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Python powered Fortran preprocessor

Project description

Fypp is a Python powered preprocessor. It can be used for any programming languages but its primary aim is to offer a Fortran preprocessor, which helps to extend Fortran with condititional compiling and template metaprogramming capabilities. Instead of introducing its own expression syntax, it uses Python expressions in its preprocessor directives, offering the consistency and versatility of Python when formulating metaprogramming tasks. It puts strong emphasis on robustness and on neat integration into developing toolchains.

The project is hosted on github.

Detailed DOCUMENTATION is available on

Fypp is released under the BSD 2-clause license.

Main features

  • Definition, evaluation and removal of variables:

    #:if DEBUG > 0
      print *, "Some debug information"
    #:set LOGLEVEL = 2
    print *, "LOGLEVEL: ${LOGLEVEL}$"
    #:del LOGLEVEL
  • Macro definitions and macro calls:

    #:def ASSERT(cond)
      #:if DEBUG > 0
        if (.not. ${cond}$) then
          print *, "Assert failed in file ${_FILE_}$, line ${_LINE_}$"
          error stop
        end if
    #:enddef ASSERT
    ! Invoked via direct call (argument needs no quotation)
    @:ASSERT(size(myArray) > 0)
    ! Invoked as Python expression (argument needs quotation)
    $:ASSERT('size(myArray) > 0')
  • Conditional output:

    program test
    #:if defined('WITH_MPI')
      use mpi
    #:elif defined('WITH_OPENMP')
      use openmp
      use serial
  • Iterated output (e.g. for generating Fortran templates):

    interface myfunc
    #:for dtype in ['real', 'dreal', 'complex', 'dcomplex']
      module procedure myfunc_${dtype}$
    end interface myfunc
  • Inline directives:

    logical, parameter :: hasMpi = #{if defined('MPI')}# .true. #{else}# .false. #{endif}#
  • Insertion of arbitrary Python expressions:

    character(*), parameter :: comp_date = "${time.strftime('%Y-%m-%d')}$"
  • Inclusion of files during preprocessing:

    #:include "macrodefs.fypp"
  • Using Fortran-style continutation lines in preprocessor directives:

    #:if var1 > var2 &
        & or var2 > var4
      print *, "Doing something here"
  • Passing (unquoted) multiline string arguments to callables:

    #! Callable needs only string argument
    #:def DEBUG_CODE(code)
      #:if DEBUG > 0
    #:enddef DEBUG_CODE
    #! Pass code block as first positional argument
    #:block DEBUG_CODE
      if (size(array) > 100) then
        print *, "DEBUG: spuriously large array"
      end if
    #:endblock DEBUG_CODE
    #! Callable needs also non-string argument types
    #:def REPEAT_CODE(code, repeat)
      #:for ind in range(repeat)
    #:enddef REPEAT_CODE
    #! Pass code block as positional argument and 3 as keyword argument "repeat"
    #:block REPEAT_CODE(repeat=3)
    this will be repeated 3 times
    #:endblock REPEAT_CODE
  • Preprocessor comments:

    #! This will not show up in the output
    #! Also the newline characters at the end of the lines will be suppressed
  • Suppressing the preprocessor output in selected regions:

    #! Definitions are read, but no output (e.g. newlines) will be produced
    #:include "macrodefs.fypp"
  • Explicit request for stopping the preprocessor:

    #:if DEBUGLEVEL < 0
      #:stop 'Negative debug level not allowed!'
  • Easy check for macro parameter sanity:

    #:def mymacro(RANK)
      #! Macro only works for RANK 1 and above
      #:assert RANK > 0
    #:enddef mymacro
  • Line numbering directives in output:

    program test
    #:if defined('MPI')
    use mpi

    transformed to

    # 1 "test.fypp" 1
    program test
    # 3 "test.fypp"
    use mpi
    # 5 "test.fypp"

    when variable MPI is defined and Fypp was instructed to generate line markers.

  • Automatic folding of generated lines exceeding line length limit


Fypp needs a working Python 3 interpreter (Python 3.5 or above).

When you install Fypp, you obtain the command line tool fypp and the Python module Latter you can import if you want to access the functionality of Fypp directly from within your Python scripts.

Installing via conda

The last stable release of Fypp can be easily installed as conda package by issuing

conda install -c conda-forge fypp

Installing via pip

You can also use Pythons command line installer pip in order to download the stable release from the Fypp page on PyPI and install it on your system.

If you want to install Fypp into the module system of the active Python 3 interpreter (typically the case when you are using a Python virtual environment), issue

pip3 install fypp

Alternatively, you can install Fypp into the user space (under ~/.local) with

pip3 install --user fypp

Installing via MSYS2 pacman

On Windows you can use the MSYS2 toolchain to install Fypp in a MinGW terminal. To install Fypp use:

pacman -S mingw-w64-x86_64-python-fypp

Make sure the selected architecture is matching your current MinGW terminal. For all supporting MinGW architectures visit check the package index here.

Manual install

For a manual install, you can download the source code of the stable releases from the Fypp project website.

If you wish to obtain the latest development version, clone the projects repository:

git clone

and check out the master branch.

The command line tool is a single stand-alone script. You can run it directly from the source folder


or after copying it from the bin folder to any location listed in your PATH environment variable, by just issuing


The python module can be found in FYP_SOURCE_FOLDER/src.


The Fypp command line tool reads a file, preprocesses it and writes it to another file, so you would typically invoke it like:

fypp source.fpp source.f90

which would process source.fpp and write the result to source.f90. If input and output files are not specified, information is read from stdin and written to stdout.

The behavior of Fypp can be influenced with various command line options. A summary of all command line options can be obtained by:

fypp -h

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