What part are you trying to delay, the expression evaluations, or the string building part?

There was a recent discussion on python-ideas starting at https://mail.python.org/archives/list/python-ideas@python.org/message/LYAC7JC5253QISKDLRMUCN27GZVIUWZC/ that might interest you.

Eric

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.

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Eric V. Smith