into a list with the same length as s:

In [11]:
def f(x): 
    return x ** 2
lst2 = list(map(f, range(10)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
In [12]:
list(map(str.capitalize, ['banana', 'apple', 'orange']))
['Banana', 'Apple', 'Orange']

Often, this is combined with the anonymous function lambda:

In [13]:
list(map(lambda x: x ** 2, range(10) ))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
In [14]:
list(map(lambda s: s.capitalize(), ['banana', 'apple', 'orange']))
['Banana', 'Apple', 'Orange']


The filter function lst2 = filter( f, lst) applies the function f to all elements in a sequence s. The function f should return True or False. This makes a list which will contain only those elements s*i* of the sequence s for which f(s*i*) has returned True.

In [15]:
def greater_than_5(x):
    if x > 5:
            return True
            return False

list(filter(greater_than_5, range(11)))
[6, 7, 8, 9, 10]

The usage of lambda can simplify this significantly:

In [16]:
list(filter(lambda x: x > 5, range(11)))
[6, 7, 8, 9, 10]
In [17]:
known_names = ['smith', 'miller', 'bob']
list(filter( lambda name : name in known_names, \
     ['ago', 'smith', 'bob', 'carl']))
['smith', 'bob']

List comprehension

List comprehensions provide a concise way to create and modify lists without resorting to use of map(), filter() and/or lambda. The resulting list definition tends often to be clearer than lists built using those constructs. Each list comprehension consists of an expression followed by a for clause, then zero or more for or if clauses. The result will be a list resulting from evaluating the expression in the context of the for and if clauses which follow it. If the expression would evaluate to a tuple, it must be parenthesized.

Some examples will make this clearer:

In [18]:
freshfruit = ['  banana', '  loganberry ', 'passion fruit  ']
[weapon.strip() for weapon in freshfruit]
['banana', 'loganberry', 'passion fruit']
In [19]:
vec = [2, 4, 6]
[3 * x for x in vec]
[6, 12, 18]
In [20]:
[3 * x for x in vec if x > 3]
[12, 18]
In [21]:
[3 * x for x in vec if x < 2]
In [22]:
[[x, x ** 2] for x in vec]
[[2, 4], [4, 16], [6, 36]]

We can also use list comprehension to modify the list of integers returned by the range command so that our subsequent elements in the list increase by non-integer fractions:

In [23]:
[x*0.5 for x in range(10)]
[0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5]

Let’s now revisit the examples from the section on filter

In [24]:
[x for x in range(11) if x>5 ]
[6, 7, 8, 9, 10]
In [25]:
[name for name in ['ago','smith','bob','carl'] \
      if name in known_names]
['smith', 'bob']

and the examples from the map section

In [26]:
[x ** 2 for x in range(10) ]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
In [27]:
[fruit.capitalize() for fruit in ['banana', 'apple', 'orange'] ]
['Banana', 'Apple', 'Orange']

all of which can be expressed through list comprehensions.

More details


The reduce function takes a binary function f(x,y), a sequence s, and a start value a0. It then applies the function f to the start value a0 and the first element in the sequence: a1 = f(a,s[0]). The second element (s[1]) of the sequence is then processed as follows: the function f is called with arguments a1 and s[1], i.e. a2 = f(a1,s[1]). In this fashion, the whole sequence is processed. Reduce returns a single number.

This can be used, for example, to compute a sum of numbers in a sequence if the function f(x,y) returns x+y:

In [28]:
from functools import reduce
In [29]:
def add(x,y):
    return x+y

reduce(add, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 0)
In [30]:
reduce(add, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 100)

We can modify the function add to provide some more detail about the process:

In [31]:
def add_verbose(x, y):
    print("add(x=%s, y=%s) -> %s" % (x, y, x+y))
    return x+y

reduce(add_verbose, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 0)
add(x=0, y=1) -> 1
add(x=1, y=2) -> 3
add(x=3, y=3) -> 6
add(x=6, y=4) -> 10
add(x=10, y=5) -> 15
add(x=15, y=6) -> 21
add(x=21, y=7) -> 28
add(x=28, y=8) -> 36
add(x=36, y=9) -> 45
add(x=45, y=10) -> 55

It may be instructive to use an asymmetric function f, such as add_len( n, s ) where s is a sequence and the function returns n+len(s) (suggestion from Thomas Fischbacher):

In [32]:
def add_len(n,s):
    return n+len(s)

reduce(add_len, ["This","is","a","test."],0)

As before, we’ll use a more verbose version of the binary function to see what is happening:

In [33]:
def add_len_verbose(n,s):
    print("add_len(n=%d, s=%s) -> %d" % (n, s, n+len(s)))
    return n+len(s)

reduce(add_len_verbose, ["This", "is", "a", "test."], 0)
add_len(n=0, s=This) -> 4
add_len(n=4, s=is) -> 6
add_len(n=6, s=a) -> 7
add_len(n=7, s=test.) -> 12

Another way to understand what the reduce function does is to look at the following function (kindly provided by Thomas Fischbacher) which behaves like reduce but explains what it does:

