CODEWITHSUNDEEP
pythonf-string
Setop
Setop

Reputation: 2510

dynamically load f-string format

I'd like to build a tool where fstring formats are stored in a configuration file.

config = load_config()

def build_fstring(str):
  return ...   # <-- issue is there

chosen_format = config.get("chosen_format")  # returns '{k},{v}'

fstring = build_fstring(chosen_format) # may return something like 'f"{k},{v}"'

for (k,v) in d.items():
  print(fstring)  # fstring is evaluated here

My issue is that fstring is compiled before variables are known.

Is there a way to do it ?

Upvotes: 3

Views: 1083

Answers (2)

blhsing
blhsing

Reputation: 107124

According to PEP-498, f-strings are meant to "provide a way to embed expressions inside string literals", which means that f-strings are first and foremost string literals, and that trying to evaluate the value of a variable as an f-string defeats its very purpose.

For your purpose of using a variable as a string formatting template, it would be easier to use the str.format method instead:

k, v = 1, 2
chosen_format = '{k},{v}'
print(chosen_format.format(**locals()))

This outputs:

1,2

Upvotes: 6

Setop
Setop

Reputation: 2510

I found a trick using eval:

config = load_config()

chosen_format = config.get("chosen_format")  # returns '{k},{v}'

for (k,v) in d.items():
  print(eval('f"'+chosen_format+'"'))

But I not very satisfied by the cleanness of the code - print should not know how to build the content - and the performance of evaluating code for each record.

Upvotes: 0

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