Python Language Dictionary
Internal
Overview
A dictionary is a mutable collection of key-value pairs. The pairs can be accessed and modified. Each key is unique within the key set, and can be an instance of any immutable type: boolean, integer, float, tuple, string, etc. In other programming languages, the same data structure is referred to as "associative array" or "hash tables" or "hash maps".
Key Discussion
The keys 1
and True
are equivalent. Why?
Create a Dictionary
A new dictionary instance is declared using the {...}
syntax. The curly braces are placed around comma-separated key: value
pairs. The dictionary can be empty
d = {}
or it can be populated with values:
d = {'a': 'b', 1: 2}
It is good form to insert a space after :
. A comma is tolerated after the last pair.
Access a Dictionary
Access Individual Elements
Individual dictionary elements can be accessed with the []
syntax or with the get()
function.
Access with [] Syntax
The []
syntax can only be used with keys that exist in the dictionary:
d = {'a': 'b'}
print(d['a'])
An attempt to access an inexistent key will throw a KeyError
exception. To avoid the exception, use get()
instead, or test the existence of the key first.
get() Function
The get()
will return the associated value, or None
if the key does not exist.
d = {'a': 'b'}
print(d.get('a')) # will display 'a'
print(d.get('no-such-key')) # will display None
The get()
function allows for a second argument which will be returned instead of None
in case the key does not exist:
d = {}
print(d.get('no-such-key', 'alternative')) # will display "alternative"
Test the Existence of a Key
The existence of the key can be tested with in
:
if 'some-key' in d:
print("key exists")
get()
can also be used:
if d.get('some-key') is not None:
print("key exists")
Get All Keys
d = {'a': 'b', 'c': 'd'}
print(d.keys())
In Python 3, the keys()
will return a dict_keys()
, which is an iterable view of keys. This is useful with large dictionaries because the runtime does not use the time and the memory to create and store a list that might not be used. In case you need a list, use list()
to wrap the result of keys()
.
list(d.keys())
Get All Values
Modify a Dictionary
Modify Individual Elements
Modification with [] Syntax
Individual elements can be modified with the []
syntax. If the key does not exist, the key-value pair will be added to the dictionary. If the key exists, the associated value will be updated:
d = {}
d['a'] = 'b'
print(d['a']) # will display "a"
d['a'] = 'c'
print(d['a']) # will display "c"
Delete Individual Element
d = {'a': 'b', 'c': 'd'}
del d['a']
print(d) # will display {'c': 'd'}
Note that if the key being deleted does not exist, the []
syntax will throw an KeyError
exception.
Delete All Items
Use the clear()
function:
d = {...}
d.clear()