Python C++ extension compiler warning

Question

I have something like this in a C++ extension:

char* kwlist[] = {"x", "W"};
if (!PyArg_ParseTupleAndKeywords(args, kwrds, "OO", kwlist, &x, &W)) return NULL;

The compiler complains that:

warning: conversion from string literal to 'char *' is deprecated [-Wdeprecated-writable-strings]

What is the best way to get rid of this warning?

I tried:

char const* kwlist[] = {"x", "W"};

But that fails with:

candidate function not viable: no known conversion from 'const char [2]' to 'char *'

Answer 1

It looks like you're trying to be pedantically const-correct to an extent that Python's C API is not, even if indirectly by turning up the warnings on your compiler. You have a few options here. (Note that I wasn't able to reproduce you warnings in my compiler, so do double-check my recommendations actually resolve things.)

• Cast away the warning on creation:

  char* kwlist[] = { const_cast<char*>("x"), const_cast<char*>("W") };
  if (!PyArg_ParseTupleAndKeywords(args, kwrds, "OO", kwlist, &x, &W)) return NULL;
  

• Cast away the error on use:

  char const* kwlist[] = { "x", "W" };
  if (!PyArg_ParseTupleAndKeywords(args, kwrds, "OO", const_cast<char **>(kwlist), &x, &W)) return NULL;
  

• Make things mutable as required:

  char kw_x[] = "x";
  char kw_W[] = "W";
  char* kwlist[] = { kw_x, kw_W };
  if (!PyArg_ParseTupleAndKeywords(args, kwrds, "OO", kwlist, &x, &W)) return NULL;
  

• Silence the specific warnings, either globally or locally:

  The exact code for local warning suppression depends on the compiler. I've included two common cases below:

  GCC (thanks Matt Joiner!)

  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wdeprecated-writable-strings"
  takes_ppchar(kwlist);         /* no diagnostic for this one */
  #pragma GCC diagnostic pop
  

  Visual Studio

  #pragma warning(push)
  #pragma warning(disable : ####)
  code_causing_warning(####)
  #pragma warning(pop)
  

Casting away const is not universally safe, so if PyArg_ParseTupleAndKeywords actually uses the contents in a non-const manner, either of the first two approaches can legitimately result in run-time failures.

If you strongly suspect (or better yet verify) that PyArg_ParseTupleAndKeywords uses your parameter in a const-safe manner (requesting more permissions than it uses), most approaches are less verbose than the technically correct third approach.