I double checked my hierarchy / structure and finally I found my mistake!
The structure below works well and I think it's correct one.

Test Plugin/
    test_plugin.pyp
    res/
        c4d_symbols.h
        description/
            ttestplugin.h
            ttestplugin.res
        strings_en-US/
            description  -------------------- This folder needed! --------------------
                ttestplugin.str

@m_adam Oh, that's a little update for many users but huge for tiny developers like me!
Thank you so much for the heads up.

@m_magalhaes Thank you for your checking, Manuel.

I didn't think it's a bug! And yep, your workaround with bracket notation works fine.
So these bracket notation style is safer than using BaseContainer in any case?
Anyway, let us know if you confirm this is a bug.

Hey folks,

I encountered Undo related issue again. It might be due to my lack of understanding for undo things but let me ask you a question.

I wrote a test tag plugin with python which follows another object's position with a little offset.
It simply works but when I undo, the object that has the tag doesn't react until I click somewhere (see below).

Is it possible to solve this issue with python?
I know undo things are prohibited in execute() of tagdata, so is there any workaround?

Please help me if you know the solution.

Here's quick code for this.

testtag.pyp

import os
import c4d
from c4d import plugins, bitmaps, utils


PLUGIN_ID = *******


class TestTagData(plugins.TagData):
    def Init(self, node):
        pd = c4d.PriorityData()
        if pd is None:
            raise MemoryError("Failed to create a priority data.")
        pd.SetPriorityValue(lValueID = c4d.PRIORITYVALUE_MODE, data = c4d.CYCLE_INITIAL)
        node[c4d.EXPRESSION_PRIORITY] = pd
        return True

    def Execute(self, tag, doc, op, bt, priority, flags):
        bc = tag.GetDataInstance()
        obj = bc.GetLink(c4d.TTEST_TARGET_OBJECT)
        if obj is None:
            return False
        mrx = obj.GetMg()
        mrx.off += c4d.Vector(100, 0, 0)
        op.SetMg(mrx)
        return c4d.EXECUTIONRESULT_OK


if __name__ == "__main__":
    plugins.RegisterTagPlugin(id = PLUGIN_ID, str = "Test Tag", info = c4d.TAG_EXPRESSION | c4d.TAG_VISIBLE, g = TestTagData, description = "ttest", icon = None)

ttest.res

CONTAINER Ttest
{
    NAME Ttest;
    INCLUDE Texpression;

    GROUP ID_TAGPROPERTIES
    {
        LINK TTEST_TARGET_OBJECT
        {
            ANIM ON;
            ACCEPT { Obase; };
        }
}

@Cairyn Thank you so much for helping me again!

@m_magalhaes Thank you for your explanation! It's really helpful to know several solutions for a problem.
C4D python doc is not user friendly for newbies like me but this community is super friendly and pretty awesome!

So I tried some changes based on a hint @Cairyn points out, my code seems to work now.
But I'm still not sure this is a proper way to add undo for dynamic descriptions, so please let me know if you know the correct / better way!

test_tag.pyp

import os
import c4d
from c4d import plugins


PLUGIN_ID = *******
LINK_NO = 1100
FLOAT_NO = 1200


class TestTagData(c4d.plugins.TagData):
    def Init(self, node):
        self.InitAttr(node, int, c4d.TTESTTAG_CONTROLLERS_NUM)  #-------------------- Changed --------------------
        bc = node.GetDataInstance()  #-------------------- Changed --------------------
        bc.SetInt32(c4d.TTESTTAG_CONTROLLERS_NUM, 0)  #-------------------- Changed --------------------
        pd = c4d.PriorityData()
        if pd is None:
            raise MemoryError("Failed to create a priority data.")
        pd.SetPriorityValue(lValueID = c4d.PRIORITYVALUE_MODE, data = c4d.CYCLE_EXPRESSION)
        node[c4d.EXPRESSION_PRIORITY] = pd
        return True

    def Execute(self, tag, doc, op, bt, priority, flags):
        return c4d.EXECUTIONRESULT_OK

