Tools II: Code Formatters


Teaching: 3 min
Exercises: 2 min
  • How to format code with no effort on the part of the coder?

  • Know how to install and use a code formatter

Why does formatting matter?

Rules to choose a code formatter

  1. Choose one
  2. Stick with it

We chose black because it has very few options with which to fiddle.

Formatting example

Using Visual Studio Code:

  1. Put the following into a file and save it. If you are prompted to install the Python extension then be sure to do so.

    x = {  'a':37,'b':42,
    y = 'hello '+       'world'
    class foo  (     object  ):
       def f    (self   ):
           return       y **2
       def g(self, x :int,
           y : int=42
           ) -> int:
           return x--y
    def f  (   a ) :
       return      37+-a[42-a :  y*3]
  2. Ensure that you have activated your “course” conda environment using the selector in the bottom panel of VS Code
  3. Open Settings
    • macOS via ⌘ + , or menus: Code > Preferences > Settings
    • Windows/Linux via Ctrl + , or menus: File > Preferences > Settings
  4. Search for “python formatting provider” and choose “black”
  5. Search for “format on save” and check the box to enable
  6. Save the file again: it should be reformatted automagically
  7. Now paste the code again but before saving delete a ‘:’ somewhere. When saving, the code will likely not format. It is syntactically invalid. The formatter cannot make sense of the code and thus can’t format it.


After saving, the code should be automatically formatted to:

x = {"a": 37, "b": 42, "c": 927}
y = "hello " + "world"

class foo(object):
    def f(self):
        return y ** 2

    def g(self, x: int, y: int = 42) -> int:
        return x - -y

Ah! much better!

Still, the sharp-eyed user will notice at least one issue with this code. Formatting code does not make it less buggy!

Key Points

  • Code formatting means how the code is typeset

  • It influences how easily the code is read

  • It has no impact on how the code runs

  • Almost all editors and IDEs have some means to set up an automatic formatter

  • 5 minutes to set up the formatter is redeemed across the time of the project i.e. the cost is close to nothing