TexText.make_number_changeable will not work properly

[Slarper]

Describe the bug

Today I try the UpdatersExample in example_scenes.py.

class UpdatersExample(Scene):
    def construct(self):

        square = Square()
        square.set_fill(BLUE_E, 1)

        # On all frames, the constructor Brace(square, UP) will
        # be called, and the mobject brace will set its data to match
        # that of the newly constructed object
        brace = always_redraw(Brace, square, UP)

        label = TexText("Width = 0.00")
        number = label.make_number_changeable("0.00", replace_all=True)

        # This ensures that the method deicmal.next_to(square)
        # is called on every frame
        label.always.next_to(brace, UP)
        # You could also write the following equivalent line
        # label.add_updater(lambda m: m.next_to(brace, UP))

        # If the argument itself might change, you can use f_always,
        # for which the arguments following the initial Mobject method
        # should be functions returning arguments to that method.
        # The following line ensures thst decimal.set_value(square.get_y())
        # is called every frame
        number.f_always.set_value(square.get_width)
        # You could also write the following equivalent line
        # number.add_updater(lambda m: m.set_value(square.get_width()))

        self.add(square, brace, label)

        # Notice that the brace and label track with the square
        self.play(
            square.animate.scale(2),
            rate_func=there_and_back,
            run_time=2,
        )
        self.wait()
        self.play(
            square.animate.set_width(5, stretch=True),
            run_time=3,
        )
        self.wait()
        self.play(
            square.animate.set_width(2),
            run_time=3
        )
        self.wait()

        # In general, you can alway call Mobject.add_updater, and pass in
        # a function that you want to be called on every frame.  The function
        # should take in either one argument, the mobject, or two arguments,
        # the mobject and the amount of time since the last frame.
        now = self.time
        w0 = square.get_width()
        square.add_updater(
            lambda m: m.set_width(w0 * math.sin(self.time - now) + w0)
        )
        self.wait(4 * PI)

and it raises IndexError: list index out of range
(I am not sure if others encounter it)

During 3 hours further exploring, I believe that it raises because the label = TexText("Width = 0.00") has a self.submobjects=[]. So the next line number = label.make_number_changeable("0.00", replace_all=True) will raise Error here in last 2 lines.

    def make_number_changeable(
        self,
        value: float | int | str,
        index: int = 0,
        replace_all: bool = False,
        **config,
    ) -> VMobject:
        substr = str(value)
        parts = self.select_parts(substr)
        if len(parts) == 0:
            log.warning(f"{value} not found in Tex.make_number_changeable call")
            return VMobject()
        if index > len(parts) - 1:
            log.warning(f"Requested {index}th occurance of {value}, but only {len(parts)} exist")
            return VMobject()
        if not replace_all:
            parts = [parts[index]]

        from manimlib.mobject.numbers import DecimalNumber

        decimal_mobs = []
        for part in parts:
            if "." in substr:
                num_decimal_places = len(substr.split(".")[1])
            else:
                num_decimal_places = 0
            decimal_mob = DecimalNumber(
                float(value),
                num_decimal_places=num_decimal_places,
                **config,
            )
            decimal_mob.replace(part)
            decimal_mob.match_style(part)
            if len(part) > 1:
                self.remove(*part[1:])
+++++    self.replace_submobject(self.submobjects.index(part[0]), decimal_mob)  <======== the part[0] will raise Error, because it has no submobjects!
            decimal_mobs.append(decimal_mob)

So how did the empty part generate?we can see parts = self.select_parts(substr)
the select_parts will fall back into select_unisolated_substring in this case, finally produced by last 2 lines of it.

    def select_unisolated_substring(self, pattern: str | re.Pattern) -> VGroup:
        if isinstance(pattern, str):
            pattern = re.compile(re.escape(pattern))
        result = []
        for match in re.finditer(pattern, self.string):
            index = match.start()
            start = self.substr_to_path_count(self.string[:index])
            substr = match.group()
            end = start + self.substr_to_path_count(substr)
+++    result.append(self[start:end]) // <====== here, the self[start:end] has no elements(submobjects)!
        return VGroup(*result)

because StringMObject's get_item_ magic function will eventually fall back into MObject's get_item magic function:

    def __getitem__(self, value: int | slice) -> Mobject:
        if isinstance(value, slice):
            GroupClass = self.get_group_class()
            return GroupClass(*self.split().__getitem__(value))
        return self.split().__getitem__(value)

    def split(self) -> list[Self]:
        return self.submobjects

and self.submobjects of label = TexText("Width = 0.00") is []. that's all.

there are 2 ways to solve problems. First, modify TexText's init function to initialize its submobjects. Second, modify StringMObject's getitem magic to return correct submobjects before it falls back into MObject's.

Code:
To reproduce the bug:

git clone https://github.com/3b1b/manim.git
cd manim
pip install -e .
manimgl example_scenes.py UpdatersExample

Wrong display or Error traceback:

Additional context