A minimal example of calling a python function via a callback passed to a fortran subroutine.

Makefile

.PHONY: venv

VENV_PATH=venv
TIMESTAMP_FILE=$(VENV_PATH)/.f2pytest-creation-timestamp

test: venv test.f90
	. $(VENV_PATH)/bin/activate && \
	    python -m numpy.f2py -c test.f90 -m test

venv: $(TIMESTAMP_FILE)

$(TIMESTAMP_FILE):
	virtualenv -p python3 $(VENV_PATH)
	. $(VENV_PATH)/bin/activate && pip install numpy scipy
	touch $(TIMESTAMP_FILE)

test-callback.py

import test

def func(x):
    print('x is {}'.format(x))
    return x * 2

test.test(func)

test.f90

subroutine test(py_func)
integer x,y,py_func
external py_func
x = 42
y = py_func(x)
end subroutine