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Limitations
###########
Design choices
^^^^^^^^^^^^^^
pybind11 strives to be a general solution to binding generation, but it also has
certain limitations:
- pybind11 casts away ``const``-ness in function arguments and return values.
This is in line with the Python language, which has no concept of ``const``
values. This means that some additional care is needed to avoid bugs that
would be caught by the type checker in a traditional C++ program.
- The NumPy interface ``pybind11::array`` greatly simplifies accessing
numerical data from C++ (and vice versa), but it's not a full-blown array
class like ``Eigen::Array`` or ``boost.multi_array``. ``Eigen`` objects are
directly supported, however, with ``pybind11/eigen.h``.
Large but useful features could be implemented in pybind11 but would lead to a
significant increase in complexity. Pybind11 strives to be simple and compact.
Users who require large new features are encouraged to write an extension to
pybind11; see `pybind11_json <https://github.com/pybind/pybind11_json>`_ for an
example.
Known bugs
^^^^^^^^^^
These are issues that hopefully will one day be fixed, but currently are
unsolved. If you know how to help with one of these issues, contributions
are welcome!
- Intel 20.2 is currently having an issue with the test suite.
`#2573 <https://github.com/pybind/pybind11/pull/2573>`_
- Debug mode Python does not support 1-5 tests in the test suite currently.
`#2422 <https://github.com/pybind/pybind11/pull/2422>`_
- PyPy3 7.3.1 and 7.3.2 have issues with several tests on 32-bit Windows.
Known limitations
^^^^^^^^^^^^^^^^^
These are issues that are probably solvable, but have not been fixed yet. A
clean, well written patch would likely be accepted to solve them.
- Type casters are not kept alive recursively.
`#2527 <https://github.com/pybind/pybind11/issues/2527>`_
One consequence is that containers of ``char *`` are currently not supported.
`#2245 <https://github.com/pybind/pybind11/issues/2245>`_
- The ``cpptest`` does not run on Windows with Python 3.8 or newer, due to DLL
loader changes. User code that is correctly installed should not be affected.
`#2560 <https://github.com/pybind/pybind11/issue/2560>`_
Python 3.9.0 warning
^^^^^^^^^^^^^^^^^^^^
Combining older versions of pybind11 (< 2.6.0) with Python on exactly 3.9.0
will trigger undefined behavior that typically manifests as crashes during
interpreter shutdown (but could also destroy your data. **You have been
warned**).
This issue was `fixed in Python <https://github.com/python/cpython/pull/22670>`_.
As a mitigation for this bug, pybind11 2.6.0 or newer includes a workaround
specifically when Python 3.9.0 is detected at runtime, leaking about 50 bytes
of memory when a callback function is garbage collected. For reference, the
pybind11 test suite has about 2,000 such callbacks, but only 49 are garbage
collected before the end-of-process. Wheels (even if built with Python 3.9.0)
will correctly avoid the leak when run in Python 3.9.1, and this does not
affect other 3.X versions.
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