Hello World program compilable (/runnable) as both .py and .c(pp). Makes use of the #define preprocessor directive of C/C++ to make Python code invisible to the C compiler while using multiline comments from Python (""") to make C(++)-code invisible to the Python interpreter. Both programs just output "Hello World from both!", while each program…

cpp-py

#// This comment is invisible to both, Python and C(++).

#/*
#  This multiline comment is also invisible to both.
#*/

#include <stdio.h>

#define pass void printstring(char* x) { printf("%s", x); printf("\n"); }
pass

#/*

print "Only visible to Python."

#*/

#define testing "\
"""

void onlyvisibletocpp() {
  printf("Only visible to C(++).\n");
}

#undef testing
#define testing "\
"""

#define def void
#define x ) {};
#define print
def printmessage(
#define unused \
x):
    print x

#undef print
#define printmessage(x) printstring((char*)x);

#undef pass
#define pass int main(int argc, char** argv) { \
printmessage("Hello World from both!")

pass

#undef pass
#define pass onlyvisibletocpp(); return 0; }

pass