Summary

Fix dependency patching when run across multiple PEP 517 hooks.

Review Request #15028 — Created May 7, 2026 and submitted May 12, 2026, 8:53 p.m.

Information

Owner

david

Repository

buildthings

Branch

master

Bugs

Depends On

Reviewers

Groups

beanbag-misc

People

Description

Since switching to the buildthings backend, running tox in djblets was
crashing with an assertion "Attempted to patch dependencies too late!"

The cause here is that pyproject_api invokes build hooks multiple times
in a single worker process, so the earlier get_requires_for_build_*
hooks instantiate a _ConfigExpander (via Distribution) before
prepare_metadata_for_build_wheel calls
patch_setuptools_package_deps(). The "already initialized" assertion
fired in that case, even though the next _ConfigExpander instance (the
one we actually want to patch) has not been built yet.

This change gets rid of the assertion, and clears the post-funcs list
before appending so deps don't stack across hook calls in the same
worker.

Testing Done


Built a package and put it in my $PIP_FIND_LINKS. Was able to run

tox successfully.
Ran unit tests.

Commits

Summary	ID
Fix dependency patching when run across multiple PEP 517 hooks. Since switching to the buildthings backend, running `tox` in djblets was crashing with an assertion "Attempted to patch dependencies too late!" The cause here is that PEP 517 frontends like pyproject_api keep one worker process across hooks, so the earlier `get_requires_for_build_*` hooks instantiate a `_ConfigExpander` (via `Distribution`) before `prepare_metadata_for_build_wheel` calls `patch_setuptools_package_deps()`. The "already initialized" assertion fired in that case, even though the next `_ConfigExpander` instance (the one we actually want to patch) has not been built yet. This change gets rid of the assertion, and clears the post-funcs list before appending so deps don't stack across hook calls in the same worker. Testing Done: - Built a package and put it in my `$PIP_FIND_LINKS`. Was able to run `tox` successfully. - Ran unit tests.	qmzolnttkkvuotwruqlkmqtmzusrqymw

Summary

Fix dependency patching when run across multiple PEP 517 hooks.

Since switching to the buildthings backend, running `tox` in djblets was crashing with an assertion "Attempted to patch dependencies too late!" The cause here is that PEP 517 frontends like pyproject_api keep one worker process across hooks, so the earlier `get_requires_for_build_*` hooks instantiate a `_ConfigExpander` (via `Distribution`) before `prepare_metadata_for_build_wheel` calls `patch_setuptools_package_deps()`. The "already initialized" assertion fired in that case, even though the next `_ConfigExpander` instance (the one we actually want to patch) has not been built yet. This change gets rid of the assertion, and clears the post-funcs list before appending so deps don't stack across hook calls in the same worker. Testing Done: - Built a package and put it in my `$PIP_FIND_LINKS`. Was able to run `tox` successfully. - Ran unit tests.

qmzolnttkkvuotwruqlkmqtmzusrqymw

Issues

Description	From	Last Updated
Typo: "Unfortanetly"	chipx86	May 12, 2026, 8:52 p.m.
This is where I started deep-diving a couple days ago and got distracted. I looked at all the tools in …	chipx86	May 12, 2026, 8:43 a.m.
This is mostly a duplicate of the docs. We can probably simplify this.	chipx86	May 12, 2026, 8:43 a.m.
Typo: "OTHrewise"	chipx86	May 12, 2026, 8:43 a.m.
If we global this, we can just assign to a new list.	chipx86	May 12, 2026, 8:44 a.m.
Going to bump to 1.1 for the dependency fixes. Same below.	chipx86	May 12, 2026, 8:44 a.m.

flake8 passed.

JSHint passed.

buildthings/setuptools_patches.py (Diff revision 1)

The issue has been resolved. Show all issues

This is where I started deep-diving a couple days ago and got distracted.

