Python: Is it possible to use the string.format method without arguments?

Question

Just curious: Is there a way to (ab)use the string.formatmethod without any arguments?

Say I want a string like this:

'foo   bar    '   # Desired string

I could create a template string like:

'foo{:3}bar{:4}'

And then use it with dummy parameters like:

>>> 'foo{:3}bar{:4}'.format('', '')
'foo   bar    '
>>> 'foo{:3}bar{:4}'.format(*'  ')
'foo   bar    '

Since omitting parameters will give this error:

>>> 'foo{:3}bar{:4}'.format()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: tuple index out of range

I can even create a single dummy variable like:

'foo{s:3}bar{s:4}'.format(s='')

But is there an even more elegant way to create parameterized strings using string.format?

It's reasonable, but in my example the template already has all required formatting information. Moving part of the formatting to the variables doesn't improve readability in my opinion. This question is of course a bit nitpicky already as there are plenty of options to solve the actual problem. — bohrax
– bohrax, Commented Sep 13, 2018 at 20:06
Maybe not quite what you are looking for, but in Python 3 you could do f'foo{" "*3}bar{" "*4}'. — 0x5453
– 0x5453, Commented Sep 13, 2018 at 20:06
@0x5453 It's better for readability I think, but unfortunately I haven't the luxury of f-strings yet. — bohrax
– bohrax, Commented Sep 13, 2018 at 20:09

user2357112 · Accepted Answer · 2018-09-13 20:16:48Z

2

Literally speaking, yes, it's possible to use str.format without arguments:

>>> 'asdf'.format()
'asdf'

No replacement fields, no arguments necessary. That's not what you're looking for, though.

If you want to pass empty strings for everything, that's not possible with positional replacement fields. It's possible with named replacement fields and format_map:

>>> class Defaults(object):
...     def __getitem__(self, key):
...         return ''
... 
>>> 'asdf {a} fdsa {b}'.format_map(Defaults())
'asdf  fdsa '

We also could have used collections.defaultdict(str) instead of writing our own class, but for this use case, I don't like how defaultdict would unnecessarily populate a (default)dict.

If you really want to do it with positional replacement fields, the best you can do is pass a ton of arguments and hope you have enough for every field. str.format will ignore extra arguments:

>>> 'asdf {} fdsa {}'.format(*['']*100)
'asdf  fdsa '

If you're okay with using a slightly different formatting API, string.Formatter lets subclasses customize value retrieval:

import string
class MyFormatter(string.Formatter):
    def get_value(self, key, args, kwargs):
        # Yes, no * or ** on args or kwargs. That's what the signature looks like.
        try:
            return super(MyFormatter, self).get_value(key, args, kwargs)
        except (IndexError, KeyError):
            return ''

Demo:

>>> MyFormatter().format('asdf {} fdsa {}')
'asdf  fdsa '

edited Sep 13, 2018 at 20:16

answered Sep 13, 2018 at 20:08

user2357112

286k32 gold badges490 silver badges569 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

bohrax Over a year ago

Thanks, I believe that you answered both my literal question and the actual one. string.Formatter seems like the most elegant solution to me.

Collectives™ on Stack Overflow

Python: Is it possible to use the string.format method without arguments?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related