Only enumerate extension entry points when required

[akx]

On my machine, this saves 33 milliseconds off import time for the library, which could be significant for e.g. CLI applications.

(master) $ hyperfine 'python -m markdown' Benchmark 1: python -m markdown Time (mean ± σ): 113.1 ms ± 11.5 ms [User: 84.8 ms, System: 19.2 ms] Range (min … max): 100.1 ms … 151.7 ms 19 runs (late-extensions) $ hyperfine 'python -m markdown' Benchmark 1: python -m markdown Time (mean ± σ): 80.7 ms ± 4.7 ms [User: 61.0 ms, System: 13.1 ms] Range (min … max): 73.9 ms … 94.8 ms 30 runs 

There is also a small benefit to the overall test suite run time:

(master) $ hyperfine 'python -m unittest discover tests' Benchmark 1: python -m unittest discover tests Time (mean ± σ): 1.177 s ± 0.074 s [User: 1.073 s, System: 0.051 s] Range (min … max): 1.075 s … 1.250 s 10 runs late-extensions) $ hyperfine 'python -m unittest discover tests' Benchmark 1: python -m unittest discover tests Time (mean ± σ): 1.085 s ± 0.029 s [User: 0.990 s, System: 0.046 s] Range (min … max): 1.034 s … 1.127 s 10 runs 

Going forward, one could consider making the importlib.metadata backport an optional dependency – it's only required if one wants to load an extension by name.

[waylan]

I will merge this if a note is added to the release notes.