subclass string.Formatter

Question

Following a remark here: How to define a new string formatter, I tried subclassing string.Formatter. Here is what I\'ve done. Unfortunately I seem to have broke

Answer 1

str.format does auto numbering, while string.Formatter does not.

Modifying __init__ and overriding get_value will do the trick.

def __init__(self):
    super(CustFormatter, self).__init__()
    self.last_number = 0

def get_value(self, key, args, kwargs):
    if key == '':
        key = self.last_number
        self.last_number += 1
    return super(CustFormatter, self).get_value(key, args, kwargs)

BTW, above code does not strictly mimic str.format behavior. str.format complains if we mix auto numbering with manual number, but above does not.

>>> '{} {1}'.format(1, 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: cannot switch from automatic field numbering to manual field specification
>>> '{0} {}'.format(1, 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: cannot switch from manual field specification to automatic field numbering

Answer 2

Good news: you have not done anything wrong. Bad news: that's how string.Formatter behaves, it does not support {}-like positional format. So, the last call will fail even without any subclassing. Good news: that's can be fixed by overriding the parse method:

import string

class CF(string.Formatter):
    def parse(self, s):
        position = 0
        for lit, name, spec, conv in super(CF, self).parse(s):
            if not name:
                name = str(position)
                position += 1
            yield lit, name, spec, conv

Bad news... Ah, no, that's basically it:

>>> CF().format('{} {}!', 'Hello', 'world')
'Hello world!'