Analogous of %= using new style interpolation

Question

Consider the strings

string_0 = '%s test'
string_1 = '{} test'

Imagine now the desired output is to return the formatted string using the variable x = 'A'.

For the first case there is an easy and elegant solution:

string_0 %= x
print(string_0)
# A test

Is there something similar for the second case? E.g.

string_1 f= x    # Unfortunately does not work
print(string_1)
# A test

Addressing the comments/responses

I realise string_1 is not an f-string but Python won't allow f-strings with empty expression
I know f'{x} test' works perfectly well but requires knowing x before the creation.
The format solutions is also something I'm familiar with but when comparing s = s.format(x) to s %= x I find not very smooth

PS Anticipating the responses, I wanted to edit the above part in my question but was not able due to the simultaneous edit.

If you have the joy of using modern python versions (3.6 and beyond), just do `string_2 = f"{x} test"` — byxor, Oct 25 '19 at 10:32
@MadPhysicist Well Yeah: `SyntaxError: f-string: empty expression not allowed` — user101, Oct 25 '19 at 10:33
@carlsegg97. It's not a dupe. Three title similarity is a mistake in nomenclature on OP's part. — Mad Physicist, Oct 25 '19 at 10:43
@MadPhysicist Yeah I'm OP :D And what I meant is I believe you misunderstood the question slightly and hence modified the title: `string_1` is not an f-string because something like `string_1 = f'{} test'` will yield the above exception — user101, Oct 25 '19 at 10:47
@user101. Sorry for the misunderstanding. I looked at your code instead of the title. I'll go get some sleep now. — Mad Physicist, Oct 25 '19 at 10:49
@user101. Either way, the answer seems to be to use `str.format` instead of f-strings. May I close your question then? — Mad Physicist, Oct 25 '19 at 10:50

score 1 · Answer 1 · answered Oct 25 '19 at 10:34

1

New-style string formatting doesn't use an operator and therefore doesn't have an __i*__-method overloaded (as old-style formatting does with __imod__).

You could directly use an f-string:

string_1 = f"{x} test"

Other than that, you'll have to be explicit and use

string_1 = string_1.format(x)

answered Oct 25 '19 at 10:34

L3viathan

26,748
2
58
81

Thanks, the `__i*__` section helped me understanding it better – user101 Oct 25 '19 at 10:42

Mad Physicist · Answer 2 · 2019-10-25T10:41:47.590

1

Since strings are immutable, a %= b is pretty much exactly equivalent to a = a % b. The new style equivalent to the old style % operator is the format method:

string_1 = string_1.format(x)

edited Oct 25 '19 at 10:41

answered Oct 25 '19 at 10:34

Mad Physicist

107,652
25
181
264

Analogous of %= using new style interpolation

2 Answers2