[REF-3225] implement __format__ for immutable vars

[adhami3310]

For new-style immutable vars, the format string mechanism will use a tagged marker that can be used to reconstruct the original Var from the formatted string.

[linear]

REF-3225 `ImmutableVar.__format__` should save a reference to the current Var instead of serializing the `VarData`

[masenf]

code looks good, going to test it now

[masenf]

Currently the ImmutableVar cannot be created from an f-string with vars inside of it

import reflex as rx


class S(rx.State):
    foo: str = "foo"


# Create an new style Var with data.
iv = rx._x.vars.ImmutableVar.create("foo", _var_data=S.foo._var_data)

# Create an f-string using the new style Var.
f_s = f"{iv}bar"

# Create an old style Var from the f-string.
n_v = rx.Var.create(f_s)
print(n_v, repr(n_v))

# Create a new style Var from the f-string.
n_iv = rx._x.vars.ImmutableVar.create(f_s)
print(n_iv, repr(n_iv))

It seems there is a bit of code in rx.Var.__post_init__ which is trying to assign to self._var_name and self._var_data, but this does not work for immutable instances.

Instead we can change the code to call __init__ again with the updated values, and this should be fine in nearly every case. It sort of breaks immutability, but if we consider __post_init__ to be just an extension of __init__, then it makes sense.

diff --git a/reflex/vars.py b/reflex/vars.py
index ad7c223a..f55aa025 100644
--- a/reflex/vars.py
+++ b/reflex/vars.py
@@ -502,8 +502,10 @@ class Var:
         # Decode any inline Var markup and apply it to the instance
         _var_data, _var_name = _decode_var(self._var_name)
         if _var_data:
-            self._var_name = _var_name
-            self._var_data = VarData.merge(self._var_data, _var_data)
+            self.__init__(
+                _var_name=_var_name,
+                _var_data=VarData.merge(self._var_data, _var_data),
+            )
 
     def _replace(self, merge_var_data=None, **kwargs: Any) -> BaseVar:
         """Make a copy of this Var with updated fields.

changed post_init behavior in a different file