3.6. Dictionaries

A dictionary is a mapping from a set of keys to a set of values. The data type of a key or a value is not restricted in any way, and they can be either integers, floats, strings, lists, or even another dictionary (although in this case it is typically the value that is a dictionary, not the key). Unlike a list, a dictionary begins with curly braces ({}), where each key-value pair is specified using the colon : operator. For example:

>>> scorecard = {'class':'CS 323','hw':92,'midterm':85,'final':95,'grade':'A'}
>>> scorecard
{'final': 95, 'grade': 'A', 'midterm': 85, 'class': 'CS 323', 'hw': 92}

The square bracket operator ([]) can be used to retrieve the value associated with a given key. If the key is not already present in the dictionary, then a new key-value pair is added to it, otherwise the original value is modified to the newly specified value. Similar to lists, entries within a dictionary can also be deleted using the del operator.

>>> scorecard['class']
'CS 323'
>>> scorecard['hw']
92
>>> scorecard['class-participation'] = 0    # Very bad! :-(
>>> scorecard
{'midterm': 85, 'grade': 'A', 'class-participation': 0, 'hw': 92, 'class': 'CS 323', 'final': 95}
>>> del scorecard['midterm']
>>> scorecard
{'grade': 'A', 'class-participation': 0, 'hw': 92, 'class': 'CS 323', 'final': 95}

Python provides an items function in every dictionary which returns a list of all stored key-value pairs, and can be used to conveniently loop over all the individual entries. Note that the variable names key and value are not fixed, and one can choose any other name as well. Since both the keys and the values can be of any arbitrary data type, we explicitly convert them into a string for ease of display. If only the keys are desired, but not the values, then the function keys can be used instead, which returns a list of all keys in the dictionary. Likewise, if only the values are desired but not the keys, then the function values can be used which returns a list of all values in the dictionary.

>>> for key,value in scorecard.items():
...     print 'Key: '+str(key)+', Value: '+str(value)
...
Key: grade, Value: A
Key: class-participation, Value: 0
Key: hw, Value: 92
Key: class, Value: CS 323
Key: final, Value: 95
>>> scorecard.keys()
['grade', 'class-participation', 'hw', 'class', 'final']
>>> scorecard.values()
['A', 0, 92, 'CS 323', 95]