"True" f-strings can only be literal strings within a python file, so for the sake of semantics, what you have is actually a regular string which you're calling .format on. If you want the same functionality of "true" f-strings (pulling local variables for the {replacement_values}), you need to give the format method a dictionary of those values, which can be easily obtained with the builtin functions: locals(), and globals().

This brings up one of the age-old questions of programming: Are global variables bad? In short... they have their uses, but if you use them as a crutch, sometimes it will break. The problem you mentioned in the comments is exactly one such example. You presumably had variable definitions for this template sprinkled throughout your code, and one of them had a slightly different name, or you missed one that was supposed to be filled in. This is why I would actually recommend you don't use globals or locals in favor of creating your own dictionary of inputs. This is basically what you already had in your question, but there are a few ways to clean up the text to make it not look so bad in you .py file:

1. Honestly, just leave it as you had it, or just split the args to format over multiple lines. There's nothing wrong with a long function args section, and it's extremely clear what the intent is.

with open('my_template.txt') as f:
    filled_template = f.read().format(
        name_string=name_string, 
        firstname_string=firstname_string,
        # ...
    )

2. Create your own dictionary of input values which you can then pass and unpack into format. This is helpful if you have other important things to do next to the line where you're filling the template, and you don't want them to get visually lost.

f_args = {}

f_args["name_string"] = input("enter a name")
f_args["firstname_string"] = f_args["name_string"].split()[0] #extract the first name from a full name

with open('my_template.txt') as f:
    filled_template = f.read().format(**f_args) #unpack the args dict into the `format` call

#### or if you already have the values, and just want to collect them...

f_args = {
    "name_string":name_string, 
    "firstname_string":firstname_string,
    # ...
}