How f-strings Handle Double Curly Braces

Today I was about setting up a test case for parsing a YAML file which is templated with Jinja variables.

Something like this…

f"""
pipeline:
    - test
jobs:
    test:
        package-repositories:
            - type: apt
              url: "https://{{auth}}@example.org"
"""

My test case failed…

My Pydantic model, which uses yaml.safe_load under the hood to load the above configuration, returned the URL, even before the value got replaced, as https://{auth}@example.org.

One pair of the double curly braces was stripped away!

Oh no! Is my complete concept broken? Can’t I use Pydantic with yaml.safe_load and Jinja templates? Are my deadlines in danger?

I tried to reproduce the issue in the REPL - hm, safe_load does not touch the curly braces. What’s going on? Different Python versions? Different PyYAML versions?

f-strings in da house

As I wrote the above string into a temp file in my test fixture, next I checked whether either Path.write_text does nasty things or the test framework.

And… actually it’s Python itself - or more specific f-strings!

Turns out double curly braces are the way to write literal curly braces inside an f-string!

From the official Python documentation:

The parts of the string outside curly braces are treated literally, except that any doubled curly braces ‘{{’ or ‘}}’ are replaced with the corresponding single curly brace.

f-strings in da house#

f-strings in da house