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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 <>`_ for an
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 <>`_
- Debug mode Python does not support 1-5 tests in the test suite currently.
`#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 <>`_
One consequence is that containers of ``char *`` are currently not supported.
`#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 <>`_
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
This issue was `fixed in Python <>`_.
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.