On Thu, 24 Jun 2021 at 17:25, Eric Nieuwland wrote:

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)

This isn't quite right as the semantics between f-strings and str.format() are not actually the same (though this isn't well documented): >>> f'{1 + 2}' '3' >>> str(DelayedFString('{1 + 2}')) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in __str__ KeyError: '1 + 2' >>> d = dict(a=1) >>> f'{d["a"]}' '1' >>> str(DelayedFString('{d["a"]}')) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in __str__ KeyError: '"a"' Basically, f-strings rely on eval-like semantics. Martin

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/LYAC7J... that might interest you. Eric On 6/24/2021 5:37 AM, Eric Nieuwland wrote:

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-evalu... https://stackoverflow.com/questions/42497625/how-to-postpone-defer-the-evalu... 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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I didn’t make myself clear, sorry. The code example was just to give an initial what I was suggesting. The errors you show demonstrate the example is incomplete. I’d like them to work.

On 24 Jun 2021, at 18:37, Martin (gzlist) wrote:

On Thu, 24 Jun 2021 at 17:25, Eric Nieuwland wrote:

...

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)

This isn't quite right as the semantics between f-strings and str.format() are not actually the same (though this isn't well documented):

...

...

...

f'{1 + 2}' '3' str(DelayedFString('{1 + 2}')) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in __str__ KeyError: '1 + 2'

...

...

...

d = dict(a=1) f'{d["a"]}' '1' str(DelayedFString('{d["a"]}')) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in __str__ KeyError: '"a"'

Basically, f-strings rely on eval-like semantics.

Martin

As pointed out already, f-strings and format are subtly different (not counting that one can eval and the other cannot). Besides quoting, the f-sting mini language has diverged from format's

...

...

spam="Spam" f"{spam=}" "spam='Spam'" "{spam=}".format(spam=spam) Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'spam='

I created a package some time ago to do exactly this https://pypi.org/project/f-yeah/ On Thu, Jun 24, 2021 at 10:07 AM Luciano Ramalho wrote:

I don't think that would be a good idea since we already have .format() which covers that use case and is more flexible than f-strings (it supports positional arguments, as well as *args and **kwargs).

I think keeping f-strings simple is a better idea.

Best,

Luciano

On Thu, Jun 24, 2021 at 1:30 PM Eric Nieuwland wrote:

...

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-evalu... 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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https://mail.python.org/archives/list/python-dev@python.org/message/GT5DNA7R...

...

Code of Conduct: http://python.org/psf/codeofconduct/

-- Luciano Ramalho | Author of Fluent Python (O'Reilly, 2015) | http://shop.oreilly.com/product/0636920032519.do | Technical Principal at ThoughtWorks | Twitter: @ramalhoorg _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/LVPDNGUR... Code of Conduct: http://python.org/psf/codeofconduct/

24.06.21 12:37, Eric Nieuwland пише:

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 ‘%’.

You can just use lambda: delayed_fstring = lambda: f"The current name is {name}" print(delayed_fstring()) If you want to get rid of () when convert to string, you can use a wrapper: class LazyStr: def __init__(self, str_func): self._str_func = str_func def __str__(self): return self._str_func() delayed_fstring = LazyStr(lambda: f"The current name is {name}") print(delayed_fstring) but it works only if str() is directly or indirectly called for the value. You cannot pass it open() as a file name, because open() expects only str, bytes, integer of path-like object. Most builtin functions will not and could not work with LazyStr. As for evaluating variables in different scope as in your example, it just does not work in Python. Python uses static binding, not dynamic binding. First than introducing "delayed f-strings" you need to introduce dynamic binding in normal functions.