Week 1: F2PY Frontend

F2PY frontend: The Old and the New

We will look at the current F2PY frontend, and understand the ongoing re-implementation of it using the argparse python library.


F2PY is a command line tool that provides easy connection between Fortran languages and Python. It facilitates creating Python C/API extension modules that can be called and imported in Python. It gives you the speed of Fortran and the flexibility of Python.

The frontend of a command line tool

A command line works by parsing the input on the command line, and then calling correct functions based on the input. The input on the command line is generally a list of positional arguments (like files, folders, keys or data) and flags (like --help or --version). For example, if you type

git --version

then the command line tool will call the git program, and pass the --version flag to it as an argument. Internally, an array argv containing the argument is created and passed to git program. In this case argv will look like this: ['--version'].

git add app.py

then the command line tool will call the git program, and pass the add and app.py to it as an argument. Internally, git will recognize add as an option, and app.py as a positional argument to add option. The argv array passed to git program will look like this: ['add', 'app.py'].

F2PY’s frontend

Suppose you have a fortran source file fib.f:

      REAL*8 A(N)
      DO I=1,N
         IF (I.EQ.1) THEN
            A(I) = 0.0D0
         ELSEIF (I.EQ.2) THEN
            A(I) = 1.0D0
            A(I) = A(I-1) + A(I-2)

User can pass fortran source files and functions to F2PY and perform three major tasks:

  1. Generate the signature file which can be used to generate the C/API extension module, done by -h flag.
f2py fib.f -h fibsign.pyf

It will produce a header file fibsign.pyf which the user can edit to optimize wrapper functions, to make them “smarter” and more “pythonic”.

  1. Generate the C/API extension module, either from a signature file or from a fortran source file, done by -m flag.
f2py fib.f -m fib
# OR
f2py fibsign.pyf -m fib

The first one will use create C wrapper from fib.f directly, f2py will intelligently sense input and output arguments and create the wrapper. The second one will use the signature file to create the modified wrapper.

  1. Compile the C/API file and fortran source file to produce a shared library that can be imported in Python, done by -c flag.
f2py fib.f -c fib.c -m fib

Here -m flag provides the name of the module, without which the C file generate will be named like “untitled.cpython-310-x86_64-linux-gnu.so”, and you would have to import it using import untitled (which is pretty funny).

The current F2PY frontend

The current F2PY frontend can be found in numpy/f2py/f2py2e.py. This handwritten parser was written by Dr. Pearu Peterson way before argparse was introduced. It is not easy to understand for a beginner, so I will try to explain it in a few steps.

There are two important functions that run the entire program - run_main() and run_compile().

Argument Activates function
-c run_compile()
Everything else run_main()


As I said above, f2py performs three major tasks. This function handles the first two tasks - generating the signature file and generating the C/API extension module. It is called by the f2py if the argv array doesn’t contain the -c flag. It uses scaninputline() to parse the argv array to get fortran source files and functions, and set a lot of internal variables accordingly (visible in this line).

run_compile then calls crackfortran.py using callcrackfortran() function. If you have studied about compiler, you might know about abstract syntax trees (AST). An AST is a tree structure that represents the source code. Here, crackfortran generates a similar struture out of the fortran source code, which is then used by buildmodules to either generate the signature file or the C wrapper.


This is a really tricky function to understand. It is called by the f2py if the argv array contains the -c flag. It parses the command line again without the use of scaninputline(), basically filtering out all the possible flags and options, and then searching for the fortran, .pyf, or object files. It then passes the source files and related flags to numpy.disutils where C wrapper is generated and shared library is compiled and linked to produce a Python extension module.

This function will receive a major haul while modernising F2PY. All the flag filtering is needs to be done by another function, which should be common to both run_compile() and run_main(). This function also uses regex to parse the command line which is hard to understand. It is essentially a bohemoth that does the entire job from parsing the command line to creating extension module in a single shot.

The new F2PY frontend

Argument Activates parser
Source files and functions main parser
-c build helper parser
-h, -m Wrapper generation parser

The NumPy team is aiming to make F2PY to more user and developer friendly. F2PY will be soon shifting to a modern argparse based CLI. An ongoing implementation can be found in Rohit Goswami’s fork (the core function to handle flags is remaining). The new frontend will leverage subparser functionality of argparse to handle the three major tasks.

The main parser

The main parser will accept fortran source files and functions to generate C wrapper for and general flags related to module name (-m), documentation generation(--rest-doc), incuding other header files (--include-paths), verbosity (--verbose, --quiet) etc.

The build helper subparser will activated by -c flag to generate the shared object file which can be imported in Python. It will handle all the build related options like fortran compiler and its flags (--fcompiler, --f77exec, f77flags), architecture optimization (--arch), linking (--link-<name>)etc.

The wrapper generation subparser will activated by the -h flag to generate the signature file and -m to generate the wrapper file.

Why do we need a new argparse implementation?

One may raise a question about why we need a new argparse implementation. If the CLI has been working fine for more than a decade, why not leave it as it is? I am still a newbie to f2py and fortran, but there are a lot of ways we can improve F2PY, like adding the derived types support, improving syntax tree mechanism. The current development team will pass on F2PY to new developers, and it will be worthwhile to modernise and make its functionalities granular. A better codebase and documentation will facilitate easier development and maintenance. Making it more modular will increase flexibility and will allow other programs to use specific functionalities of F2PY exclusively (like how SciPy uses --build-dir flag to generate shared object file from its pyf files). It will also be much easier to enhace the functionality of the CLI. A developer could just go add a new flag to the appropriate subparser and link to the handling function. Modernising F2PY will be a effortful task, given the features we have to develop keeping backwards compatibility. However it is essential for the future development and maintainence of this incredible tool.