Julien Danjou: An Introduction to Functional Programming with Python

Many Python developers are unaware of the extent to which you can use functional programming in Python, which is a shame: with few exceptions, functional programming allows you to write more concise and efficient code. Moreover, Python’s support for functional programming is extensive.

Here I'd like to talk a bit about how you can actually have a functional approach to programming with our favorite language.

Pure Functions

When you write code using a functional style, your functions are designed to have no side effects: instead, they take an input and produce an output without keeping state or modifying anything not reflected in the return value. Functions that follow this ideal are referred to as purely functional.

Let’s start with an example of a regular, non-pure function that removes the last item in a list:

def remove_last_item(mylist):
    """Removes the last item from a list."""
    mylist.pop(-1)  # This modifies mylist

This function is not pure: it has a side effect as it modifies the argument it is given. Let's rewrite it as purely functional:

def butlast(mylist):
    """Like butlast in Lisp; returns the list without the last element."""
    return mylist[:-1]  # This returns a copy of mylist

We define a butlast() function (like butlast in Lisp) that returns the list without the last element without modifying the original list. Instead, it returns a copy of the list that has the modifications in place, allowing us to keep the original. The practical advantages of using functional programming include the following:

• Modularity. Writing with a functional style forces a certain degree of
  separation in solving your individual problems and makes sections of code
  easier to reuse in other contexts. Since the function does not depend on any
  external variable or state, call it from a different piece of code is
  straightforward.
• Brevity. Functional programming is often less verbose than other paradigms.
• Concurrency. Purely functional functions are thread-safe and can run
  concurrently. Some functional languages do this automatically, which can be
  a big help if you ever need to scale your application, though this is not
  quite the case yet in Python.
• Testability. Testing a functional program is incredibly easy: all you need
  is a set of inputs and an expected set of outputs. They are idempotent,
  meaning that calling the same function over and over with the same arguments
  will always return the same result.

Note that concepts such as list comprehension in Python are already functionals in their approach, as they are designed to avoid side effects. We'll see in the following that some of the functional functions Python provide can actually be expressed as list comprehension!

Python Functional Functions

You might repeatedly encounter the same set of problems when manipulating data using functional programming. To help you deal with this situation efficiently, Python includes a number of functions for functional programming. Here, we'll see with a quick overview some of these built-in functions that allows you to build fully functional programs. Once you have an idea of what’s available, I encourage you to research further and try out functions where they might apply in your own code.

Applying Functions to Items with map

The map() function takes the form map(function, iterable) and applies function to each item in iterable to return an iterable map object:

>>> map(lambda x: x + "bzz!", ["I think", "I'm good"])
<map object at 0x7fe7101abdd0>
>>> list(map(lambda x: x + "bzz!", ["I think", "I'm good"]))
['I thinkbzz!', "I'm goodbzz!"]

You could also write an equivalent of map() using list comprehension, which
would look like this:

>>> (x + "bzz!" for x in ["I think", "I'm good"])
<generator object <genexpr> at 0x7f9a0d697dc0>
>>> [x + "bzz!" for x in ["I think", "I'm good"]]
['I thinkbzz!', "I'm goodbzz!"]

Filtering Lists with filter

The filter() function takes the form filter(function or None, iterable) and filters the items in iterable based on the result returned by function. This will return iterable filter object:

>>> filter(lambda x: x.startswith("I "), ["I think", "I'm good"])
<filter object at 0x7f9a0d636dd0>
>>> list(filter(lambda x: x.startswith("I "), ["I think", "I'm good"]))
['I think']

You could also write an equivalent of filter() using list comprehension, like
so:

>>> (x for x in ["I think", "I'm good"] if x.startswith("I "))
<generator object <genexpr> at 0x7f9a0d697dc0>
>>> [x for x in ["I think", "I'm good"] if x.startswith("I ")]
['I think']

Getting Indexes with enumerate

The enumerate() function takes the form enumerate(iterable[, start]) and returns an iterable object that provides a sequence of tuples, each consisting of an integer index (starting with start, if provided) and the corresponding item in iterable. This function is useful when you need to write code that refers to array indexes. For example, instead of writing this:

i = 0
while i < len(mylist):
    print("Item %d: %s" % (i, mylist[i]))
    i += 1

You could accomplish the same thing more efficiently with enumerate(), like so:

for i, item in enumerate(mylist):
    print("Item %d: %s" % (i, item))

Sorting a List with sorted

The sorted() function takes the form sorted(iterable, key=None, reverse=False) and returns a sorted version of iterable. The key argument allows you to provide a function that returns the value to sort on:

>>> sorted([("a", 2), ("c", 1), ("d", 4)])
[('a', 2), ('c', 1), ('d', 4)]
>>> sorted([("a", 2), ("c", 1), ("d", 4)], key=lambda x: x[1])
[('c', 1), ('a', 2), ('d', 4)]

Finding Items That Satisfy Conditions with any and all

The any(iterable) and all(iterable) functions both return a Boolean depending on the values returned by iterable. These simple functions are equivalent to the following full Python code:

def all(iterable):
    for x in iterable:
        if not x:
            return False
    return True

def any(iterable):
    for x in iterable:
        if x:
            return True
    return False

These functions are useful for checking whether any or all of the values in an iterable satisfy a given condition. For example, the following checks a list for two conditions:

mylist = [0, 1, 3, -1]
if all(map(lambda x: x > 0, mylist)):
    print("All items are greater than 0")
if any(map(lambda x: x > 0, mylist)):
    print("At least one item is greater than 0")

The key difference here, as you can see, is that any() returns True when at least one element meets the condition, while all() returns True only if every element meets the condition. The all() function will also return True for an empty iterable, since none of the elements is False.

Combining Lists with zip

The zip() function takes the form zip(iter1 [,iter2 [...]]) and takes multiple sequences and combines them into tuples. This is useful when you need to combine a list of keys and a list of values into a dict. Like the other functions described here, zip() returns an iterable. Here we have a list of keys that we map to a list of values to create a dictionary:

>>> keys = ["foobar", "barzz", "ba!"]
>>> map(len, keys)
<map object at 0x7fc1686100d0>
>>> zip(keys, map(len, keys))
<zip object at 0x7fc16860d440>
>>> list(zip(keys, map(len, keys)))
[('foobar', 6), ('barzz', 5), ('ba!', 3)]
>>> dict(zip(keys, map(len, keys)))
{'foobar': 6, 'barzz': 5, 'ba!': 3}

What's Next?

While Python is often advertised as being object oriented, it can be used in a very functional manner. A lot of its built-in concepts, such as generators and list comprehension, are functionally oriented and don’t conflict with an object-oriented approach. Python provides a large set of builtin functions that can help you keeping your code with no side effects. That also limits the reliance on a program’s global state, for your own good.

In the next blog post, we'll see how you can leverage Python functools and itertools module to enhance your functional adventure. Stay tuned!

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