Using python-ctypes to interface fortran with python

Question

Experience: fortran for about 3 months python - intermediate : never used the ctypes module in python before this I was looking for a way to use the fortran code for my doctoral work in python - subsequently using the computation for visualizations on-the-fly for visualizations using matplotlib. THIS POST helped (this tells that fortran code can be used/invoked in python using the ctypes module - and given that the fortran functions have alternate names bound to them - this much makes sense to me logically although i do not know how this works in detail. But we DO choose our battles wisely!). Then this SO post deals with calling fortran functions from python as well. The next logical step was to look up the documentation for the python module ctypes. This talks about how the shared library can be accessed using python at an API level. I had all the pieces to make a minimal working example, which another answer has already done. But i wanted to look at the output mechanism and mathemat…

Accepted Answer

In Fortran, arguments are passed by reference. Fortran character arrays aren't null terminated; the length is passed by value as an implicit `long int` argument. Also, Python's `float` type is a `double`, so you may want to use Fortran `real(8)` for consistency.

test.f90:

```
function prnt(s)          ! byref(s), byval(length) [long int, implicit]
    character(len=*):: s  ! variable length input
    logical :: prnt
    write(*, "(A)") s     ! formatted, to remove initial space
    prnt = .true.
end function prnt

function sin_2(r)         ! byref(r)
    real:: r, sin_2       ! float; use real(8) for double
    sin_2 = sin(r)**2
end function sin_2
```

Remember to set ctypes `argtypes` for functions, and `restype` where appropriate. In this case `sin_2` takes a float pointer and returns a float.

ctypes example:

```
>>> from ctypes import *
>>> test = CDLL('./test.so')

>>> test.prnt_.argtypes = [c_char_p, c_long]
>>> test.sin_2_.argtypes = [POINTER(c_float)]
>>> test.sin_2_.restype = c_float

>>> s = 'Mary had a little lamb'
>>> test.prnt_(s, len(s))
Mary had a little lamb
1

>>> x = c_float(4.56)
>>> test.sin_2_(byref(x))
0.9769567847251892
```

Using python-ctypes to interface fortran with python

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