How to create immutable dictionary that won't change its values even if they changed out of dictionary score?

Question

I have a group of functions whose values are pandas dataframe that change over time when updating the dataframes and these functions are initially stored in a dictionary which we use when needing a specific function that will return the wanted dataframe.

What I need is a way that will keep these data unchanged (fixed) across time even after the update. I tried MappingProxyType to create an immutable dictionary but it didn't help. Here's my trial

from types import MappingProxyType

def func_x():
    x = pd.DataFrame({"col1": [1,2,3], "col2": [4,5,6]})
    return x


def func_y():
    y = pd.DataFrame({"col3": [7,8,9], "col4": [10,20,30]})
    return y


def func_z():
    z = pd.DataFrame({"col5": [40,50,60], "col6": [70,80,90]})
    return z


def collect_funcs_proxy(func):
    collection_proxy = MappingProxyType(
        {"func_x": func_x, "func_y": func_y, "func_z": func_z}
    )
    return collection_proxy[func]()

I mean, when changing the values of x inside func_x, I need collect_funcs_proxy to keep the old values of func_x ignoring the new update.

Is there any way I can achieve this using MappingProxyType or something else?