AFAICT pybuild is broken on forky if you use DEB_BUILD_OPTIONS=terse.

This works on trixie but fails on forky (it builds twice, once with terse and once without, to prove terse is the problem):

    mmdebstrap trixie /dev/null --include=build-essential,debhelper,dh-python,flit,pybuild-plugin-pyproject,python3-all,python3-setuptools 'deb https://deb.debian.org/debian trixie main' 'deb-src https://deb.debian.org/debian trixie main' --customize-hook='set -x; chroot $1 apt-get build-dep python-idna; DEB_BUILD_OPTIONS=terse chroot $1 apt-get --build source python-idna; chroot $1 apt-get --build source python-idna'

    mmdebstrap forky /dev/null --include=build-essential,debhelper,dh-python,flit,pybuild-plugin-pyproject,python3-all,python3-setuptools 'deb https://deb.debian.org/debian forky main' 'deb-src https://deb.debian.org/debian forky main' --customize-hook='set -x; chroot $1 apt-get build-dep python-idna; DEB_BUILD_OPTIONS=terse chroot $1 apt-get --build source python-idna; chroot $1 apt-get --build source python-idna'

The problem is here:

     debian/rules binary
    dh binary --with python3 --buildsystem=pybuild
    I: pybuild plugin_pyproject:142: Building wheel for python3.14 with "build" module
    I: pybuild base:385: python3.14 -m build --skip-dependency-check --no-isolation --wheel --outdir /python-idna-3.11/.pybuild/cpython3_3.14_idna
    E: pybuild pybuild:485: build: plugin pyproject failed with:
    I: pybuild plugin_pyproject:142: Building wheel for python3.13 with "build" module
    I: pybuild base:385: python3.13 -m build --skip-dependency-check --no-isolation --wheel --outdir /python-idna-3.11/.pybuild/cpython3_3.13_idna
    E: pybuild pybuild:485: build: plugin pyproject failed with:
    dh_auto_build: error: pybuild --build -i python{version} -p "3.14 3.13" --quiet --parallel=4 returned exit code 13
    make: *** [debian/rules:6: binary] Error 13
    dpkg-buildpackage: error: debian/rules binary subprocess failed with exit status 2
    W: Download is performed unsandboxed as root as file 'python-idna_3.11-1.dsc' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
    E: Build command 'cd python-idna-3.11 && dpkg-buildpackage -b -uc' failed.

A full transcript is attached.

I encountered this with my own in-house package.
I picked python-idna at random as another flit-based package.
https://codesearch.debian.net/search?q=flit_core

Popping pybuild into a debugger and stepping through, the assertion in
the middle of this block fails:

/--- dhpython/tools.py:262 [execute()]

     with ExitStack() as stack:
         output: TextIO | int | None
         if log_output is False:
             output = None
         elif log_output is None:
             output = PIPE
         elif log_output:
             if isinstance(log_output, str):
                 log_output = stack.enter_context(
                     open(log_output, "a", encoding="utf-8")
                 )
             assert isinstance(log_output, TextIO)
             log_output.write(f"\n# command executed on
{datetime.now().isoformat()}")
             log_output.write(f"\n$ {command}\n")
             log_output.flush()
             output = log_output

---/

log_output is initially a str:

(Pdb) p log_output
'/build/python-idna-W7e44C/python-idna-3.11/.pybuild/cpython3_3.14/build_wheel_cmd.log'


but its rebirth as a TextIO does not work as intended:

 > /usr/share/dh-python/dhpython/tools.py(291)execute()
-> assert isinstance(log_output, TextIO)
(Pdb)
AssertionError
 > /usr/share/dh-python/dhpython/tools.py(291)execute()
-> assert isinstance(log_output, TextIO)
(Pdb) p log_output
<_io.TextIOWrapper
name='/build/python-idna-W7e44C/python-idna-3.11/.pybuild/cpython3_3.14/build_wheel_cmd.log'
mode='a' encoding='utf-8'>


it's actually a TextIOWrapper; changing the assertion to

	assert isinstance(log_output, io.TextIOBase)

is enough to get it working. I guess this is a case where the typehint
TextIO is not quite enough for the isinstance check.

The failed assertion should probably generate some more useful noise too
(perhaps raising a TypeError or ValueError if the log_output is neither
the desired type nor coercible into a usable type?

regards
Stuart

Hello,

Bug #1135722 in dh-python reported by you has been fixed in the
Git repository and is awaiting an upload. You can see the commit
message below and you can check the diff of the fix at:

https://salsa.debian.org/python-team/tools/dh-python/-/commit/1b4dab511830bf798aead0627256235305116db5
------------------------------------------------------------------------
Fix crash with DEB_BUILD_OPTIONS=terse

Closes: #1135722
------------------------------------------------------------------------

(this message was generated automatically)
-- 
Greetings

https://bugs.debian.org/1135722

We believe that the bug you reported is fixed in the latest version of
dh-python, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1135722@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Stefano Rivera <stefanor@debian.org> (supplier of updated dh-python package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Sun, 24 May 2026 18:48:39 -0400
Source: dh-python
Architecture: source
Version: 7.20260524
Distribution: unstable
Urgency: medium
Maintainer: Piotr Ożarowski <piotr@debian.org>
Changed-By: Stefano Rivera <stefanor@debian.org>
Closes: 1133706 1135722 1137486
Changes:
 dh-python (7.20260524) unstable; urgency=medium
 .
   * Fix crash in "pybuild --interpreter python3.x" (Closes: #1137486)
   * Fix crash with DEB_BUILD_OPTIONS=terse (Closes: #1135722, LP: #2150227)
   * Don't require substvars with debhelper compat level >= 14 (Closes: #1133706)
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