iter, values, item in dictionary does not work

dictionary, python

Solution

You are using Python 3; use `dict.items()` instead.

The Python 2 `dict.iter*` methods have been renamed in Python 3, where `dict.items()` returns a dictionary view instead of a list by default now. Dictionary views act as iterables in the same way `dict.iteritems()` do in Python 2.

From the Python 3 What's New documentation:

- `dict` methods `dict.keys()`, `dict.items()` and `dict.values()` return “views” instead of lists. For example, this no longer works: `k = d.keys(); k.sort()`. Use `k = sorted(d)` instead (this works in Python 2.5 too and is just as efficient).

- Also, the `dict.iterkeys()`, `dict.iteritems()` and `dict.itervalues()` methods are no longer supported.

Also, the `.next()` method has been renamed to `.__next__()`, but dictionary views are not iterators. The line `graph.iteritems().next()` would have to be translated instead, to:

current = next(iter(graph.items()))

which uses `iter()` to turn the items view into an iterable and `next()` to get the next value from that iterable.

You'll also have to rename the `next` variable in the `while` loop; using that replaces the built-in `next()` function which you need here. Use `next_` instead.

The next problem is that you are trying to use `current` as a key in `cycles`, but `current` is a tuple of an integer and a list of integers, making the whole value not hashable. I think you wanted to get just the next key instead, in which case `next(iter(dict))` would give you that:

while graph:
    current = next(iter(graph))
    cycle = [current]
    cycles[current] = cycle
    while current in graph:
        next_ = graph[current][0]
        del graph[current][0]
        if len(graph[current]) == 0:
            del graph[current]
        current = next_
        cycle.append(next_)

This then produces some output:

>>> cycles
{0: [0, 3, 2, 1, 0], 2: [2, 6, 5, 4, 2], 6: [6, 8, 7, 9, 6]}

Problem

Original source