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10.5: Multiple assignment with dictionaries

Combining items, tuple assignment, and for, you can see a nice code pattern for traversing the keys and values of a dictionary in a single loop:

for key, val in list(d.items()):
    print(val, key)

This loop has two iteration variables because items returns a list of tuples and key, val is a tuple assignment that successively iterates through each of the key-value pairs in the dictionary.

For each iteration through the loop, both key and value are advanced to the next key-value pair in the dictionary (still in hash order).

The output of this loop is:

10 a
22 c
1 b

Again, it is in hash key order (i.e., no particular order).

If we combine these two techniques, we can print out the contents of a dictionary sorted by the value stored in each key-value pair.

To do this, we first make a list of tuples where each tuple is (value, key). The items method would give us a list of (key, value) tuples, but this time we want to sort by value, not key. Once we have constructed the list with the value-key tuples, it is a simple matter to sort the list in reverse order and print out the new, sorted list.

>>> d = {'a':10, 'b':1, 'c':22}
>>> l = list()
>>> for key, val in d.items() :
...     l.append( (val, key) )
>>> l
[(10, 'a'), (22, 'c'), (1, 'b')]
>>> l.sort(reverse=True)
>>> l
[(22, 'c'), (10, 'a'), (1, 'b')]

By carefully constructing the list of tuples to have the value as the first element of each tuple, we can sort the list of tuples and get our dictionary contents sorted by value.