Added example for go routines

examples/goroutines/foo.py

@@ -0,0 +1,17 @@
+import sys
+
+def print_odds(limit=10):
+    """
+    Print odds numbers < limit
+    """
+    for i in range(limit):
+        if i%2:
+            sys.stderr.write("odds: {}\n".format(i))
+
+def print_even(limit=10):
+    """
+    Print even numbers < limit
+    """
+    for i in range(limit):
+        if i%2 == 0:
+            sys.stderr.write("even: {}\n".format(i))

examples/goroutines/main.go

@@ -0,0 +1,118 @@
+package main
+
+import (
+	"errors"
+	"log"
+	"os"
+	"runtime"
+	"sync"
+
+	python3 "github.com/go-python/cpy3"
+)
+
+func main() {
+	var err error
+
+	// At the end of all, if there was an error
+	// prints the error and exit with the non-zero code
+	defer func() {
+		if err != nil {
+			log.Printf("%+v", err)
+			os.Exit(1)
+		}
+	}()
+
+	// Undo all initializations made by Py_Initialize() and subsequent
+	defer python3.Py_Finalize()
+
+	// Prints any python error if it was here
+	// no needs to call it after each single check of PyErr_Occurred()
+	defer python3.PyErr_Print()
+
+	// Initialize the Python interpreter and
+	// since version 3.7 it also create the GIL explicitly by calling PyEval_InitThreads()
+	// so you don’t have to call PyEval_InitThreads() yourself anymore
+	python3.Py_Initialize() // create the GIL, the GIL is locked by the main thread
+
+	if !python3.Py_IsInitialized() {
+		err = errors.New("error initializing the python interpreter")
+		return
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+
+	fooModule := python3.PyImport_ImportModule("foo") // new reference, a call DecRef() is needed
+	if fooModule == nil && python3.PyErr_Occurred() != nil {
+		err = errors.New("error importing the python module")
+		return
+	}
+	defer fooModule.DecRef()
+
+	odds := fooModule.GetAttrString("print_odds") // new reference, a call DecRef() is needed
+	if odds == nil && python3.PyErr_Occurred() != nil {
+		err = errors.New("error getting the attribute print_odds")
+		return
+	}
+	defer odds.DecRef()
+
+	even := fooModule.GetAttrString("print_even") // new reference, a call DecRef() is needed
+	if even == nil && python3.PyErr_Occurred() != nil {
+		err = errors.New("error getting the attribute print_even")
+		return
+	}
+	defer even.DecRef()
+
+	limit := python3.PyLong_FromGoInt(50) // new reference, will stolen later, a call DecRef() is NOT needed
+	if limit == nil && python3.PyErr_Occurred() != nil {
+		err = errors.New("error creating python long object")
+		return
+	}
+
+	args := python3.PyTuple_New(1) // new reference, a call DecRef() is needed
+	if args == nil && python3.PyErr_Occurred() != nil {
+		err = errors.New("error creating python tuple object")
+		return
+	}
+	defer args.DecRef()
+
+	ret := python3.PyTuple_SetItem(args, 0, limit) // steals reference to limit
+
+	// Cleans the Go variable, because now a new owner is caring about related PyObject
+	// no action, such as a call DecRef(), is needed here
+	limit = nil
+
+	if ret != 0 {
+		err = errors.New("error setting a tuple item")
+		limit.DecRef()
+		limit = nil
+		return
+	}
+
+	// Save the current state and release the GIL
+	// so that goroutines can acquire it
+	state := python3.PyEval_SaveThread() // release the GIL, the GIL is unlocked for using by goroutines
+
+	go func() {
+		runtime.LockOSThread()
+		_gstate := python3.PyGILState_Ensure() // acquire the GIL, the GIL is locked by the 1st goroutine
+		odds.Call(args, python3.PyDict_New())
+		python3.PyGILState_Release(_gstate) // release the GIL, the GIL is unlocked for using by others
+
+		wg.Done()
+	}()
+
+	go func() {
+		runtime.LockOSThread()
+		_gstate := python3.PyGILState_Ensure() // acquire the GIL, the GIL is locked by the 2nd goroutine
+		even.Call(args, python3.PyDict_New())
+		python3.PyGILState_Release(_gstate) // release the GIL, the GIL is unlocked for using by others
+
+		wg.Done()
+	}()
+
+	wg.Wait()
+
+	// Restore the state and lock the GIL
+	python3.PyEval_RestoreThread(state) // acquire the GIL, the GIL is locked by the main thread
+}