Why does the existance of an output file speed up resolution?

[notatallshaw]

Python 3.10 uv 0.1.31

Create a requitement.in file as such:

click >= 7.0
click-loglevel ~= 0.2
dandi >= 0.24.0
psutil ~= 5.9
pyyaml
selenium

No compile these requirements without cache to an output file, then again with the existing output file, then again without the output file:

$ time uv pip compile requirements.in --exclude-newer 2023-10-01T00:00:00Z --no-cache -o test_out.txt --quiet
real    0m9.926s
user    0m6.522s
sys     0m1.117s

$ time uv pip compile requirements.in --exclude-newer 2023-10-01T00:00:00Z --no-cache -o test_out.txt --quiet
real    0m1.334s
user    0m0.575s
sys     0m0.177s

$ rm -rf test_out.txt

$ time uv pip compile requirements.in --exclude-newer 2023-10-01T00:00:00Z --no-cache -o test_out.txt --quiet
real    0m9.899s
user    0m6.395s
sys     0m1.226s

What's going on here? Why would a file that you write to speed up resolution?

This might not be a bug but it feels wrong, it should be at least documented in the help which currently just says:

  -o, --output-file <OUTPUT_FILE>
          Write the compiled requirements to the given `requirements.txt` file

[charliermarsh]

Because we use the existing resolution in the output file as the starting point for the subsequent resolution, unless you run with the upgrade flag.

[notatallshaw]

Okay, I tested and this seems to be the same behavior as pip-compile:

$ time pip-compile requirements.in --pip-args "--no-cache-dir" --output-file test_out2.txt --quiet --strip-extras
real    0m30.315s
user    0m12.760s
sys     0m0.456s

$ time pip-compile requirements.in --pip-args "--no-cache-dir" --output-file test_out2.txt --quiet --strip-extras
real    0m21.981s
user    0m7.968s
sys     0m0.471s

$ rm test_out2.txt
$ time pip-compile requirements.in --pip-args "--no-cache-dir" --output-file test_out2.txt --quiet --strip-extras
real    0m29.986s
user    0m12.799s
sys     0m0.400s

I even tested that it doesn't upgrade a valid solution, I just find this very unintuitive, as it's called an output file not a sync file, but it matches the tool you are mirroring so ¯\(ツ)/¯.