Python Language Tutorial => Dictionary method changes

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In Python 3, many of the dictionary methods are quite different in behaviour from Python 2, and many were removed as well: has_key, iter* and view* are gone. Instead of d.has_key(key), which had been long deprecated, one must now use key in d.

In Python 2, dictionary methods keys, values and items return lists. In Python 3 they return view objects instead; the view objects are not iterators, and they differ from them in two ways, namely:

they have size (one can use the len function on them)
they can be iterated over many times

Additionally, like with iterators, the changes in the dictionary are reflected in the view objects.

Python 2.7 has backported these methods from Python 3; they're available as viewkeys, viewvalues and viewitems. To transform Python 2 code to Python 3 code, the corresponding forms are:

d.keys(), d.values() and d.items() of Python 2 should be changed to list(d.keys()), list(d.values()) and list(d.items())
d.iterkeys(), d.itervalues() and d.iteritems() should be changed to iter(d.keys()), or even better, iter(d); iter(d.values()) and iter(d.items()) respectively
and finally Python 2.7 method calls d.viewkeys(), d.viewvalues() and d.viewitems() can be replaced with d.keys(), d.values() and d.items().

Porting Python 2 code that iterates over dictionary keys, values or items while mutating it is sometimes tricky. Consider:

d = {'a': 0, 'b': 1, 'c': 2, '!': 3}
for key in d.keys():
    if key.isalpha():
        del d[key]

The code looks as if it would work similarly in Python 3, but there the keys method returns a view object, not a list, and if the dictionary changes size while being iterated over, the Python 3 code will crash with RuntimeError: dictionary changed size during iteration. The solution is of course to properly write for key in list(d).

Similarly, view objects behave differently from iterators: one cannot use next() on them, and one cannot resume iteration; it would instead restart; if Python 2 code passes the return value of d.iterkeys(), d.itervalues() or d.iteritems() to a method that expects an iterator instead of an iterable, then that should be iter(d), iter(d.values()) or iter(d.items()) in Python 3.

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Python Language Incompatibilities moving from Python 2 to Python 3 Dictionary method changes

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