Hi, I'm Thomas

NYC-based developer, student, and CS enthusiast.

Making Python code beautiful

04 October 2020

A bit of Python code can be written so well that your boss and coworkers will marvel at its beauty. Keep reading to discover easy changes that will improve your code’s speed and style!

Treat this as a collection of useful concepts. Even you Python pros will find tips to apply to your code.

Tuple unpacking

Let’s start simple. Say you wanted to swap the values that two variables store. In many languages (such as C), you would need to use a temporary variable, like this:

int originalB = b;
b = a;
a = originalB;

But in Python, this is all shortened to just one readable line:

a, b = b, a

That’s it! This is faster and more space-effective. Maybe your code is prettier already.

Python pros — did you know that you can use * to assign a list of values to a variable?

a, b, *several, c, d = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
print(a)  # 1
print(several)  # [3, 4, 5, 6, 7, 8]

Line length

Your lines should be at most 80–90 characters long. Use parentheses, not backslashes, to fix longer lines:

long_variable_name = long_function_name(long_arg, another_arg) + another_function() - final_function() / 3

long_variable_name = long_function_name(long_arg, another_arg) \
                        + another_function() - final_function() / 3

long_variable_name = (
    long_function_name(long_arg, another_arg)
    + another_function()
    - final_function() / 3

This leaves more whitespace and aligns the operators (e.g. + ), making it easier for somebody (including your future self) to understand the code.

Python pros — You can also look into linters, which automatically format your code for you! Linters go beyond line length: nested lists, spacing, etc. I use the Black linter and I love it.

List comprehension

Here’s a quick one. I cringe whenever I see Python code that creates an empty list, then appends to it using a for loop:

even_numbers = []
for n in range(20):
    if n % 2 == 0:

If you ever catch yourself doing that, change it to this:

even_numbers = [n for n in range(20) if n % 2 == 0]

It’s faster and it’s a lot cleaner.

Context managers

Please, don’t read and write files without using with!

logfile = open('october.log', 'w')
logfile.write('Bad code...\n')

# BAD: logfile is never closed

First of all, file pointers are meant to be closed! Secondly, there’s a beautiful Python syntax that lets you forget the opening and closing:

with open('october.log', 'w') as logfile:
    logfile.write('Much better!\n')

# GOOD: logfile is closed automatically

Python pros — You can actually write your own context managers. These are useful for code that has a setup and an ending action. The general structure looks like this:

from functools import contextmanager  # standard library

def my_context_manager():
    # setup code: runs just before entering the
    # "with" statement
        yield var  # or just "yield" if
                   # you don't need to pass
                   # a variable on
        # ending code: runs right after exiting the
        # "with" statement. It will run even if code
        # inside the "with" statement raised an exception!

Say you wanted to have a context manager that printed to let you know when your processes start and end:

That’s it! Now you can use it like this:

Positional-only arguments

Often, the names of a function’s arguments are irrelevant. In this case, you want those arguments to always be passed positionally:

  • Positionally: is_capitalized("world")
  • Not positionally: is_capitalized(string="world")

In Python 3.8, you can force arguments to your functions to be positional! This can make code cleaner and more readable.

To achieve this functionality, use a / as an argument:

def is_capitalized(string, /):
    return string[0].isupper()

All arguments that precede the / will need to be passed positionally:

# True

# TypeError: is_capitalized() got some
# positional-only arguments passed as ...

Going further

There is a lot that you can do to make your code look neater. I encourage you to add your thoughts on the matter as a comment on this article.

If you want to look at tips from others, here are two good places to get started:

Raymond Hettinger’s PyCon 2013 talk about idiomatic Python code. In it, he goes over many little changes you to your programs that make them nicer. The talk was part of the inspiration for this article. Note, the talk shows code in Python 2. The top comment on the video explains the few changes you’ll need to make for Python 3.

Conor Hoekstra’s PyCon 2020 talk about refactoring Python code. In it, he shows how you can shrink a 60-line file to one-third the size. Note, at 14:51 in the video, Conor turns a try-except into an if-else. I discourage this, since it violates Python’s easier-to-ask-permission-than-forgiveness (EAFP) principle. Learn more about EAFP in this article by Brett Cannon from Microsoft.