When should iteritems() be used instead of items()?

python python-3.x
user248237 picture user248237 · Dec 22, 2012 · Viewed 225.9k times · Source

Is it legitimate to use items() instead of iteritems() in all places? Why was iteritems() removed from Python 3? Seems like a terrific and useful method. What's the reasoning behind it?

Edit: To clarify, I want to know what is the correct idiom for iterating over a dictionary in a generator-like way (one item at a time, not all into memory) in a way that is compatible with both Python 2 and Python 3?

Answer

Jon Clements picture Jon Clements · Dec 22, 2012

In Python 2.x - .items() returned a list of (key, value) pairs. In Python 3.x, .items() is now an itemview object, which behaves different - so it has to be iterated over, or materialised... So, list(dict.items()) is required for what was dict.items() in Python 2.x.

Python 2.7 also has a bit of a back-port for key handling, in that you have viewkeys, viewitems and viewvalues methods, the most useful being viewkeys which behaves more like a set (which you'd expect from a dict).

Simple example:

common_keys = list(dict_a.viewkeys() & dict_b.viewkeys())

Will give you a list of the common keys, but again, in Python 3.x - just use .keys() instead.

Python 3.x has generally been made to be more "lazy" - i.e. map is now effectively itertools.imap, zip is itertools.izip, etc.

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