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libpython depends on libstdcxx >=15 but links no C++ runtime

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
65/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
backend, build-system

Research direction

Read the libpython output's build requirements and compare them with the python output's ignore_run_exports_from setting; the issue says the 3.11 through 3.14 branches contain the same line. Check the package recipe and build metadata to confirm libpython's runtime dependencies on Linux and macOS. Done when libpython carries the C++ compiler run-export and environments capped below libstdcxx 15 can solve the stated package cases.

Written by the indexing model from the issue text.

Description

Since the split in #894, every libpython build depends on libstdcxx >=15 on Linux (and libcxx on macOS), but the library links no C++ runtime:

$ objdump -p lib/libpython3.15.so.1.0 | grep NEEDED   # libpython-3.15.0-hdc7f604_100_cp315, linux-64
  NEEDED libm.so.6 libpthread.so.0 libdl.so.2 libutil.so.1 libgcc_s.so.1 libc.so.6 ld-linux-x86-64.so.2
$ otool -L lib/libpython3.15.dylib                    # osx-arm64
  @rpath/libpython3.15.dylib, /usr/lib/libSystem.B.dylib

The requirement is a run-export of {{ compiler('cxx') }} in the libpython output's build requirements. The python output already drops the same run-export with ignore_run_exports_from.

This blocks environments that cap libstdc++. PythonCall.jl (Julia embedding Python, via CondaPkg) caps libstdcxx at the version Julia bundles, which is below 15 on Julia 1.10 and 1.12. Under libstdcxx <15:

  • python + libpython: unsolvable
  • python=3.14.8 or python=3.13.16: unsolvable, since they pin libpython exactly
  • plain python: resolves to an older build from before the split (3.14.6)

From 3.15 on, python no longer depends on libpython, so embedders like PythonCall have to request libpython explicitly, which makes this harder to avoid.

Fixes: #949 (main) and #950 (3.14). The 3.11, 3.12 and 3.13 branches have the same line.

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Forks
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Avg merge
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Merged PRs (30d)
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