[Python-Dev] More imformative iterator representations

Raymond Hettinger raymond.hettinger at verizon.net
Tue Apr 6 21:22:19 EDT 2004

Armin's note reminded me that the only thing I do not like about using
iterators is not being able to see their contents at the interactive
prompt.  So, I posted an idea about changing the print part of the
read-eval-print loop to display more information:

>>> enumerate('abcdefgh') 
<enumerate object at 0x401a102c:  (0, 'a') (1, 'b') (2, 'c') ...>

There a couple of problems with the approach.  One is how to identify an
iterator object -- the presence of __iter__() and next() is a good but
not perfect indicator.

The other issue was trying to leave the iterator undisturbed while
fetching the first three entries.  The least imperfect solution I found
was to replace the iterator with an equivalent object:
firstfew=list(islice(it, 3)); it=chain(firstfew, it).  Unfortunately,
now the "it" variable is a chain object rather than the original
iterator and the object id is different.

An alternative to the read-eval-print approach is providing a custom
__repr__() method for the builtin iterators.  The good news is that this
approach is very clean.  The bad news is that it won't help user defined
iterators or builtin iterators like enumerate() that do not know their
future until next() is called.  So, this approach is only applicable to
a few iterators; it is no problem for lists, tuples, dictionaries, sets,
sequence iterators, xrange iterator objects, reversed objects,
itertools.repeat(), itertools.count(), and collections.deque()

Eventhough this solution cannot be applied uniformly for all iterators,
I think is worth considering for places where it does apply:

    >>> reversed('abcdefgh')
    reversed(['h', 'g', 'f', ...])

This could make iterators more pleasant to use interactively.  What do
you guys think?

Raymond Hettinger


Q:  What if .next() is labor intensive to compute?
A:  It is only being proposed for objects where the entries are
immediately accessible.

Q:  Are there any other candidate output formats?
A:  Yes, we could add length information for a more mathematical style

    reversed(['h', 'g', 'f', ... to 8 objects])

Q:  What if .next() returns three objects whose representations consume
a lot of visual space?
A:  This is not unique to iterators.  The same issue occurs to a larger
extent when displaying lists.  It is sometimes a PITA but usually not a

Q:  What prevents enumerate(), imap(), izip(), etc. from implementing
the more informative representations?
A:  They could, but it is messy to go through copy=list(islice(it,3))
and it=chain(copy,it) for every input.

Q:  What if calling next() initiates some sort of activity that we
wished to defer until later?
A:  This would be a rare situation for interactive work, but if known to
be an issue, printing the repr() is easily avoided by making an
assignment:  _ = launch1000ships().  

Q:  Do you really want this?
A:  It is nice to have but not essential.

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