05. Mappings dictionaries

The third category of Python collections is mappings. A mapping is a mutable unordered collection of key-value pairs. Programmers use mappings as associative arrays, lookup tables and hash tables.


To declare a dictionary, use the dict keyword.

>>> dict((('A', 'adenine'),
...    ('C', 'cytosine'),
...    ('G', 'guanine'),
...    ('T', 'thymine')
...    ))

Another way to create a dictionary is between curly braces.

>>> {'A': 'adenine', 'C': 'cytosine', 'G': 'guanine', 'T': 'thymine'}
{'A': 'adenine', 'C': 'cytosine', 'G': 'guanine', 'T': 'thymine'}

Dictionary Operations

The most basic operatiosn of dictionaries act like any other sequence or list, with brackets and indices. Assume we have a dictionary d.

Return the value at key.
d[key] = value
Set (or replace) new key with value.
d[key] += -= *= /= //= %= **
Augmented assignment operators work on dictionaries as well.
del d[key]
Delete a particular key from the dictionary.

Dictionary functions

Here are a list of methods used with dictionaries. As an example, we will use a dictionary d with a key-value pairing. Any default values (what is returned in case a value is not found) will be denoted as dflt.

d.get(key[, dflt])
Get the value that corresponds to the key.
No error if d does
d.setdefault(key[, dflt])
If key exists, do nothing. If not, set to dflt.
d.pop(key[, dflt]
For each key in d,
Returns sequence of keys in d.
Returns sequence of values in d.
Returns a sequence containing (key, value) tuples in d.

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