== Python Button Evaluation ==

Users can write any valid Python expression (that evals to an int or float) inside Blender's gui number buttons (preceded by #) and have them evaluated to their actual int or float value.

Button Evaluation has access to the same modules as PyDrivers.

For example:

#1.0*9-2.3
#ob("Camera").LocZ
#1.0/ob("Cube").LocX
#math.sin(math.pi) -- or simply #m.sin(m.pi)
etc

source/blender/blenkernel/BKE_bad_level_calls.h

@@ -71,6 +71,8 @@ void BPY_free_screen_spacehandlers (struct bScreen *sc);
 struct Object **BPY_pydriver_get_objects(struct IpoDriver *driver);
 float BPY_pydriver_eval(struct IpoDriver *driver);
 void BPY_pydriver_update(void);
+/* button python evaluation */
+int BPY_button_eval(char *expr, double *value);
 
 /* writefile.c */
 struct Oops;

source/blender/blenkernel/bad_level_call_stubs/stubs.c

@@ -114,12 +114,15 @@ float BPY_pydriver_eval(struct IpoDriver *driver)
 {
 	return 0;
 }
-
 /* depsgraph.c: */
 struct Object **BPY_pydriver_get_objects(struct IpoDriver *driver)
 {
 	return 0;
 }
+int BPY_button_eval(char *expr, double *value)
+{
+	return 0;
+}
 
 /* writefile.c */
 	/* struct Oops; */

source/blender/python/BPY_extern.h

@@ -85,6 +85,8 @@ extern "C" {
 	float BPY_pydriver_eval(struct IpoDriver *driver);
 	struct Object **BPY_pydriver_get_objects(struct IpoDriver *driver);
 
+	int BPY_button_eval(char *expr, double *value);
+
 /* format importer hook */
 	int BPY_call_importloader( char *name );
 

source/blender/python/BPY_interface.c

@@ -1144,6 +1144,74 @@ float BPY_pydriver_eval(IpoDriver *driver)
 	return result;
 }
 
+/* Button Python Evaluation */
+
+/* Python evaluation for gui buttons:
+ *	users can write any valid Python expression (that evals to an int or float)
+ *	inside Blender's gui number buttons and have them evaluated to their
+ *	actual int or float value.
+ *
+ *	The global dict used for pydrivers is also used here, so all imported
+ *	modules for pydrivers (including the pydrivers.py Blender text) are
+ *	available for button py eval, too. */
+
+static int bpy_button_eval_error(char *expr) {
+
+	if (bpy_pydriver_oblist)
+		bpy_pydriver_freeList();
+
+	if (bpy_pydriver_Dict) { /* free the persistent global dict */
+		/* it's the same dict used by pydrivers */
+		PyDict_Clear(bpy_pydriver_Dict);
+		Py_DECREF(bpy_pydriver_Dict);
+		bpy_pydriver_Dict = NULL;
+	}
+
+	fprintf(stderr, "\nError in button evaluation:\nThis is the failed Python expression:\n'%s'\n\n", expr);
+
+	PyErr_Print();
+
+	return -1;
+}
+
+int BPY_button_eval(char *expr, double *value)
+{
+	PyObject *retval, *floatval;
+
+	if (!value || !expr || expr[0]=='\0') return -1;
+
+	*value = 0.0; /* default value */
+
+	if (!bpy_pydriver_Dict) {
+		if (bpy_pydriver_create_dict() != 0) {
+			fprintf(stderr,
+				"Button Python Eval error: couldn't create Python dictionary");
+			return -1;
+		}
+	}
+
+	retval = PyRun_String(expr, Py_eval_input, bpy_pydriver_Dict,
+		bpy_pydriver_Dict);
+
+	if (retval == NULL) {
+		return bpy_button_eval_error(expr);
+	}
+	else {
+		floatval = PyNumber_Float(retval);
+		Py_DECREF(retval);
+	}
+
+	if (floatval == NULL) 
+		return bpy_button_eval_error(expr);
+	else {
+		*value = (float)PyFloat_AsDouble(floatval);
+		Py_DECREF(floatval);
+	}
+
+	return 0; /* successful exit */
+}
+
+
 /*****************************************************************************/
 /* ScriptLinks                                                        */
 /*****************************************************************************/

source/blender/src/interface.c

@@ -99,6 +99,8 @@
 
 #include "BSE_view.h"
 
+#include "BPY_extern.h" /* for BPY_button_eval */
+
 #include "mydevice.h"
 #include "interface.h"
 #include "blendef.h"
@@ -2007,7 +2009,7 @@ static int ui_act_as_text_but(uiBut *but)
 	min= but->min;
 	max= but->max;
 	but->min= 0.0;
-	but->max= 15.0;
+	but->max= UI_MAX_DRAW_STR - 1; /* for py strings evaluation */
 	temp= but->type;
 	but->type= TEX;
 	textleft= but->flag & UI_TEXT_LEFT;
@@ -2024,8 +2026,15 @@ static int ui_act_as_text_but(uiBut *but)
 	but->max= max;
 	if(textleft==0) but->flag &= ~UI_TEXT_LEFT;
 
-	if( but->pointype==FLO ) value= atof(str);
-	else value= atoi(str);
+	if(str[0] == '#') {
+		if(BPY_button_eval(str+1, &value)) { /* str+1 to skip the # sign */
+			error("Invalid Python expression, check console");
+			retval = 0; /* invalidate return value if eval failed */
+		}
+	}
+	else value = atof(str);
+
+	if(but->pointype!=FLO) value= (int)value;
 
 	if(value<min) value= min;
 	if(value>max) value= max;