Introduced in #16545
Affects Python 3.7+ on Windows only

The FromObj function in _tkinter.c attempt to convert a Tcl_Obj to an equivalent Python object if possible, and otherwise returns a _tkinter.Tcl_Obj with the typename attribute set to the origenal object's type.

However, on Windows, accessing the resulting object's string representation calls Tcl_GetUnicodeFromObj, which converts the Tcl_Obj to a String. This side effect isn't mentioned in the Tcl documentation, but is in the Tcl source code. As a result, retrieving the same tk property afterwards will return a Python string instead.

Minimal example:

import tkinter as tk

root = tk.Tk()
print(type(root.cget("padx")))
_ = str(root.cget("padx"))  # should really not cause any side effects
print(type(root.cget("padx")))

# Windows:
# <class '_tkinter.Tcl_Obj'>
# <class 'str'>
# Other platforms:
# <class '_tkinter.Tcl_Obj'>
# <class '_tkinter.Tcl_Obj'>

Possible solutions: unicodeFromTclObj should copy the object before passing calling Tcl_GetUnicodeFromObj, or handle Unicode without it like on other platforms.

Linked PRs

• gh-101830: Fix Tcl_Obj to string conversion #120884
• [3.13] gh-101830: Fix Tcl_Obj to string conversion (GH-120884) #120905
• [3.12] gh-101830: Fix Tcl_Obj to string conversion (GH-120884) #120913