The Python 3.1 problem

Table of Contents

Or, a variation on the Norway problem #

Short version: put quotes around version numbers in YAML.

The Norway problem is when you put this in YAML:

countries:
  - GB
  - IE
  - FR
  - DE
  - NO

But get this out:

>>> import yaml
>>> with open("countries.yml") as f:
...     yaml.safe_load(f)
...
{'countries': ['GB', 'IE', 'FR', 'DE', False]}

😱

Use quotes:

countries:
  - "GB"
  - "IE"
  - "FR"
  - "DE"
  - "NO"

>>> with open("countries.yml") as f:
...     yaml.safe_load(f)
...
{'countries': ['GB', 'IE', 'FR', 'DE', 'NO']}

🇳🇴 ✅

A similar problem will affect the Python community in October 2021, when Python 3.10 comes out.

Why?

When 3.10 is added to YAML, for example in CI test matrix config, it’s interpreted as a float. This:

python-version: [3.6, 3.7, 3.8, 3.9, 3.10, pypy3]

Turns into this:

>>> import yaml
>>> with open("versions.yml") as f:
...     yaml.safe_load(f)
...
{'python-version': [3.6, 3.7, 3.8, 3.9, 3.1, 'pypy3']}

CI failed! It’s not 2009! Python 3.1 not found!

😱

Relatedly, 3.10-dev without quotes works because it’s interpreted as a string. But when deleting -dev, 3.10 is interpreted as a float.

Version numbers are strings, not floats. Use quotes:

python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "pypy3"]

>>> import yaml
>>> with open("versions.yml") as f:
...     yaml.safe_load(f)
...
{'python-version': ['3.6', '3.7', '3.8', '3.9', '3.10', 'pypy3']}

🐍 ✅