conditional string in format string (f"{...}")

Question

As you know over Python 3.6, there is a feature known as format string literals. str(obj['my_str_index']) can be either None or a string value. I've tried the below one but it doesn't yield 'null' text if it is None.

foo = "soner test " \
f"{str(obj['my_str_index']) if str(obj['my_str_index']) is not None else 'null'}

Does this answer your question? [Using f-string with format depending on a condition](https://stackoverflow.com/questions/51885913/using-f-string-with-format-depending-on-a-condition) — clubby789, Jan 15 '20 at 11:08
@JammyDodger it is what actually tried but it doesn't yield my wish. Thanks for your help by the bye. — Soner from The Ottoman Empire, Jan 15 '20 at 11:09
Do you absolutely need to do it this way? it's borderlining abusing the f-string mechanism. I think an explicit if condition will be much more readable — DeepSpace, Jan 15 '20 at 11:10
@DeepSpace highly likely in future nobody needs to touch the code, that is, it can be considered as just two-click script. — Soner from The Ottoman Empire, Jan 15 '20 at 11:11

Amadan · Answer 1 · 2020-01-15T11:29:07.857

4

str(None) is not None, but "None". So without that useless and harmful stringification:

foo = "soner test " \
f"{str(obj['my_str_index']) if obj['my_str_index'] is not None else 'null'}"

EDIT: A more legible way (note that interpolation in an f-string automatically stringifies, so we don't need str at all):

index = obj['my_str_index']
if index is None:
    index = "none"
foo = f"soner test {index}"

EDIT: Another way, with the walrus (limited to 3.8+):

foo = f"soner test {'null' if (index := obj['my_str_index']) is None else index}"

edited Jan 15 '20 at 11:29

answered Jan 15 '20 at 11:09

Amadan

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Now combine it with Python 3.8's walrus operator so `obj['my_str_index']` is not evaluated twice... – DeepSpace Jan 15 '20 at 11:12
@DeepSpace Oh, I was not even close to arguing that this is the best way to do this. :) – Amadan Jan 15 '20 at 11:13
@Armadan if you have better idea, you can post it sir. I can learn ((: – Soner from The Ottoman Empire Jan 15 '20 at 11:13
@DeepSpace if you think there is a better way, I'm very glad to learn, thanks. – Soner from The Ottoman Empire Jan 15 '20 at 11:14
`index = obj['my_str_index'] or "none"` – Chris_Rands Jan 15 '20 at 11:22
@Chris_Rands If this was Ruby, that would have been my answer. In Python, too many things that are not `None` register as falsy. – Amadan Jan 15 '20 at 11:23

DeepSpace · Answer 2 · 2020-01-15T11:51:06.913

You can greatly simplify the condition. This will work (in most cases, see the caveat below) and in my opinion a bit more readable

foo = f"soner test {obj['my_str_index'] or 'null'}"

You don't have to worry about str(...) because the interpolation mechanism calls the objects __str__ method implicitly (if it does not have __format__).

The only caveat with this approach is that foo will contain null if obj['my_str_index'] is any of the falsey values (None, 0 and any empty sequence).

conditional string in format string (f"{...}")

2 Answers2