Prevent circular import of matrix space

[soehms]

The issue occurs randomly when testing ticket #34185 on a system with the optional package meataxe being present. For example

sage -tp --hide=meataxe src/sage/knots/
Traceback (most recent call last):
  File "/home/sebastian/devel/sage/src/bin/sage-runtests", line 157, in <module>
    err = DC.run()
  File "/home/sebastian/devel/sage/local/var/lib/sage/venv-python3.8/lib/python3.8/site-packages/sage/doctest/control.py", line 1439, in run
    if f.is_present():
  File "/home/sebastian/devel/sage/local/var/lib/sage/venv-python3.8/lib/python3.8/site-packages/sage/features/__init__.py", line 182, in is_present
    res = self._is_present()
  File "/home/sebastian/devel/sage/local/var/lib/sage/venv-python3.8/lib/python3.8/site-packages/sage/features/join_feature.py", line 63, in _is_present
    test = f._is_present()
  File "/home/sebastian/devel/sage/local/var/lib/sage/venv-python3.8/lib/python3.8/site-packages/sage/features/__init__.py", line 950, in _is_present
    importlib.import_module(self.name)
  File "/usr/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1174, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "sage/matrix/matrix_gfpn_dense.pyx", line 1, in init sage.matrix.matrix_gfpn_dense (build/cythonized/sage/matrix/matrix_gfpn_dense.c:21852)
  File "sage/matrix/matrix_dense.pyx", line 1, in init sage.matrix.matrix_dense (build/cythonized/sage/matrix/matrix_dense.c:7897)
  File "sage/matrix/matrix2.pyx", line 1, in init sage.matrix.matrix2 (build/cythonized/sage/matrix/matrix2.c:127958)
  File "sage/matrix/matrix1.pyx", line 1, in init sage.matrix.matrix1 (build/cythonized/sage/matrix/matrix1.c:25542)
  File "sage/matrix/matrix0.pyx", line 28, in init sage.matrix.matrix0 (build/cythonized/sage/matrix/matrix0.c:45663)
  File "/home/sebastian/devel/sage/local/var/lib/sage/venv-python3.8/lib/python3.8/site-packages/sage/modules/free_module.py", line 182, in <module>
    import sage.matrix.matrix_space
  File "/home/sebastian/devel/sage/local/var/lib/sage/venv-python3.8/lib/python3.8/site-packages/sage/matrix/matrix_space.py", line 40, in <module>
    from . import matrix_generic_dense
  File "sage/matrix/matrix_generic_dense.pyx", line 1, in init sage.matrix.matrix_generic_dense (build/cythonized/sage/matrix/matrix_generic_dense.c:6637)
AttributeError: partially initialized module 'sage.matrix.matrix_dense' has no attribute 'Matrix_dense' (most likely due to a circular import)

Of course this also occurs (randomly) with option --hide=all or --hide=optional.

Part of Meta-ticket #33580: make all modules importable without sage.all

Component: doctest framework

Keywords: matrix, space, circular, import

Author: Sebastian Oehms

Branch/Commit: 53d8d11

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/34283

[soehms]

Branch: u/soehms/circular_import_matrix_space_34283

[soehms]

Commit: d4cf504

[soehms]

Author: Sebastian Oehms

[soehms]

Changed keywords from none to matrix, space, circular, import

[soehms]

New commits:

d4cf504

34283: initial

[mkoeppe]

comment:3

I would suggest to move the imports even closer to their uses, to match the other imports done in this function

[sagetrac-git]

Changed commit from d4cf504 to 53d8d11

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

53d8d11

34283: fix according to review

[soehms]

comment:6

Replying to @mkoeppe:

I would suggest to move the imports even closer to their uses, to match the other imports done in this function

Done!

[mkoeppe]

Reviewer: Matthias Koeppe

[soehms]

comment:8

Thanks!

[vbraun]

Changed branch from u/soehms/circular_import_matrix_space_34283 to 53d8d11