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It was my understanding that python will print the repr of the output, but this is apparently not always the case. For example:

In ipython:

In [1]: type([])
Out[1]: list

In [2]: set([3,1,2])
Out[2]: {1, 2, 3}

In python:

>>> type([])
<type 'list'>
>>> set([3,1,2])
set([1, 2, 3])

What transformation does ipython apply on the output?

wim
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1 Answers1

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Instead of repr or standard pprint module IPython uses IPython.lib.pretty.RepresentationPrinter.pretty method to print the output.

Module IPython.lib.pretty provides two functions that use RepresentationPrinter.pretty behind the scenes.

IPython.lib.pretty.pretty function returns the string representation of an object:

>>> from IPython.lib.pretty import pretty
>>> pretty(type([]))
'list'

IPython.lib.pretty.pprint function prints the representation of an object:

>>> from IPython.lib.pretty import pprint
>>> pprint(type([]))
list

IPython uses its own pretty printer because the standard Python pprint module "does not allow developers to provide their own pretty print callbacks."

Nigel Tufnel
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    Is it documented somewhere that `IPython.lib.pretty.pretty` is indeed the function which is used? I did `IPython.lib.pretty.pretty = lambda *args, **kwargs: 'potato'` and there was no change in the output within ipython. – wim Jan 14 '14 at 11:30
  • @wim, good catch, I've edited my answer. `IPython.lib.pretty.RepresentationPrinter.pretty = lambda *args, **kwargs: sys.stdout.write('12')` will print 12 for any input. – Nigel Tufnel Jan 14 '14 at 12:05
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    note that this is only the case if the `%pprint` magic is ON. I have it turned off by default, and get the behavior the OP is seeing at the regular Python prompt. – MattDMo Jan 14 '14 at 14:56