[issue21542] pprint doesn't work well for counters, sometimes shows them like a dict

[akira]

akira added the comment:

If it fits on a line then it seems Counter's repr is used:

  >>> pprint(Counter({i:i*i for i in range(10)}))
  Counter({9: 81, 8: 64, 7: 49, 6: 36, 5: 25, 4: 16, 3: 9, 2: 4, 1: 1, 0: 0})

Otherwise It is shown as a dict (Counter is a dict subclass) if it is too 
large (multi-line):

  >>> pprint(Counter({i:i*i for i in range(10)}), width=20)
  {0: 0,
   1: 1,
   2: 4,
   3: 9,
   4: 16,
   5: 25,
   6: 36,
   7: 49,
   8: 64,
   9: 81}
 
the behaviour is weird but pprint doesn't promise that custom objects such 
as Counter that can't be created using Python literals will be printed in a 
reversible manner.

It seems there is a special support for some objects:

  >>> pprint(frozenset({i for i in range(10)}))
  frozenset({0, 1, 2, 3, 4, 5, 6, 7, 8, 9})
  >>> pprint(frozenset({i for i in range(10)}), width=20)
  frozenset({0,
             1,
             2,
             3,
             4,
             5,
             6,
             7,
             8,
             9})

Perhaps the support for Counter could be added using functools.singledispatch
and/or __prettyprint__ hook from issue #7434

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nosy: +akira

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<http://bugs.python.org/issue21542>
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