1Limitations
2###########
3
4Design choices
5^^^^^^^^^^^^^^
6
7pybind11 strives to be a general solution to binding generation, but it also has
8certain limitations:
9
10- pybind11 casts away ``const``-ness in function arguments and return values.
11  This is in line with the Python language, which has no concept of ``const``
12  values. This means that some additional care is needed to avoid bugs that
13  would be caught by the type checker in a traditional C++ program.
14
15- The NumPy interface ``pybind11::array`` greatly simplifies accessing
16  numerical data from C++ (and vice versa), but it's not a full-blown array
17  class like ``Eigen::Array`` or ``boost.multi_array``. ``Eigen`` objects are
18  directly supported, however, with ``pybind11/eigen.h``.
19
20Large but useful features could be implemented in pybind11 but would lead to a
21significant increase in complexity. Pybind11 strives to be simple and compact.
22Users who require large new features are encouraged to write an extension to
23pybind11; see `pybind11_json <https://github.com/pybind/pybind11_json>`_ for an
24example.
25
26
27Known bugs
28^^^^^^^^^^
29
30These are issues that hopefully will one day be fixed, but currently are
31unsolved. If you know how to help with one of these issues, contributions
32are welcome!
33
34- Intel 20.2 is currently having an issue with the test suite.
35  `#2573 <https://github.com/pybind/pybind11/pull/2573>`_
36
37- Debug mode Python does not support 1-5 tests in the test suite currently.
38  `#2422 <https://github.com/pybind/pybind11/pull/2422>`_
39
40- PyPy3 7.3.1 and 7.3.2 have issues with several tests on 32-bit Windows.
41
42Known limitations
43^^^^^^^^^^^^^^^^^
44
45These are issues that are probably solvable, but have not been fixed yet. A
46clean, well written patch would likely be accepted to solve them.
47
48- Type casters are not kept alive recursively.
49  `#2527 <https://github.com/pybind/pybind11/issues/2527>`_
50  One consequence is that containers of ``char *`` are currently not supported.
51  `#2245 <https://github.com/pybind/pybind11/issues/2245>`_
52
53- The ``cpptest`` does not run on Windows with Python 3.8 or newer, due to DLL
54  loader changes. User code that is correctly installed should not be affected.
55  `#2560 <https://github.com/pybind/pybind11/issue/2560>`_
56
57Python 3.9.0 warning
58^^^^^^^^^^^^^^^^^^^^
59
60Combining older versions of pybind11 (< 2.6.0) with Python on 3.9.0 will
61trigger undefined behavior that typically manifests as crashes during
62interpreter shutdown (but could also destroy your data. **You have been
63warned**).
64
65This issue has been
66`fixed in Python <https://github.com/python/cpython/pull/22670>`_.  As a
67mitigation until 3.9.1 is released and commonly used, pybind11 (2.6.0 or newer)
68includes a temporary workaround specifically when Python 3.9.0 is detected at
69runtime, leaking about 50 bytes of memory when a callback function is garbage
70collected. For reference; the pybind11 test suite has about 2,000 such
71callbacks, but only 49 are garbage collected before the end-of-process. Wheels
72built with Python 3.9.0 will correctly avoid the leak when run in Python 3.9.1.
73