Pass value to string imported from another module using f-string

Question

This will be easier to explain with an example. Let's say I have a settings.py file that looks like this:

settings.py

a_string='This string should contain {value} <<< here.'

I then import settings into my .py file, where I have a function mystring that looks something like this (note the function does not work as I would like it to):

import settings

def mystring(value='100'):
    b_string=f'{settings.a_string}'
    return b_string

print(mystring())
# I would like this to return:
# "This string should contain 100 <<< here."

I have tried placing the f-string formatting inside settings.py, such as:

a_string="f'This string should contain {value} <<< here.'"

to no avail. No luck if I place it in the function or function call either.

I feel like I am missing something obvious, but I just can't see it, so any pointers would be much appreciated.

You can't do that with f-strings. They are evaluated at definition time. after a = f'...' is executed, a is as "normal" as any other string — DeepSpace
– DeepSpace, Commented Feb 19, 2020 at 14:33
@DeepSpace That's true, but in this code, a_string is not an f-string, it's just a regular string containing the literal text {value}. — kaya3
– kaya3, Commented Feb 19, 2020 at 14:53
@kaya3 true, but OP specifically asked about doing this with f-string, to which I replied it's impossible (while not considering any hack involving eval or exec) — DeepSpace
– DeepSpace, Commented Feb 19, 2020 at 14:55
Thanks guys, the question is indeed around f-string, rather than other formatting ways e.g. .format() — 0buz
– 0buz, Commented Feb 19, 2020 at 14:58

bb1950328 · Accepted Answer · 2020-02-19 14:45:10Z

4

Method 1: use str.format():

b_string=settings.a_string.format(value=value)

Method 2: use eval :

fstr = "the value is {value}"
def func(value=100):
    b_str = eval('f"' + fstr + '"')
    return b_str

WARNING: Only use this method if you know what fstr contains. If fstr is something like "open('virus.txt', 'w').write('evil')", your code gets vulnerable.

edited Feb 19, 2020 at 14:45

answered Feb 19, 2020 at 14:32

bb1950328

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2 Comments

DeepSpace Over a year ago

using eval just to arbitrarily be able to use an f-string is hideous and IMHO shouldn't even be listed as a considerable 'solution'

bb1950328 Over a year ago

@DeepSpace but its more universal than the first method because you don't have to pass all the values.

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