Can custom formatting through the % operator be done in Python?

Question

Is it possible to have a class be formatted in its own specific way with the % operator, in Python? I am interested in a case where the format string would be something like %3z (not simply %s or %r, which are handled by the __str__() and __repr__() methods). Said differently: Python 2.6+ allows classes to define a __format__() method: is there an equivalent for the % operator?

I tried to define the __rmod()__ method, hoping that str.__mod__() would return NotImplemented and that __rmod()__ would be called, but "%3z" % … returns ValueError: unsupported format character 'z' instead…

I'm pretty sure this can't be done. Is there any reason why you're avoiding using format? — mgilson
– mgilson, Commented May 7, 2013 at 12:57
There is a reason: the formatting will be applied to the numbers with uncertainty of my uncertainties package, which works with all version of Python starting with 2.3 (versions 2.3 to 2.5 only have % and not format()). — Eric O. Lebigot
– Eric O. Lebigot, Commented May 9, 2013 at 3:37

lvc · Accepted Answer · 2013-05-07 13:47:32Z

2

There isn't. This customizability is one of the major benefits of the format method over % formatting. If it wasn't novel, PEP 3101 wouldn't need to discuss this aspect of it in so much detail.

If you need to support older versions of Python than have new-style string formatting, the best you can do is to implement a custom conversion function on your class, and expect clients to call it like this:

'%4f %s' % (5, myobj.str('<'))

answered May 7, 2013 at 13:47

lvc

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Community · Accepted Answer · 2017-05-23 10:25:45Z

-1

You might find this helpful: operator overloading in python

__mod__ and __rmod__ are included here: http://docs.python.org/2/reference/datamodel.html#emulating-numeric-types

Don't bother trying to reinvent Python's string formatting mini-language...use it to your advantage: http://docs.python.org/2/library/string.html#format-specification-mini-language

EDIT: you probably got at least this far, but since we're here to code:

class moddableObject(object):
    def __mod__(self, arg)
        return 'got format arg: {}'.format(arg)

moddable = moddableObject()
print moddable % 'some string'

edited May 23, 2017 at 10:25

CommunityBot

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answered May 7, 2013 at 13:06

mdscruggs

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2 Comments

lvc Over a year ago

OP clearly already knows how operator overloading works - he has specifically tried implementing moddableObject.__rmod__ to find that 'some string' % moddable doesn't call it (since str.__mod__ raises an exception rather than returning NotImplemented).

mdscruggs Over a year ago

hence my comment, "you probably got at least this far, but since we're here to code"...I put it there so others could see a quick example (and so I could try it out myself).

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