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

`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`?

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'

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

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>

Iteration over values¶

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

The entire power of Python¶

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

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