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Python Functions, In Practice

I have used Python for the past one and a half years. I can tell you that Python is not weird, or wonderful, but rather weirdly wonderful.

Coming out of college, I thought I knew what “professional” code looked like. It looked like Java code. There’s objects. There’s object-oriented design patterns. Don’t use inheritance, use composition - unless you want something to not be flexible. Divide up your code into modular pieces of state (objects) that communicate to each other. Blah blah blah.

Then I started using JavaScript, and discovered Lodash. Lodash was my first experience with functional programming. At first, it was alien. It grew on me, though; not having to worry about state made it a lot easier to test code and assume that the test case matched production use cases. Lodash was my gateway drug to learning about immutable data types like Immutable.js, functional state stores like Redux, and full-blown functional programming like Clojure, Common Lisp, and Elixir. I’m not a functional fanatic (yet), but I do love me some stateless backends.

And then there’s Python.

The thing about Python is, it’s an object-oriented language. Of course it is. Here’s literally the first two sentences of the Python 3 data model reference:

Objects are Python’s abstraction for data. All data in a Python program is represented by objects or by relations between objects.

Wow. They’re really pushing this object-oriented thing here. Hard.

So when I first used Python in production, I treated it like Java. It’s an object-oriented language, right? Then, one day during code review, I stumble across something like this:

x = [
    for i
    in iterable
    if cond(i)

It’s a list comprehension, Python’s answer to lambdas, map(), filter(), and reduce() (not that Python doesn’t have those built in). It definitely does not look object-oriented. It’s downright functional. Here’s the imperative version of the above:

x = []
for i in iterable:
  If cond(i):

And here’s the above in a more traditional functional statement:

x = list(map(lambda i : expr(i), filter(lambda i: cond(i), iterable)))

Replace iterable with whatever iterable you want (e.g., range(0, 10)), expr(i) with whatever expression you want (e.g., i * i), and cond(i) with whatever conditional you want (e.g., i % 2 == 0) to test it out yourself in your Python shell.

One of the senior developers said experienced Python SWEs prefer list comprehensions because they are easier to read. I knew I wasn’t one then because I could not grok that at all. After a few months though, I learned to love it.

It didn’t stop there. I played around with Flask and mongoDB, and you can pretty much make a stateless application in Python (if you’re extremely careful about your database design). You have functools. You have operator. It’s all there. And if it’s not because you’re using Python 2.x, you can use from __future__ import absolute_import and then it’s all there.

Why does Python have this ability?

Here’s Guido van Rossum, Python’s benevolent dictator for life (BDFL), in his own words:

I have never considered Python to be heavily influenced by functional languages, no matter what people say or think. I was much more familiar with imperative languages such as C and Algol 68 and although I had made functions first-class objects, I didn’t view Python as a functional programming language.

Boom. Functions are first-class objects.

…what’s a first class object?

In “Fluent Python”, first class objects are defined as objects that have the following properties:

Created at runtime Assigned to a variable or element in a data structure Passed as an argument to a function Returned as the result of a function

Here’s an example:

username$ python
Python 2.7.13 |Anaconda custom (x86_64)| (default, Sep 21 2017, 17:38:20)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def factorial(n):
…          ‘’’returns n!’’’
…          return 1 if n < 2 else n * factorial(n-1)
>>> factorial(5)

You can create a function in a console session (i.e. “runtime”).

>>> fact = []
>>> fact.append(factorial)
>>> fact[0](5)
>>> fact = factorial
>>> fact(5)

You can assign a function to a variable in a data structure. You can also just straight up assign it to a variable. You can execute it either way.

>>> def exec_factorial(factorial):
…          ‘’’executes factorial function with n = 5’’’
...          return factorial(5)
>>> exec_factorial(fact)

You can pass a function into another function as an argument.

>>> def return_factorial():
...          ‘’’returns the factorial function’’’
...          return fact
>>> fact = return_factorial()
>>> fact(5)

Finally, you can return a function from a function.

Additionally, other Pythonic attributes may convince you Pythons are first order functions:

>>> factorial.__doc__
‘returns n!’
>>> set(dir(object())).issubset(dir(factorial))
>>> factorial.__hash__()

Not only does a function definition have object-like attributes, it has signature Python object attributes, like ‘_doc_’. You can programmatically verify that a base object’s attributes are a subset of a function’s attributes. You can even hash a function!

>>> type(factorial)
<class ‘function’>

And here, we find out that function definitions are instances of the class ‘function’.

Mind. Blown.

All of this makes functions in Python extremely powerful. And Python does encourage this! For example, Python has native decorators, which are syntactic sugar for higher-order functions. It provides a really nice, standardized syntax for wrapping a function with other business logic, so that you can do stuff before and after said function without having to write it in a bunch of locations.

Case in point, here’s an extremely basic example:

>>> def decorator(func):
...     def wrapper(x):
...             print("Before calling " + func.__name__)
...             func(x)
...             print("After calling " + func.__name__)
...     return wrapper
>>> @decorator
... def foo(x):
...     print("Hi, foo has been called with " + str(x))
>>> foo('firstName lastName')
Before calling foo
Hi, foo has been called with firstName lastName
After calling foo

See how you return a function that wraps another function, and all you need to invoke that wrapping is to add @${DECORATOR_NAME} at the function you want to wrap? It really is that simple.

In practice, I use pytest frequently as part of my day job, and the two parts of pytest I use most are two fixtures: pytest.fixture() and pytest.mark.parametrize(). Those two decorators have saved me much pain and suffering while writing comprehensive tests. Much more on that in a later post.

This quirk (or feature) of Python is mostly due to Python’s usefulness-first philosophy. Guido’s first bullet point in his post about Python’s design philosophy is “Borrow ideas from elsewhere whenever it makes sense”. As a result of functions having all the same privileges as objects, you can apply both object-oriented patterns, functional programming patterns, or a mixture of the two, in the same language. This is powerful whether you are replacing bash scripts, constructing REST APIs, or simply gluing different services together, as I have found through personal experience.

Much thanks to “Fluent Python” for providing the technical chops of this post. If you’re a Python developer and you haven’t bought this book, go out and buy it now. It contains insights I’d never thought I’d know.