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Iterate over keys and values in dictionary


key is just a variable name.

for key in d: will simply loop over the keys in the dictionary, rather than the keys and values. To loop over both key and value you can use the following:

for key, value in d.iteritems():

Test for yourself, change the word key to poop.


For Python 3.x, iteritems() has been replaced with simply items(), which returns a set-like view backed by the dict, like iteritems() but even better. This is also available in 2.7 as viewitems(). The operation items() will work for both 2 and 3, but in 2 it will return a list of the dictionary's (key, value) pairs, which will not reflect changes to the dict that happen after the items() call. If you want the 2.x behavior in 3.x, you can call list(d.items()).


It's not that key is a special word, but that dictionaries implement the iterator protocol. You could do this in your class, e.g. see this question for how to build class iterators.

In the case of dictionaries, it's implemented at the C level. The details are available in PEP 234. In particular, the section titled "Dictionary Iterators":

  • Dictionaries implement a tp_iter slot that returns an efficient iterator that iterates over the keys of the dictionary. [...] This means that we can write

    for k in dict: ...

    which is equivalent to, but much faster than

    for k in dict.keys(): ...

    as long as the restriction on modifications to the dictionary (either by the loop or by another thread) are not violated.

  • Add methods to dictionaries that return different kinds of iterators explicitly:

    for key in dict.iterkeys(): ...
    
    for value in dict.itervalues(): ...
    
    for key, value in dict.iteritems(): ...

    This means that for x in dict is shorthand for for x in dict.iterkeys().


Append multiple values per key in Python dictionary


If I can rephrase your question, what you want is a dictionary with the years as keys and an array for each year containing a list of values associated with that year, right? Here's how I'd do it:

years_dict = dict()

for line in list:
    if line[0] in years_dict:
        # append the new number to the existing array at this slot
        years_dict[line[0]].append(line[1])
    else:
        # create a new array in this slot
        years_dict[line[0]] = [line[1]]

What you should end up with in years_dict is a dictionary that looks like the following:

{
    "2010": [2],
    "2009": [4,7],
    "1989": [8]
}

In general, it's poor programming practice to create "parallel arrays", where items are implicitly associated with each other by having the same index rather than being proper children of a container that encompasses them both.



You would be best off using collections.defaultdict (added in Python 2.5). This allows you to specify the default object type of a missing key (such as a list).

So instead of creating a key if it doesn't exist first and then appending to the value of the key, you cut out the middle-man and just directly append to non-existing keys to get the desired result.

A quick example using your data:

>>> from collections import defaultdict
>>> data = [(2010, 2), (2009, 4), (1989, 8), (2009, 7)]
>>> d = defaultdict(list)
>>> d
defaultdict(<type 'list'>, {})
>>> for year, month in data:
...     d[year].append(month)
... 
>>> d
defaultdict(<type 'list'>, {2009: [4, 7], 2010: [2], 1989: [8]})

This way you don't have to worry about whether you've seen a digit associated with a year or not. You just append and forget, knowing that a missing key will always be a list. If a key already exists, then it will just be appended to.





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