Dictionary methods

clear

Method clear allows to clear dictionary:

In [1]: london = {'name': 'London1', 'location': 'London Str'}

In [2]: london.clear()

In [3]: london
Out[3]: {}

copy

Method copy allows to create a full copy of dictionary.

If one dictionary is equal to other:

In [4]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [5]: london2 = london

In [6]: id(london)
Out[6]: 25489072

In [7]: id(london2)
Out[7]: 25489072

In [8]: london['vendor'] = 'Juniper'

In [9]: london2['vendor']
Out[9]: 'Juniper'

In this case london2 is another name that refers to dictionary london. And when you change london dictionary, london2 dictionary changes as well because it’s a link to the same object.

Therefore, if you want to make a copy of dictionary, use copy method:

In [10]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [11]: london2 = london.copy()

In [12]: id(london)
Out[12]: 25524512

In [13]: id(london2)
Out[13]: 25563296

In [14]: london['vendor'] = 'Juniper'

In [15]: london2['vendor']
Out[15]: 'Cisco'

get

If you query a key that is not present in dictionary, an error occurs:

In [16]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [17]: london['ios']
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-17-b4fae8480b21> in <module>()
----> 1 london['ios']

KeyError: 'ios'

Method get queries for key and if there is no key, returns None instead.

In [18]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [19]: print(london.get('ios'))
None

Method get() also allows you to specify another value instead of None:

In [20]: print(london.get('ios', 'Ooops'))
Ooops

setdefault

Method setdefault searches for key and if there is no key, instead of error it creates a key with None value.

In [21]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [22]: ios = london.setdefault('ios')

In [23]: print(ios)
None

In [24]: london
Out[24]: {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco', 'ios': None}

If key is present, setdefault returns value that corresponds to it:

In [25]: london.setdefault('name')
Out[25]: 'London1'

The second argument allows to specify which value should correspond to key:

In [26]: model = london.setdefault('model', 'Cisco3580')

In [27]: print(model)
Cisco3580

In [28]: london
Out[28]:
{'name': 'London1',
 'location': 'London Str',
 'vendor': 'Cisco',
 'ios': None,
 'model': 'Cisco3580'}

Method setdefault replaces this expression:

In [30]: if key in london:
    ...:     value = london[key]
    ...: else:
    ...:     london[key] = 'somevalue'
    ...:     value = london[key]
    ...:

keys, values, items

Methods keys, values, items:

In [24]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [25]: london.keys()
Out[25]: dict_keys(['name', 'location', 'vendor'])

In [26]: london.values()
Out[26]: dict_values(['London1', 'London Str', 'Cisco'])

In [27]: london.items()
Out[27]: dict_items([('name', 'London1'), ('location', 'London Str'), ('vendor', 'Cisco')])

All three methods return special view objects that contains keys, values, and key-value pairs of dictionary, respectively.

A very important feature of view is that they change together with dictionary. And in fact, they just give you a way to look at objects, but they don’t make a copy of them.

Example of uning keys:

In [28]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [29]: keys = london.keys()

In [30]: print(keys)
dict_keys(['name', 'location', 'vendor'])

Now keys variable corresponds to view dict_keys, in which three keys: name, location and vendor.

But if we add another key-value pair to dictionary, keys object will also change:

In [31]: london['ip'] = '10.1.1.1'

In [32]: keys
Out[32]: dict_keys(['name', 'location', 'vendor', 'ip'])

If you want to get a simple list of keys that will not be changed with dictionary changes, it is enough to convert view to list:

In [33]: list_keys = list(london.keys())

In [34]: list_keys
Out[34]: ['name', 'location', 'vendor', 'ip']

del

Remove key and value:

In [35]: london = {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco'}

In [36]: del london['name']

In [37]: london
Out[37]: {'location': 'London Str', 'vendor': 'Cisco'}

update

Method update allows you to add contents of one dictionary to another dictionary:

In [38]: r1 = {'name': 'London1', 'location': 'London Str'}

In [39]: r1.update({'vendor': 'Cisco', 'ios':'15.2'})

In [40]: r1
Out[40]: {'name': 'London1', 'location': 'London Str', 'vendor': 'Cisco', 'ios': '15.2'}

Values can be updated in the same way:

In [41]: r1.update({'name': 'london-r1', 'ios':'15.4'})

In [42]: r1
Out[42]:
{'name': 'london-r1',
 'location': 'London Str',
 'vendor': 'Cisco',
 'ios': '15.4'}