F2PY and build systems#

In this section we will cover the various popular build systems and their usage with f2py.

Changed in version NumPy: 1.26.x

The default build system for f2py has traditionally been through the enhanced numpy.distutils module. This module is based on distutils which was removed in Python 3.12.0 in October 2023. Like the rest of NumPy and SciPy, f2py uses meson now, see Status of numpy.distutils and migration advice for some more details.

All changes to f2py are tested on SciPy, so their CI configuration is always supported.

Note

See 1 Migrating to meson for migration information.

Basic concepts#

Building an extension module which includes Python and Fortran consists of:

  • Fortran source(s)

  • One or more generated files from f2py

    • A C wrapper file is always created

    • Code with modules require an additional .f90 wrapper

    • Code with functions generate an additional .f wrapper

  • fortranobject.{c,h}

    • Distributed with numpy

    • Can be queried via python -c "import numpy.f2py; print(numpy.f2py.get_include())"

  • NumPy headers

    • Can be queried via python -c "import numpy; print(numpy.get_include())"

  • Python libraries and development headers

Broadly speaking there are three cases which arise when considering the outputs of f2py:

Fortran 77 programs

  • Input file blah.f

  • Generates

    • blahmodule.c

    • blah-f2pywrappers.f

When no COMMON blocks are present only a C wrapper file is generated. Wrappers are also generated to rewrite assumed shape arrays as automatic arrays.

Fortran 90 programs

  • Input file blah.f90

  • Generates:

    • blahmodule.c

    • blah-f2pywrappers.f

    • blah-f2pywrappers2.f90

The f90 wrapper is used to handle code which is subdivided into modules. The f wrapper makes subroutines for functions. It rewrites assumed shape arrays as automatic arrays.

Signature files

  • Input file blah.pyf

  • Generates:

    • blahmodule.c

    • blah-f2pywrappers2.f90 (occasionally)

    • blah-f2pywrappers.f (occasionally)

Signature files .pyf do not signal their language standard via the file extension, they may generate the F90 and F77 specific wrappers depending on their contents; which shifts the burden of checking for generated files onto the build system.

Changed in version NumPy: 1.22.4

f2py will deterministically generate wrapper files based on the input file Fortran standard (F77 or greater). --skip-empty-wrappers can be passed to f2py to restore the previous behaviour of only generating wrappers when needed by the input .

In theory keeping the above requirements in hand, any build system can be adapted to generate f2py extension modules. Here we will cover a subset of the more popular systems.

Note

make has no place in a modern multi-language setup, and so is not discussed further.

Build systems#

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