I looked at all the tools in this space that I could find, including uv, and every one of them with the exception of tox's own pyproject_api invokes each hook in a separate subprocess. Most use pyproject_hooks, which is the official Python module for this work, and the rest roll their own using the same one-process-per-hook approach.

tox is the odd one out. Let's document that for now, so we can track that.

buildthings/setuptools_patches.py (Diff revision 1)

The issue has been resolved. Show all issues

This is mostly a duplicate of the docs. We can probably simplify this.

buildthings/setuptools_patches.py (Diff revision 1)
The issue has been resolved. Show all issues
```
Typo: "OTHrewise"
```
buildthings/setuptools_patches.py (Diff revision 1)
The issue has been resolved. Show all issues
```
If we global this, we can just assign to a new list.
```
tests/test_setuptools_patches.py (Diff revision 1)
The issue has been resolved. Show all issues
```
Going to bump to 1.1 for the dependency fixes.
Same below.
```

Description:

		Since switching to the buildthings backend, running `tox` in djblets was
		crashing with an assertion "Attempted to patch dependencies too late!"

~		The cause here is that PEP 517 frontends like pyproject_api keep one
~		worker process across hooks, so the earlier `get_requires_for_build_*`
	~	The cause here is that `pyproject_api` invokes build hooks multiple times
	~	in a single worker process, so the earlier `get_requires_for_build_*`
		hooks instantiate a `_ConfigExpander` (via `Distribution`) before
		`prepare_metadata_for_build_wheel` calls
		`patch_setuptools_package_deps()`. The "already initialized" assertion
		fired in that case, even though the next `_ConfigExpander` instance (the
		one we actually want to patch) has not been built yet.

		This change gets rid of the assertion, and clears the post-funcs list
		before appending so deps don't stack across hook calls in the same
		worker.

Commits:

	Summary	ID
	Fix dependency patching when run across multiple PEP 517 hooks. Since switching to the buildthings backend, running `tox` in djblets was crashing with an assertion "Attempted to patch dependencies too late!" The cause here is that PEP 517 frontends like pyproject_api keep one worker process across hooks, so the earlier `get_requires_for_build_*` hooks instantiate a `_ConfigExpander` (via `Distribution`) before `prepare_metadata_for_build_wheel` calls `patch_setuptools_package_deps()`. The "already initialized" assertion fired in that case, even though the next `_ConfigExpander` instance (the one we actually want to patch) has not been built yet. This change gets rid of the assertion, and clears the post-funcs list before appending so deps don't stack across hook calls in the same worker. Testing Done: - Built a package and put it in my `$PIP_FIND_LINKS`. Was able to run `tox` successfully. - Ran unit tests.	qmzolnttkkvuotwruqlkmqtmzusrqymw
	Fix dependency patching when run across multiple PEP 517 hooks. Since switching to the buildthings backend, running `tox` in djblets was crashing with an assertion "Attempted to patch dependencies too late!" The cause here is that PEP 517 frontends like pyproject_api keep one worker process across hooks, so the earlier `get_requires_for_build_*` hooks instantiate a `_ConfigExpander` (via `Distribution`) before `prepare_metadata_for_build_wheel` calls `patch_setuptools_package_deps()`. The "already initialized" assertion fired in that case, even though the next `_ConfigExpander` instance (the one we actually want to patch) has not been built yet. This change gets rid of the assertion, and clears the post-funcs list before appending so deps don't stack across hook calls in the same worker. Testing Done: - Built a package and put it in my `$PIP_FIND_LINKS`. Was able to run `tox` successfully. - Ran unit tests.	qmzolnttkkvuotwruqlkmqtmzusrqymw

Diff:

Revision 2 (+318 -30)

Show changes

	dev-requirements.txt
	buildthings/setuptools_patches.py
	tests/test_setuptools_patches.py

Checks run (2 succeeded)

flake8 passed.

JSHint passed.

Ship it!

buildthings/setuptools_patches.py (Diff revisions 1 - 2)
The issue has been resolved. Show all issues
```
Typo: "Unfortanetly"
```
1. david
  
  May 12, 2026, 8:52 p.m.
  Wow, that's quite a typo. Wonder how I did that.

Status:: Completed
Change Summary:: Pushed to master (2125935)