Fix Python – partial string formatting


Asked By – P3trus

Is it possible to do partial string formatting with the advanced string formatting methods, similar to the string template safe_substitute() function?

For example:

s = '{foo} {bar}'
s.format(foo='FOO') #Problem: raises KeyError 'bar'

Now we will see solution for issue: partial string formatting


You can trick it into partial formatting by overwriting the mapping:

import string

class FormatDict(dict):
    def __missing__(self, key):
        return "{" + key + "}"

s = '{foo} {bar}'
formatter = string.Formatter()
mapping = FormatDict(foo='FOO')
print(formatter.vformat(s, (), mapping))


FOO {bar}

Of course this basic implementation only works correctly for basic cases.

This question is answered By – Sven Marnach

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