    def Message(self, node, type, data):
        if type == c4d.MSG_DESCRIPTION_COMMAND:
            if data["id"][0].id == c4d.TTESTTAG_BUTTON_ADD:
                doc = c4d.documents.GetActiveDocument()
                doc.StartUndo()
                doc.AddUndo(c4d.UNDOTYPE_CHANGE_SMALL, node)
                bc = node.GetDataInstance()  #-------------------- Changed --------------------
                bc.SetInt32(c4d.TTESTTAG_CONTROLLERS_NUM, bc.GetInt32(c4d.TTESTTAG_CONTROLLERS_NUM) + 1)  #-------------------- Changed --------------------
                doc.EndUndo()
            elif data["id"][0].id == c4d.TTESTTAG_BUTTON_REMOVE:
                bc = node.GetDataInstance()  #-------------------- Changed --------------------
                controllers_num = bc.GetInt32(c4d.TTESTTAG_CONTROLLERS_NUM)  #-------------------- Changed --------------------
                if controllers_num > 0:  #-------------------- Changed --------------------
                    doc = c4d.documents.GetActiveDocument()
                    doc.StartUndo()
                    doc.AddUndo(c4d.UNDOTYPE_CHANGE_SMALL, node)
                    bc.SetInt32(c4d.TTESTTAG_CONTROLLERS_NUM, controllers_num - 1)  #-------------------- Changed --------------------
                    doc.EndUndo()
        return True

    def GetDDescription(self, node, description, flags):
        if not description.LoadDescription(node.GetType()):
            return False
        singleId = description.GetSingleDescID()
        groupId = c4d.DescID(c4d.DescLevel(c4d.ID_TAGPROPERTIES))
        bc = node.GetDataInstance()  #-------------------- Changed --------------------
        controllers_num = bc.GetInt32(c4d.TTESTTAG_CONTROLLERS_NUM)  #-------------------- Changed --------------------
        if controllers_num > 0:
            for i in range(controllers_num):
                linkId = c4d.DescID(c4d.DescLevel(LINK_NO + (i + 1)))
                if singleId is None or linkId.IsPartOf(singleId)[0]:
                    link_bc = c4d.GetCustomDataTypeDefault(c4d.DTYPE_BASELISTLINK)
                    link_bc.SetString(c4d.DESC_NAME, "Controller " + str(i + 1))
                    link_bc.SetString(c4d.DESC_SHORT_NAME, "Controller " + str(i + 1))
                    link_bc.SetInt32(c4d.DESC_ANIMATE, c4d.DESC_ANIMATE_ON)
                    if not description.SetParameter(linkId, link_bc, groupId):
                        return False
                floatId = c4d.DescID(c4d.DescLevel(FLOAT_NO + (i + 1)))
                if singleId is None or floatId.IsPartOf(singleId)[0]:
                    float_bc = c4d.GetCustomDataTypeDefault(c4d.DTYPE_REAL)
                    float_bc.SetString(c4d.DESC_NAME, "Rate")
                    float_bc.SetString(c4d.DESC_SHORT_NAME, "Rate")
                    float_bc.SetFloat(c4d.DESC_DEFAULT, 1)
                    float_bc.SetInt32(c4d.DESC_ANIMATE, c4d.DESC_ANIMATE_ON)
                    float_bc.SetInt32(c4d.DESC_UNIT, c4d.DESC_UNIT_PERCENT)
                    float_bc.SetFloat(c4d.DESC_MIN, -1000)
                    float_bc.SetFloat(c4d.DESC_MAX, 1000)
                    float_bc.SetFloat(c4d.DESC_MINSLIDER, -1)
                    float_bc.SetFloat(c4d.DESC_MAXSLIDER, 1)
                    float_bc.SetFloat(c4d.DESC_STEP, 0.01)
                    float_bc.SetInt32(c4d.DESC_CUSTOMGUI, c4d.CUSTOMGUI_REALSLIDER)
                    if not description.SetParameter(floatId, float_bc, groupId):
                        return False
                separatorId = c4d.DescID(0)
                if singleId is None or separatorId.IsPartOf(singleId)[0]:
                    separator_bc = c4d.GetCustomDataTypeDefault(c4d.DTYPE_SEPARATOR)
                    separator_bc.SetInt32(c4d.DESC_CUSTOMGUI, c4d.CUSTOMGUI_SEPARATOR)
                    separator_bc.SetBool(c4d.DESC_SEPARATORLINE, True)
                    if not description.SetParameter(separatorId, separator_bc, groupId):
                        return False
        return (True, flags | c4d.DESCFLAGS_DESC_LOADED)


if __name__ == "__main__":
    c4d.plugins.RegisterTagPlugin(id = PLUGIN_ID, str = "Test Tag", info = c4d.TAG_EXPRESSION|c4d.TAG_VISIBLE, g = TestTagData, description = "ttesttag", icon = None)

ttesttag.res

CONTAINER Ttesttag
{
    NAME Ttesttag;
    INCLUDE Texpression;

