How to interpolate a list into an f-string in Python?

Question

Say I have a function

def foo(): return [1, 2, 3]

I want to interpolate the result of the function into a string to get "001 002 003". I've tried this:

f"{*foo():03d 03d 03d}"

But it produced SyntaxError: can't use starred expression here. Can I do this using an f-string?

The suggestion works, but the f-string is redundant and the syntax verbose. One can just use the similar answer by @ev-kounis, without f-string. It seems f-strings do not improve the printing of lists. — divenex, Feb 20 '18 at 09:52

score 7 · Answer 1 · answered Jan 27 '17 at 14:38

7

is this what you are looking for?

str_repr = ' '.join(map('{:03d}'.format, foo()))
print(str_repr)  # prints: 001 002 003

maybe the best thing about this solution is that it works with any list length and with minimal tweaking you can change the output format too.

answered Jan 27 '17 at 14:38

Ma0

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This works, but it does **not** make use of f-strings as requested. It may indeed be the best available Python solution, but it is verbose. I wish something like this `f"{foo():03d}"` would thread over the list. – divenex Feb 20 '18 at 11:07

Eugene Yarmash · Answer 2 · 2019-12-12T20:59:06.583

6

Starred expressions are only allowed in few specific contexts, such as function calls, list/tuple/set literals, etc. An f-string placeholder is not one of them, apparently. You could format each element individually and join the strings, e.g.:

lst = foo()
s = ' '.join(f'{x:03d}' for x in lst)  # '001 002 003'

Generally, to format multiple values you have to use a separate placeholder for each value.

edited Dec 12 '19 at 20:59

answered Jan 27 '17 at 14:39

Eugene Yarmash

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L3viathan · Answer 3 · 2017-01-29T21:04:29.017

2

The * operator (similar rules exist for **) can only be used inside:

a function call: foo(*bar)
a list, tuple, or set literal: ['foo', *bar]
an assignment: foo, *bar = range(10)

It is not an expression and can therefore not be used inside the braces of f-strings.

edited Jan 29 '17 at 21:04

answered Jan 27 '17 at 14:43

L3viathan

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score 1 · Answer 4 · answered Mar 12 '19 at 14:10

1

You can also use the zfill method instead:

s_repr=" ".join([str(x).zfill(3) for x in foo()]) #not very PEP8 =S

answered Mar 12 '19 at 14:10

user11191688

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score 0 · Answer 5 · answered Feb 20 '18 at 14:13

For lists, f-strings do not seem to add much to what was already possible without them. An alternative, slightly simpler, solution without f-strings and without .join, which works for any list length, is the following

a = foo()
str_repr = ("{:03d} "*len(a)).format(*a)
print(str_repr)  # prints: 001 002 003

How to interpolate a list into an f-string in Python?

5 Answers5