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pip install overrides existing build configuration #851
Description
When using meson-python with editable installs, running pip install -e a second time, reusing the build directory, but without repeating the buildtype argument overwrites the existing build configuration.
Reproduction
#!/usr/bin/env bash
mkdir repro
# Create files for meson-python
cat <<EOF > repro/pyproject.toml
[project]
name = "foo"
version = "0.0.1"
[build-system]
build-backend = "mesonpy"
requires = ["meson-python"]
EOF
cat <<EOF > repro/meson.build
project('foo', 'c', version: '0.1.0')
EOF
python -m pip install meson-python
# Create a debug build
pip install --no-build-isolation -e repro -Csetup-args=-Dbuildtype=debug -Cbuild-dir=debug
grep buildtype repro/debug/meson-logs/meson-setup.txt
# buildtype : debug
# If I decide to reuse this build directory,
# and I call `pip install` without `-Csetup-args=-Dbuildtype=debug`
# it reconfigures to have `buildtype=release` instead of reusing the current configuration.
pip install --no-build-isolation -e repro -Cbuild-dir=debug
grep buildtype repro/debug/meson-logs/meson-setup.txt
# buildtype : release
Activity
rgommers commentedon Jun 14, 2026
This is by design; you never have to re-run the editable install command to keep working on code (including compiled code), so if you do and you're changing the build options, you are asking for a new build configuration.
Do you see a problem with that?
This is where the reconfigure happens:
meson-python/mesonpy/__init__.py
Lines 814 to 819 in 4977b7d
Alvaro-Kothe commentedon Jun 14, 2026
The main issue arises when managing multiple build directories for different configurations (e.g.,
debug,release,ASAN). I want to alternate between them without accidentally wiping out existing compiled objects or unintentionally altering the build type.Right now, if I run
pip installand explicitly pass the matchingbuildtypesetup argument, everything works as expected (no reconfigure). However, if I omit that option on a subsequent run,meson-pythondefaults to areleasebuild. This triggers a destructive reconfigure on an activedebugdirectory, wiping out the existing binaries. I then have to abort the build and re-runpip installwith the explicit-Csetup-args=-Dbuildtype=debugflag to restore it.eli-schwartz commentedon Jun 14, 2026
As far as I understand, the problem is when one does not change the build options but uses -Cbuild-dir=debug (this is a big giveaway) to reinstall the editable loader while toggling between different build directories.
Despite this, meson-python blows away the existing build options by passing buildtype as a command line argument even when also passing --reconfigure, which some might call a bit opinionated. :P
This could also be solvable if it was possible to save an editable install and reinstall the editable bits, but I'm not sure that the pypa ecosystem supports this use case generically. If it was just a matter of Meson, I could have suggested using
meson devenvinside each build directory, but that lacks the editable loader, which sort of renders the whole topic moot!rgommers commentedon Jun 15, 2026
Ah, rerunning the install command makes sense if you alternate within the same environment. I've found it's usually better to have one environment per type of build - which you'd need anyway for ASan, because you need to build your dependencies with ASan as well - that works quite effectively, and this reconfigure issue doesn't come up then.
If you do want to alternate within a single environment, why not define some aliases? It's a very long invocation anyway, so something like this would help and avoid the problem of missing an argument:
We didn't do that from the start, but not passing
--reconfiguregives surprising behavior in other circumstances, that more people seemed to run into and get surprised by. In the end there's no single way to make everyone happy and make this fully intuitive, for a user base whose intuition level is both fairly low/inconsistent and shaped by stuffsetuptoolsandpipused to do.eli-schwartz commentedon Jun 15, 2026
Shower thought: maybe you could save and restore any config-settings inside a custom
-Cbuild-dirunder the rationale that custom build directories implies users want persistence.Alvaro-Kothe commentedon Jun 15, 2026
I opened #852 with the idea of not providing any default option if the build directory already exists. From what I've tested, it preserved the original configuration.
I want to point out that every alternative that @rgommers provided in #851 (comment) is valid, but I think that it's more practical to call
pip install -e . --no-build-isolation -Cbuild-dir=<builddir>to be able to alternate between build directories, while also preserving existing configurations.BUG: do not pass default options when reconfiguring
BUG: do not pass default options when reconfiguring