New python API for units handling.

Exposes all supported unit systems & types, and to_value()/to_string() functions.

Reviewed and enhanced by CampbellBarton, many thanks!

Differential Revision: https://developer.blender.org/D416

source/blender/python/generic/py_capi_utils.c

@@ -904,3 +904,73 @@ PyObject *PyC_FlagSet_FromBitfield(PyC_FlagSet *items, int flag)
 
 	return ret;
 }
+
+
+/**
+ * \return -1 on error, else 0
+ *
+ * \note it is caller's responsibility to acquire & release GIL!
+ */
+int PyC_RunString_AsNumber(const char *expr, double *value, const char *filename)
+{
+	PyObject *py_dict, *mod, *retval;
+	int error_ret = 0;
+	PyObject *main_mod = NULL;
+
+	PyC_MainModule_Backup(&main_mod);
+
+	py_dict = PyC_DefaultNameSpace(filename);
+
+	mod = PyImport_ImportModule("math");
+	if (mod) {
+		PyDict_Merge(py_dict, PyModule_GetDict(mod), 0); /* 0 - don't overwrite existing values */
+		Py_DECREF(mod);
+	}
+	else { /* highly unlikely but possibly */
+		PyErr_Print();
+		PyErr_Clear();
+	}
+
+	retval = PyRun_String(expr, Py_eval_input, py_dict, py_dict);
+
+	if (retval == NULL) {
+		error_ret = -1;
+	}
+	else {
+		double val;
+
+		if (PyTuple_Check(retval)) {
+			/* Users my have typed in 10km, 2m
+			 * add up all values */
+			int i;
+			val = 0.0;
+
+			for (i = 0; i < PyTuple_GET_SIZE(retval); i++) {
+				const double val_item = PyFloat_AsDouble(PyTuple_GET_ITEM(retval, i));
+				if (val_item == -1 && PyErr_Occurred()) {
+					val = -1;
+					break;
+				}
+				val += val_item;
+			}
+		}
+		else {
+			val = PyFloat_AsDouble(retval);
+		}
+		Py_DECREF(retval);
+
+		if (val == -1 && PyErr_Occurred()) {
+			error_ret = -1;
+		}
+		else if (!finite(val)) {
+			*value = 0.0;
+		}
+		else {
+			*value = val;
+		}
+	}
+
+	PyC_MainModule_Restore(main_mod);
+
+	return error_ret;
+}