In a recent discussion with a colleague we wondered if it would be possible to postpone the evaluation of an f-string so we could use it like a regular string and .format() or ‘%’.

I found https://stackoverflow.com/questions/42497625/how-to-postpone-defer-the-evaluation-of-f-strings and tweaked it a bit to:

import inspect

class DelayedFString(str):
def __str__(self):
vars = inspect.currentframe().f_back.f_globals.copy()
vars.update(inspect.currentframe().f_back.f_locals)
return self.format(**vars)

delayed_fstring = DelayedFString("The current name is {name}")

# use it inside a function to demonstrate it gets the scoping right
def new_scope():
names = ["foo", "bar"]
for name in names:
print(delayed_fstring)

new_scope()

While this does what it should it is very slow.

So I wondered whether it would be an idea to introduce d-strings (delayed f-strings) and make f-strings syntactic sugar for

f"The current name is {name}" = str(d"The current name is {name}")

And perhaps access to the variables and conversions specified in the d-string.