How can I add spaces around a sub-string using Python formatted strings `(f'...')` without first creating a new variable?

Question

Suppose, I want to convert the following .format()'ed string to a f'...' expression:

self.logger.debug('{:10} -- {}'.format('Jira', 'Scan: {} '.format(self.scan_id)))

I can do it easily as:

self.logger.debug(f'Jira      -- {self.scan_id}`)

However, I don't want to add the spaces (width) around 'Jira' manually. How can I do that without first having to create a new variable as in:

s='Jira'
self.logger.debug(f'{s:10} -- {self.scan_id}`)

?

@bereal - Then I would need to subtract the length of `'Jira'` from 10 to achieve the original result. — Shuzheng, May 11 '20 at 13:44
Side note, you should avoid doing string formatting directly when logging. They end may up doing unnecessary work that is discarded rather than emitted. For instance, if the logging level was set to INFO. — Dunes, May 11 '20 at 14:11
@Shuzheng Added response as an answer -- https://stackoverflow.com/a/61746377/529630 — Dunes, May 12 '20 at 07:48

Asocia · Answer 1 · 2020-08-12T08:57:14.243

7

You can directly put your string in curly brackets like this:

self.logger.debug(f'{"Jira":10} -- {self.scan_id}')

Because an f-string expects an expression inside {} and a string itself is a valid expression.

edited Aug 12 '20 at 08:57

answered May 11 '20 at 13:43

Asocia

5,935
2
21
46

score 2 · Answer 2 · answered May 12 '20 at 07:47

2

You should generally avoid doing string formatting directly in logging calls as it can waste time formatting strings that are never emitted. Instead you should prefer to use the inbuilt formatting of the logger.

An example of how you might achieve your logging call would be:

logger.debug('% -10s -- %s', 'Jira', self.scan_id)

By default, the logging module uses percent formatting (documentation). If you are more familar with brace formatting (ie.using {}) and would prefer to use that, then this answer shows you how to set it up.

answered May 12 '20 at 07:47

Dunes

37,291
7
81
97

Can't I just set the formatting using the `style='%'` parameter of `logging.Formatter.__init__`? – Shuzheng May 12 '20 at 08:41
No, because that refers to the formatter template. The linked question is about getting brace formatting working with the message template. The formatter template refers to the entire line that gets emitted, of which the message template is a part of. The formatter template might include things like level, time, logger name, etc... eg. `logging.Formatter('{asctime} {name} {levelname:8s} {message}', style='{')`. The `message` template in this answer would be `'% -10s -- %s'`. – Dunes May 12 '20 at 11:12

How can I add spaces around a sub-string using Python formatted strings `(f'...')` without first creating a new variable?

2 Answers2