    GROUP ID_TAGPROPERTIES
    {
        GROUP TTESTTAG_BUTTONS_GROUP
        {
            COLUMNS 2;
            BUTTON TTESTTAG_BUTTON_ADD { SCALE_H; }
            BUTTON TTESTTAG_BUTTON_REMOVE { SCALE_H; }
            LONG TTESTTAG_CONTROLLERS_NUM { HIDDEN; ANIM OFF; MIN 0; MAX 10000; }  //-------------------- Added --------------------
        }
    }
}

ttesttag.h

#ifndef _TTESTTAG_H_
#define _TTESTTAG_H_

enum
{
    TTESTTAG_BUTTONS_GROUP = 1001,
    TTESTTAG_BUTTON_ADD = 1002,
    TTESTTAG_BUTTON_REMOVE = 1003,
    TTESTTAG_CONTROLLERS_NUM = 1004,  //-------------------- Added --------------------
}

#endif

ttesttag.str

STRINGTABLE Ttesttag
{
    Ttesttag "Test Tag";
    TTESTTAG_BUTTONS_GROUP "";
    TTESTTAG_BUTTON_ADD "Add";
    TTESTTAG_BUTTON_REMOVE "Remove";
    TTESTTAG_CONTROLLERS_NUM "";  //-------------------- Added --------------------
}

@m_adam Thank you for confirming.
I already know symbolcache thingy, so it's no problem.

@Cairyn Thank you for your time!

Hmm, I see. I'm not sure the actual solution for the point you mention but your explanation is so helpful. Anyway, I'll try some workaround on my end.

Hi folks,

I'm still trying to make a python tag plugin which has 2 buttons in tag property. It dynamically adds new descriptions to tag property when "add" button is clicked, and it dynamically removes those descriptions when "remove" button is clicked. So far my code works but I noticed undo doesn't work for these descriptions (see below).

Is there a way to add undo for descriptions created dynamically? Please tell me how to add undo for that if it's possible for python plugin.
I tried to insert basic undo things (StartUndo(), AddUndo(), and EndUndo()) into Message() function but it doesn't seem to work.
My goal is it works like "IK-Spline" does. (see below)

"Current Status"

"My Goal"

Here's my code.

test_tag.pyp

import os
import c4d
from c4d import plugins


PLUGIN_ID = *******
LINK_NO = 1100
FLOAT_NO = 1200


class TestTagData(c4d.plugins.TagData):
    def Init(self, node):
        self.controllers_num = 0
        pd = c4d.PriorityData()
        if pd is None:
            raise MemoryError("Failed to create a priority data.")
        pd.SetPriorityValue(lValueID = c4d.PRIORITYVALUE_MODE, data = c4d.CYCLE_EXPRESSION)
        node[c4d.EXPRESSION_PRIORITY] = pd
        return True

    def Execute(self, tag, doc, op, bt, priority, flags):
        return c4d.EXECUTIONRESULT_OK

    def Message(self, node, type, data):
        if type == c4d.MSG_DESCRIPTION_COMMAND:
            if data["id"][0].id == c4d.TTESTTAG_BUTTON_ADD:
                doc = c4d.documents.GetActiveDocument()
                doc.StartUndo()                    #---------- This doesn't seem to work! ----------
                doc.AddUndo(c4d.UNDOTYPE_CHANGE_NOCHILDREN, node)
                self.controllers_num += 1
                doc.EndUndo()                    #----------------------------------------
                node.SetDirty(c4d.DIRTYFLAGS_DESCRIPTION)
            elif data["id"][0].id == c4d.TTESTTAG_BUTTON_REMOVE:
                if self.controllers_num > 0:
                    doc = c4d.documents.GetActiveDocument()
                    doc.StartUndo()                    #---------- This doesn't seem to work! ----------
                    doc.AddUndo(c4d.UNDOTYPE_CHANGE_NOCHILDREN, node)
                    self.controllers_num -= 1
                    doc.EndUndo()                    #----------------------------------------
                    node.SetDirty(c4d.DIRTYFLAGS_DESCRIPTION)
        return True

