More About Dictionaries

Dictionaries

Associative arrays …

• Stores pairs of key and value

• Keys are unique

  • ⟶ no two keys with the same value can exist in the same dictionary object

• Fast lookup

• Internally realized as a hash table

  • Keys are not sorted

  • No deterministic iteration possible

Dictionary Access

d[key] = value	Insert (or overwrite) value under key
d[key]	returns value of key (or raises exception)
d.get(key)	returns value of key (or None if not there)
d.get(key,defval)	returns value of key (or defval if not there)
del d[key]	remove entry for key (exception if not there)
d.keys()	iterable over keys
d.values()	iterable over values
d.items()	iterable over data as (key,value) tuples
len(d)	number of entries (as with all non-scalar types)
d.setdefault(key,defval)	return value if there, else insert defval and return that
d.update(other)	merge dictionary other into this
key in d	does key exist in d?
key not in d	does key not exist in d?

Examples: Simple Access

Literal, insertion, access

>>> d = {} # empty
>>> d = {'one': 1, 'two': 2}
>>> d['one']
1

Nothing there

>>> d.get('one')
1
>>> d.get('three')
None
>>> d['three']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'three'

Examples: Shortcuts

Shortcuts for what would otherwise be too much code …

Default without modification >>> d.get('three', 3) 3 >>> d.get('three') None

Default with modification >>> d.setdefault('three', 3) 3 >>> d['three'] 3

Dictionary Iteration (1)

• Iteration is a fundamental concept in Python

• … even more so in Python 3

• ⟶ compatibility alert!

Python 3

>>> d.keys()
dict_keys(['three', 'one', 'two'])
>>> list(d.keys())
['three', 'one', 'two']

Python 2

>>> d.keys()
['three', 'two', 'one']
>>> d.iterkeys()
<dictionary-keyiterator object at 0x7ff2e8753418>

Dictionary Iteration (2)

Iteration over values

>>> list(d.values())
[3, 1, 2]
>>> list(d.items())
[('three', 3), ('one', 1), ('two', 2)]

• Wait: d.item() lets me iterate over tuples …

• Why shouldn’t I use tuple unpacking then?

The entire power of Python

for key, value in d.items():
    ...

Building Dictionaries

>>> d = {}
>>> d = {1: 'one', 2: 'two'}
>>> d = dict()
>>> d = dict({1: 'one', 2: 'two'})
>>> d = dict([('one', 1), ('two', 2), ('three', 3)])