Question

Please can someone explain why I get the following error when using %r with tuples?

>>> repr((1,2))
'(1, 2)'
>>> class Foo(object):
...     def __init__(self,vals):
...             self.vals=vals
...     def __repr__(self):
...             return "Foo(%r)" % self.vals
... 
>>> foo = Foo((1,2))
>>> foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __repr__
TypeError: not all arguments converted during string formatting

What is the appropriate way for printing out __repr__? Should I be using %s and repr(self.vals) instead?

Answer 1

The % operator takes a tuple itself, so you are basically doing this:

'Foo(%r)' % (1, 2)

Wrap self.vals in a one-element tuple:

'Foo(%r)' % (self.vals,)

In principle you can use a string defined in a variable too:

REPRFORMAT % (self.vals,)

in which case you want to leave it up to that variable (perhaps taken from a configuration file?) how to format self.vals, be it %r or %s.

You could also use the .format() method instead:

return 'Foo({0!r})'.format(self.vals)

This format gives you more flexibility with the input given; you could address individual items in the vals tuple, for example:

return 'Foo(({0[0]:04d}, {0[1]:02d}))'.format(self.vals)

which would result in Foo((0001, 04)) for your example input.