f-string behaves differently then the format method for nested formatting

Question

I am doing nested formatting using format and it doesn't work as expected. however if i try to do it with f-strings it works perfectly.

example:

doing this:

values = 'first', 'second', 'third'
a = f"""cardinality and values: {'|'.join(f'val:{val}, card:{i}'for i, val in enumerate(values))} """

gives me this (the needed result):

'cardinality and values: val:first, card:0|val:scond, card:1|val:third, card:2'

however if i try to do it using format:

a = """cardinality  and values: {'|'.join('val:{val}, card:{i}'.format(val=val, i=i) for i, val in enumerate(values))} """.format(values=values)

I get the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: "'|'"

how do i do this with format ?

I need to use format instead of f-strings because another file imports this string and then formats it.

Answer 1

format is just less powerful than f-strings, so you have to simplify the format string.

Just get the comprehension from outside the format string:

a = "cardinality  and values: {} ".format('|'.join('val:{val}, card:{i}'.format(val=val, i=i) for i, val in enumerate(values)))

result:

cardinality  and values: val:first, card:0|val:second, card:1|val:third, card:2 

If you want to make a one-line "template", you could use a lambda:

a = lambda v : "cardinality  and values: {} ".format('|'.join('val:{val}, card:{i}'.format(val=val, i=i) for i, val in enumerate(v)))

now calling

print(a(values))

invokes the lambda which performs the formatting with the passed values. That's the closest to f-strings without f-strings that I can think of.