Tried to use "in" statement to check if a string exists in a array and failed using string format.

How to reproduce:

~/ ipython
Python 3.7.4 (default, Oct 12 2019, 18:55:28)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.8.0 -- An enhanced Interactive Python. Type '?' for help.

In [2]: "a" in ["a", "b"]
Out[2]: True

In [4]: z = "a"

In [5]: f"{z}" in ["a", "b"]
Out[5]: True

In [6]: z = "b"

In [7]: f"{z}" in ["a", "b"]
Out[7]: True