    def GetDDescription(self, node, description, flags):
        if not description.LoadDescription(node.GetType()):
            return False
        singleId = description.GetSingleDescID()
        groupId = c4d.DescID(c4d.DescLevel(c4d.ID_TAGPROPERTIES))
        controllers_num = self.controllers_num
        if controllers_num > 0:
            for i in range(controllers_num):
                linkId = c4d.DescID(c4d.DescLevel(LINK_NO + (i + 1)))
                if singleId is None or linkId.IsPartOf(singleId)[0]:
                    link_bc = c4d.GetCustomDataTypeDefault(c4d.DTYPE_BASELISTLINK)
                    link_bc.SetString(c4d.DESC_NAME, "Controller " + str(i + 1))
                    link_bc.SetString(c4d.DESC_SHORT_NAME, "Controller " + str(i + 1))
                    link_bc.SetInt32(c4d.DESC_ANIMATE, c4d.DESC_ANIMATE_ON)
                    if not description.SetParameter(linkId, link_bc, groupId):
                        return False
                floatId = c4d.DescID(c4d.DescLevel(FLOAT_NO + (i + 1)))
                if singleId is None or floatId.IsPartOf(singleId)[0]:
                    float_bc = c4d.GetCustomDataTypeDefault(c4d.DTYPE_REAL)
                    float_bc.SetString(c4d.DESC_NAME, "Rate")
                    float_bc.SetString(c4d.DESC_SHORT_NAME, "Rate")
                    float_bc.SetFloat(c4d.DESC_DEFAULT, 1)
                    float_bc.SetInt32(c4d.DESC_ANIMATE, c4d.DESC_ANIMATE_ON)
                    float_bc.SetInt32(c4d.DESC_UNIT, c4d.DESC_UNIT_PERCENT)
                    float_bc.SetFloat(c4d.DESC_MIN, -1000)
                    float_bc.SetFloat(c4d.DESC_MAX, 1000)
                    float_bc.SetFloat(c4d.DESC_MINSLIDER, -1)
                    float_bc.SetFloat(c4d.DESC_MAXSLIDER, 1)
                    float_bc.SetFloat(c4d.DESC_STEP, 0.01)
                    float_bc.SetInt32(c4d.DESC_CUSTOMGUI, c4d.CUSTOMGUI_REALSLIDER)
                    if not description.SetParameter(floatId, float_bc, groupId):
                        return False
                separatorId = c4d.DescID(0)
                if singleId is None or separatorId.IsPartOf(singleId)[0]:
                    separator_bc = c4d.GetCustomDataTypeDefault(c4d.DTYPE_SEPARATOR)
                    separator_bc.SetInt32(c4d.DESC_CUSTOMGUI, c4d.CUSTOMGUI_SEPARATOR)
                    separator_bc.SetBool(c4d.DESC_SEPARATORLINE, True)
                    if not description.SetParameter(separatorId, separator_bc, groupId):
                        return False
        return (True, flags | c4d.DESCFLAGS_DESC_LOADED)


if __name__ == "__main__":
    c4d.plugins.RegisterTagPlugin(id = PLUGIN_ID, str = "Test Tag", info = c4d.TAG_EXPRESSION|c4d.TAG_VISIBLE, g = TestTagData, description = "ttesttag", icon = None)

ttesttag.res

CONTAINER Ttesttag
{
    NAME Ttesttag;
    INCLUDE Texpression;

    GROUP ID_TAGPROPERTIES
    {
        GROUP TTESTTAG_BUTTONS_GROUP
        {
            COLUMNS 2;
            BUTTON TTESTTAG_BUTTON_ADD { SCALE_H; }
            BUTTON TTESTTAG_BUTTON_REMOVE { SCALE_H; }
        }
    }
}

ttesttag.h

#ifndef _TTESTTAG_H_
#define _TTESTTAG_H_

enum
{
    TTESTTAG_BUTTONS_GROUP = 1001,
    TTESTTAG_BUTTON_ADD = 1002,
    TTESTTAG_BUTTON_REMOVE = 1003,
}

#endif

ttesttag.str

STRINGTABLE Ttesttag
{
    Ttesttag "Test Tag";
    TTESTTAG_BUTTONS_GROUP "";
    TTESTTAG_BUTTON_ADD "Add";
    TTESTTAG_BUTTON_REMOVE "Remove";
}

---------- User Information ----------

Cinema 4D version: R23
OS: Windows 10
Language: Python

@Cairyn Interesting investigation but it's a bit difficult for newbie like me.
Anyway, I appreciate your help!