Here is an example using the explain_reduce function:

In [34]:
cd code/
In [35]:
from explain_reduce import explain_reduce
def f(a,b):
    return a+b

reduce(f, [1,2,3,4,5], 0)
In [36]:
explain_reduce(f, [1,2,3,4,5], 0)
Step 0: value-so-far=0 next-list-element=1
Step 1: value-so-far=1 next-list-element=2
Step 2: value-so-far=3 next-list-element=3
Step 3: value-so-far=6 next-list-element=4
Step 4: value-so-far=10 next-list-element=5
Done. Final result=15

Reduce is often combined with lambda:

In [37]:
reduce(lambda x,y:x+y, [1,2,3,4,5], 0)

There is also the operator module which provides standard Python operators as functions. For example, the function operator.__add__(a,b) is executed when Python evaluates code such as a+b. These are generally faster than lambda expressions. We could write the example above as

In [38]:
import operator
reduce(operator.__add__, [1,2,3,4,5], 0)

Use help(’operator’) to see the complete list of operator functions.

Why not just use for-loops?

Let’s compare the example introduced at the beginning of the chapter written (i) using a for-loop and (ii) list comprehension. Again, we want to compute the numbers 02, 12, 22, 32, ... up to (n − 1)2 for a given n.

Implementation (i) using a for-loop with n=10:

In [39]:
y = []
for i in range(10):
    y.append(i ** 2)

Implementation (ii) using list comprehension:

In [40]:
y = [x ** 2 for x in range(10)]

or using map:

In [41]:
y = map(lambda x: x ** 2, range(10))

The versions using list comprehension and map fit into one line of code whereas the for-loop needs 3. This example shows that functional code result in very concise expressions. Typically, the number of mistakes a programmer makes is per line of code written, so the fewer lines of code we have, the fewer bugs we need to find.

Often programmers find that initially the list-processing tools introduced in this chapter seem less intuitive than using for-loops to process every element in a list individually, but that – over time – they come to value a more functional programming style.


The functional tools described in this chapter can also be faster than using explicit (for or while) loops over list elements.

The program below computes $\\sum\_{i=0}^{N-1} i^2$ for a large value of N using 4 different methods and records execution time:

  • Method 1: for-loop (with pre-allocated list, storing of i2 in list, then using in-built sum function)

  • Method 2: for-loop without list (updating sum as the for-loop progresses)

  • Method 3: using list comprehension

  • Method 4: using numpy. (Numpy is covered in chapter 14)

The program produces the following output:

In [42]:
N = 10000000
for-loop1 3.92704701423645
for-loop2 3.2223408222198486
listcomp 2.6926040649414062
numpy 0.11613011360168457
Slowest method is 33.8 times slower than the fastest method.

The actual execution performance depends on the computer. The relative performance may depend on versions of Python and its support libraries (such as numpy) we use.

With the current version (python 3.4, numpy 1.10, on a x84 machine running OS X), we see that methods 1 and 2 (for-loop without list and with pre-allocated list) are slowest, somewhat closely followed by the slightly faster list comprehension. The fastest method is number 4 (using numpy).

For reference, here is the source code of the programme:

In [43]:
"""Compare calculation of \sum_i x_i^2 with
i going from zero to N-1.

We use (i) for loops and list, (ii) for-loop, (iii) list comprehension
and (iv) numpy.

We use floating numbers to avoid using Python's long int (which would
be likely to make the timings less representative).

import time
import numpy
N = 10000000

def timeit(f, args):
    """Given a function f and a tuple args containing
    the arguments for f, this function calls f(*args),
    and measures and returns the execution time in

    Return value is tuple: entry 0 is the time,
    entry 1 is the return value of f."""

    starttime = time.time()
    y = f(*args)    # use tuple args as input arguments
    endtime = time.time()
    return endtime - starttime, y

def forloop1(N):
    s = 0
    for i in range(N):
        s += float(i) * float(i)
    return s

def forloop2(N):
    y = [0] * N
    for i in range(N):
        y[i] = float(i) ** 2
    return sum(y)

def listcomp(N):
    return sum([float(x) * x for x in range(N)])

def numpy_(N):
    return numpy.sum(numpy.arange(0, N, dtype='d') ** 2)

#main program starts
timings = []
print("N =", N)
forloop1_time, f1_res = timeit(forloop1, (N,))
print("for-loop1", forloop1_time)
forloop2_time, f2_res = timeit(forloop2, (N,))
print("for-loop2", forloop2_time)
listcomp_time, lc_res = timeit(listcomp, (N,))
print("listcomp", listcomp_time)
numpy_time, n_res = timeit(numpy_, (N,))
print("numpy", numpy_time)

#ensure that different methods provide identical results
assert f1_res == f2_res
assert f1_res == lc_res
# Allow a bit of difference for the numpy calculation
numpy.testing.assert_approx_equal(f1_res, n_res)

print("Slowest method is {:.1f} times slower than the fastest method.".format(
In [ ]: