String format with optional dict key-value

user_1539813 0
Viewed 13535

Is there any way to format string with dict but optionally without key errors?

This works fine:

opening_line = '%(greetings)s  %(name)s !!!'
opening_line % {'greetings': 'hello', 'name': 'john'}

But let's say I don't know the name, and I would like to format above line only for 'greetings'. Something like,

 opening_line % {'greetings': 'hello'}

Output would be fine even if:

'hii %(name)s !!!'  # keeping name un-formatted

But this gives KeyError while unpacking

Is there any way?

missing-data string-formatting dictionary python
5 Answers

I faced the same problem as yours and decided to create a library to solve this problem: pyformatting.
Here is the solution to your problem with pyformatting:

>>> from pyformatting import optional_format
>>> opening_line = '{greetings}  {name} !!!'
>>> optional_format(opening_line, greetings='hii')
'hii  {name} !!!'

The only problem is pyformatting doesn't support python 2. pyformatting supports python 3.1+ If i see any feedback on the need for 2.7 support i think i will add that support.

user_121419490

You could subclass UserDict and customize __missing__ to your liking with .format_map():

from collections import UserDict


class FormatMapper(UserDict):
    def __missing__(self, key):
        return f'{key=} is MISSING'


info = FormatMapper({'greetings': 'hello', 'not_name': 'john'})
opening_line = '{greetings}  {name} !!!'

print(opening_line.format_map(info))

Output:

hello  key='name' is MISSING !!!
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