combining python with fortran, trouble with tutorial

Question

I'm following this tutorial http://www.sam.math.ethz.ch/~raoulb/teaching/PythonTutorial/combining.html

I use the same code

      program hwtest
      real*8 r1, r2 ,s
      r1 = 1.0
      r2 = 0.0
      s = hw1(r1, r2)
      write(*,*) 'hw1, result:',s
      write(*,*) 'hw2, result:'
      call hw2(r1, r2)
      call hw3(r1, r2, s)
      write(*,*) 'hw3, result:', s
      end

      real*8 function hw1(r1, r2)
      real*8 r1, r2
      hw1 = sin(r1 + r2)
      return
      end

      subroutine hw2(r1, r2)
      real*8 r1, r2, s
      s = sin(r1 + r2)
      write(*,1000) 'Hello, World! sin(',r1+r2,')=',s
 1000 format(A,F6.3,A,F8.6)
      return
      end

C     special version of hw1 where the result is
C     returned as an argument:

      subroutine hw3_v1(r1, r2, s)
      real*8 r1, r2, s
      s = sin(r1 + r2)
      return
      end

C     F2py treats s as input arg. in hw3_v1; fix this:

      subroutine hw3(r1, r2, s)
      real*8 r1, r2, s
Cf2py intent(out) s
      s = sin(r1 + r2)
      return
      end

C     test case sensitivity:

      subroutine Hw4(R1, R2, s)
      real*8 R1, r2, S
Cf2py intent(out) s
      s = SIN(r1 + r2)
      ReTuRn
      end

C end of F77 file

but when I use the command

f2py -m hw -c ../hw.f

I get the following error:

gfortran:f77: ../hw.f
../hw.f:5.13:

      s = hw1(r1, r2)                                                   
             1
Error: Return type mismatch of function 'hw1' at (1) (REAL(4)/REAL(8))
../hw.f:5.13:

      s = hw1(r1, r2)                                                   
             1
Error: Return type mismatch of function 'hw1' at (1) (REAL(4)/REAL(8))
error: Command "/usr/bin/gfortran -Wall -ffixed-form -fno-second-underscore -fPIC -O3 -funroll-loops -I/tmp/tmpRGHzqj/src.linux-x86_64-2.7 -I/usr/lib/python2.7/dist-packages/numpy/core/include -I/usr/include/python2.7 -c -c ../hw.f -o /tmp/tmpRGHzqj/fortran/hw.o" failed with exit status 1

I have no experience with Fortran, I only need to implement existing Fortran code to use in Python.

The simple answer is that the code is not portable (and thus possibly a bad tutorial). Do you want us to explain the errors (and how to fix) or to provide an alternative tutorial? — francescalus
– francescalus, Commented Dec 16, 2014 at 11:20
I used @AlexanderVogt 's answer and it works fine. I think I should be able to use the rest of the tutorial without any further problems. — Tom83B
– Tom83B, Commented Dec 16, 2014 at 11:36

Alexander Vogt · Accepted Answer · 2014-12-16 11:18:53Z

2

The interface for hw1 is missing in your code. Either specify it explicitly:

program hwtest
  real*8 r1, r2 ,s
  real*8 hw1
c [...]

Or, even better, put the functions into a module, and use the module.

BTW: Change *8 by something better suited, like:

program hwtest
  integer,parameter :: dp = kind(1.d0)
  real(dp) r1, r2 ,s
  real(dp) hw1

Or, use ISO_Fortran_env, and the pre-defined parameters REAL64 and REAL32.

answered Dec 16, 2014 at 11:18

Alexander Vogt

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1 Comment

arclight Over a year ago

It should go without saying to always use IMPLICIT NONE, requiring all variables (and in this case external functions) to be declared prior to use. This is a weakness in the example Fortran code presented in the tutorial and should be fixed upstream.

ilciavo · Accepted Answer · 2015-05-18 10:54:03Z

Just to wrap things up:

      program hwtest
      implicit none
      real*8 r1, r2 ,s
C     Adding interface for the function
      real*8 hw1
      r1 = 1.0
      r2 = 0.0
      s = hw1(r1, r2)
      write(*,*) 'hw1, result:',s
      write(*,*) 'hw2, result:'
      call hw2(r1, r2)
      call hw3(r1, r2, s)
      write(*,*) 'hw3, result:', s
      end

      real*8 function hw1(r1, r2)
      real*8 r1, r2
      hw1 = sin(r1 + r2)
      return
      end

      subroutine hw2(r1, r2)
      real*8 r1, r2, s
      s = sin(r1 + r2)
      write(*,1000) 'Hello, World! sin(',r1+r2,')=',s
1000 format(A,F6.3,A,F8.6)
      return
      end

C     special version of hw1 where the result is
C     returned as an argument:

      subroutine hw3_v1(r1, r2, s)
      real*8 r1, r2, s
      s = sin(r1 + r2)
      return
      end

C     F2py treats s as input arg. in hw3_v1; fix this:

      subroutine hw3(r1, r2, s)
      real*8 r1, r2, s
Cf2py intent(out) s
      s = sin(r1 + r2)
      return
      end

C     test case sensitivity:

      subroutine Hw4(R1, R2, s)
      real*8 R1, r2, S
Cf2py intent(out) s
      s = SIN(r1 + r2)
      ReTuRn
      end

C end of F77 file

Compile using f2pyA -m hw -c hw.f

Finally call from python as:

import sys
from hw import hw1, hw2
try:
    r1 = float(sys.argv[1]);  r2 = float(sys.argv[2])
except IndexError:
    print 'Usage:', sys.argv[0], 'r1 r2'; sys.exit(1)
print 'Fortran hw1, result:', hw1(r1, r2)
print 'Fortran hw2, result: ', hw2(r1, r2)

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