How can I set a python f-string format programmatically?

Question

I would like to set the formatting of a large integer in an fstring programmatically. Specifically, I have an integer that can span several orders of magnitude and I'd like to print it using the underscore "_" delimiter, e.g.

num = 123456789
print(f"number is {num:9_d}")
>>> number is 123_456_789   # this is ok

num = 1234
print(f"number is {num:9_d}")
>>> number is     1_234   # this has extra spaces due to hand-picking 9

How can I set the number preceding "_d" programmatically based on the number of digits in num? (Or in case I asked an XY question, how can I print the number with underscore delimiters and no extra spaces?)

Alternatively, you can left justify the number with {num:<9_d}, though this will leave trailing spaces on the right side. — Michael Cao
– Michael Cao, Commented Jul 25, 2024 at 16:39
I'm mostly just informing on f-string parameters that OP may not be aware of. — Michael Cao
– Michael Cao, Commented Jul 25, 2024 at 16:45
Ooh I didn't know I could just have dropped the length specifier 😅. Thank you. — Ziofil
– Ziofil, Commented Jul 25, 2024 at 16:52

KamilCuk · Accepted Answer · 2024-07-25 16:42:00Z

2

Like so:

v = 9
print(f"number is {num:{v}_d}")

answered Jul 25, 2024 at 16:42

KamilCuk

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inspectorG4dget · Accepted Answer · 2024-07-25 16:44:20Z

1

You could drop the length parameter altogether:

In [290]: num = 123456789

In [291]: print(f"number is {num:_d}")
number is 123_456_789

In [293]: num = 1234

In [294]: print(f"number is {num:_d}")
number is 1_234

But to answer the question as asked, you could format format string

In [295]: s = "number is {num:%d_d}" %len(str(num))  # the `%d` is used to format the length parameter, while the `_d` remains an f-string construct

In [297]: print(s.format(num=num))
number is 1_234

In [298]: print(s.format(num=123456789))
number is 123_456_789

Hope this helps

answered Jul 25, 2024 at 16:44

inspectorG4dget

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2 Comments

STerliakov Over a year ago

"Format format string" approach looks terrific, especially given that you can use variables in format specification...

olq_plo Aug 3 at 10:03

Downvoted, because the question was how to do this with f-strings, not with format.

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