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100 lines
3.9 KiB
ReStructuredText
100 lines
3.9 KiB
ReStructuredText
Frequently asked questions
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##########################
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(under construction)
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ImportError: dynamic module does not define init function
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=========================================================
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1. Make sure that the name specified in ``pybind::module`` and
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``PYBIND11_PLUGIN`` is consistent and identical to the filename of the
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extension library. The latter should not contain any extra prefixes (e.g.
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``test.so`` instead of ``libtest.so``).
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2. If the above did not fix your issue, then you are likely using an
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incompatible version of Python (for instance, the extension library was
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compiled against Python 2, while the interpreter is running on top of some
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version of Python 3)
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Limitations involving reference arguments
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=========================================
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In C++, it's fairly common to pass arguments using mutable references or
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mutable pointers, which allows both read and write access to the value
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supplied by the caller. This is sometimes done for efficiency reasons, or to
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realize functions that have multiple return values. Here are two very basic
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examples:
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.. code-block:: cpp
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void increment(int &i) { i++; }
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void increment_ptr(int *i) { (*i)++; }
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In Python, all arguments are passed by reference, so there is no general
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issue in binding such code from Python.
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However, certain basic Python types (like ``str``, ``int``, ``bool``,
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``float``, etc.) are **immutable**. This means that the following attempt
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to port the function to Python doesn't have the same effect on the value
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provided by the caller -- in fact, it does nothing at all.
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.. code-block:: python
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def increment(i):
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i += 1 # nope..
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pybind11 is also affected by such language-level conventions, which means that
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binding ``increment`` or ``increment_ptr`` will also create Python functions
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that don't modify their arguments.
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Although inconvenient, one workaround is to encapsulate the immutable types in
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a custom type that does allow modifications.
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An other alternative involves binding a small wrapper lambda function that
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returns a tuple with all output arguments (see the remainder of the
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documentation for examples on binding lambda functions). An example:
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.. code-block:: cpp
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int foo(int &i) { i++; return 123; }
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and the binding code
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.. code-block:: cpp
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m.def("foo", [](int i) { int rv = foo(i); return std::make_tuple(rv, i); });
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CMake doesn't detect the right Python version, or it finds mismatched interpreter and library versions
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======================================================================================================
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The Python detection logic of CMake is flawed and can sometimes fail to find
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the desired Python version, or it chooses mismatched interpreter and library
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versions. A longer discussion is available on the pybind11 issue tracker
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[#f1]_, though this is ultimately not a pybind11 issue.
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To force the build system to choose a particular version, delete CMakeCache.txt
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and then invoke CMake as follows:
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.. code-block:: bash
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cmake -DPYTHON_EXECUTABLE:FILEPATH=<...> \
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-DPYTHON_LIBRARY:FILEPATH=<...> \
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-DPYTHON_INCLUDE_DIR:PATH=<...> .
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.. [#f1] http://github.com/pybind/pybind11/issues/99
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Working with ancient Visual Studio 2009 builds on Windows
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=========================================================
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The official Windows distributions of Python are compiled using truly
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ancient versions of Visual Studio that lack good C++11 support. Some users
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implicitly assume that it would be impossible to load a plugin built with
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Visual Studio 2015 into a Python distribution that was compiled using Visual
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Studio 2009. However, no such issue exists: it's perfectly legitimate to
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interface DLLs that are built with different compilers and/or C libraries.
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Common gotchas to watch out for involve not ``free()``-ing memory region
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that that were ``malloc()``-ed in another shared library, using data
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structures with incompatible ABIs, and so on. pybind11 is very careful not
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to make these types of mistakes.
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