Calling Python function from Go and getting the function return value

Question

I am writing a Go program. From this Go program, I would like to call a Python function defined in another file and receive the function's return value so I can use it in subsequent processing in my Go program. I am having trouble getting any returned data back in my Go program though. Below is a minimum example of what I thought would work, but apparently doesn't:

gofile.go

package main

import "os/exec"
import "fmt"

func main() {
     fmt.Println("here we go...")
     program := "python"
     arg0 := "-c"
     arg1 := fmt.Sprintf("'import pythonfile; print pythonfile.cat_strings(\"%s\", \"%s\")'", "foo", "bar")
     cmd := exec.Command(program, arg0, arg1)
     fmt.Println("command args:", cmd.Args)
     out, err := cmd.CombinedOutput()
     if err != nil {
         fmt.Println("Concatenation failed with error:", err.Error())
     return
     }
     fmt.Println("concatentation length: ", len(out))
     fmt.Println("concatenation: ", string(out))
     fmt.Println("...done")
}

pythonfile.py

def cat_strings(a, b):
    return a + b

If I call go run gofile I get the following output:

here we go...
command args: [python -c 'import pythonfile; print pythonfile.cat_strings("foo", "bar")']
concatentation length:  0
concatenation:  
...done

A few notes:

• I'm using the -c flag in the Python invocation so I can call the function cat_strings directly. Assume cat_strings is part of a Python file full of utility functions that are used by other Python programs, hence why I don't have any if __name__ == __main__ business.
• I don't want to modify the Python file to print a + b (instead of return a + b); see the prior point about the function being part of a set of utility functions that ought to be callable by other Python code.
• The cat_strings function is fictional and for demonstration purposes; the real function is something I don't want to simply reimplement in Go. I really am interested in how I can call a Python function from Go and get the return value.

Answer 1

I managed to have some working code for this by simply removing the quote around the command itself:

package main

import "fmt"
import "os/exec"

func main() {
    cmd := exec.Command("python",  "-c", "import pythonfile; print pythonfile.cat_strings('foo', 'bar')")
    fmt.Println(cmd.Args)
    out, err := cmd.CombinedOutput()
    if err != nil { fmt.Println(err); }
    fmt.Println(string(out))
}

And sure enough, in the source, you have this function (for Windows, at least, I don't know if that works for other OSes):

// EscapeArg rewrites command line argument s as prescribed
// in http://msdn.microsoft.com/en-us/library/ms880421.
// This function returns "" (2 double quotes) if s is empty.
// Alternatively, these transformations are done:
// - every back slash (\) is doubled, but only if immediately
//   followed by double quote (");
// - every double quote (") is escaped by back slash (\);
// - finally, s is wrapped with double quotes (arg -> "arg"),
//   but only if there is space or tab inside s.
func EscapeArg(s string) string { ...

So your code is ending up passing the following command line call:

$ python -c "'import pythonfile; print pythonfile.cat_strings(\\"foo\\", \\"bar\\")'"

Which, if tested, evaluates to a string and returns nothing, hence the 0-length output.