Optimize builtin str() calls (GH-3478)

Cython/Compiler/Optimize.py

@@ -2312,6 +2312,38 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
         return ExprNodes.CachedBuiltinMethodCallNode(
             node, function.obj, attr_name, arg_list)
 
+    PyObject_String_func_type = PyrexTypes.CFuncType(
+        Builtin.str_type, [
+            PyrexTypes.CFuncTypeArg("obj", PyrexTypes.py_object_type, None)
+            ])
+
+    def _handle_simple_function_str(self, node, function, pos_args):
+        """Optimize single argument calls to str().
+        """
+        if len(pos_args) != 1:
+            if len(pos_args) == 0:
+                return ExprNodes.StringNode(node.pos, value=EncodedString(), constant_result='')
+            return node
+        arg = pos_args[0]
+
+        if arg.type is Builtin.str_type:
+            if not arg.may_be_none():
+                return arg
+
+            cname = "__Pyx_PyStr_Str"
+            utility_code = UtilityCode.load_cached('PyStr_Str', 'StringTools.c')
+        else:
+            cname = '__Pyx_PyObject_Str'
+            utility_code = UtilityCode.load_cached('PyObject_Str', 'StringTools.c')
+
+        return ExprNodes.PythonCapiCallNode(
+            node.pos, cname, self.PyObject_String_func_type,
+            args=pos_args,
+            is_temp=node.is_temp,
+            utility_code=utility_code,
+            py_name="str"
+        )
+
     PyObject_Unicode_func_type = PyrexTypes.CFuncType(
         Builtin.unicode_type, [
             PyrexTypes.CFuncTypeArg("obj", PyrexTypes.py_object_type, None)

Cython/Utility/StringTools.c

@@ -1219,3 +1219,22 @@ static CYTHON_INLINE PyObject* __Pyx_PyUnicode_Unicode(PyObject *obj) {
 #define __Pyx_PyObject_Unicode(obj) \
     (likely(PyUnicode_CheckExact(obj)) ? __Pyx_NewRef(obj) : PyObject_Unicode(obj))
 #endif
+
+
+//////////////////// PyStr_Str.proto ////////////////////
+
+static CYTHON_INLINE PyObject* __Pyx_PyStr_Str(PyObject *obj);/*proto*/
+
+//////////////////// PyStr_Str ////////////////////
+
+static CYTHON_INLINE PyObject* __Pyx_PyStr_Str(PyObject *obj) {
+    if (unlikely(obj == Py_None))
+        obj = PYIDENT("None");
+    return __Pyx_NewRef(obj);
+}
+
+
+//////////////////// PyObject_Str.proto ////////////////////
+
+#define __Pyx_PyObject_Str(obj) \
+    (likely(PyString_CheckExact(obj)) ? __Pyx_NewRef(obj) : PyObject_Str(obj))

tests/run/strfunction.pyx

@@ -5,6 +5,8 @@ __doc__ = u"""
    'test'
 """
 
+cimport cython
+
 s = str
 z = str('test')
 
@@ -39,3 +41,33 @@ def sub(string):
 
 #def csub(string):
 #    return csubs(string)
+
+
+@cython.test_fail_if_path_exists("//SimpleCallNode")
+@cython.test_assert_path_exists("//PythonCapiCallNode")
+def typed(str s):
+    """
+    >>> print(typed(None))
+    None
+    >>> type(typed(None)) is type(typed(None))
+    True
+    >>> print(typed('abc'))
+    abc
+    >>> type(typed('abc')) is type(typed('abc'))
+    True
+    """
+    return str(s)
+
+
+@cython.test_fail_if_path_exists(
+    "//SimpleCallNode",
+    "//PythonCapiCallNode",
+)
+def typed_not_none(str s not None):
+    """
+    >>> print(typed('abc'))
+    abc
+    >>> type(typed('abc')) is type(typed('abc'))
+    True
+    """
+    return str(s)