CODEWITHSUNDEEP
pythoncython
Mark Wilson
Mark Wilson

Reputation: 13

Cython - Cannot convert Python object argument to type 'float *'

I am using Cython to write a Python API to a C project.

In the C project there is the following relevant code in the header file:

#ifndef EN_API_FLOAT_TYPE
  #define EN_API_FLOAT_TYPE float
#endif

int DLLEXPORT ENgetlinkvalue(int index, int property, EN_API_FLOAT_TYPE *value);

In my .pyx file I have:

cdef extern from "epanet2.h":
    int ENgetlinkvalue(int, int, float *)

def engetlinkvalue(int index, int proprty, float *value):
    return ENgetlinkvalue(index, proprty, value)

When I run setup.py I get the following error:

def engetlinkvalue(int index, int proprty, float *value): ^

src\cyepanet.pyx:48:43: Cannot convert Python object argument to type 'float *'

I have been banging my head against the wall trying to figure this one out (oh, and also searching SO, Cython documentation, a book on Cython, ChatGPT, and Copilot)

I am sure this is an easy one for someone more experienced in C coding. For me, 99% of my experience is in Python. Thanks in advance.

Upvotes: 1

Views: 279

Answers (1)

DavidW
DavidW

Reputation: 30925

If (as you say in the comments) the float is being used as a return value, then you'd do something like

def engetlinkvalue(int index, int proprty):
    cdef float value
    errCode = ENgetlinkvalue(index, proprty, &value)
    if errCode:
        ... # raise an exception
    return value

That converts it to a nicer Python interface - you use return to send back a value (as you would in Python) and you use Python exceptions to indicate a failure.

It's often a good idea in these cases to think about the Python interface you'd like rather than trying to match the C interface. (This is pretty much what @user2357112 also says in the comments)

Upvotes: 3

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