Dictionary creation options

Literal

A dictionary can be created with help of a literal:

In [1]: r1 = {'model': '4451', 'ios': '15.4'}

dict

Construction dict allows you to create a dictionary in several ways.

If you use strings as keys you can use this option to create a dictionary:

In [2]: r1 = dict(model='4451', ios='15.4')

In [3]: r1
Out[3]: {'model': '4451', 'ios': '15.4'}

The second option of creating a dictionary with dict():

In [4]: r1 = dict([('model','4451'), ('ios','15.4')])

In [5]: r1
Out[5]: {'model': '4451', 'ios': '15.4'}

dict.fromkeys

In a situation where you need to create a dictionary with known keys but so far empty values (or identical values), fromkeys method is very convenient:

In [5]: d_keys = ['hostname', 'location', 'vendor', 'model', 'ios', 'ip']

In [6]: r1 = dict.fromkeys(d_keys)

In [7]: r1
Out[7]:
{'hostname': None,
 'location': None,
 'vendor': None,
 'model': None,
 'ios': None,
 'ip': None}

By default fromkeys sets None value. But you can also pass your own value:

In [8]: router_models = ['ISR2811', 'ISR2911', 'ISR2921', 'ASR9002']

In [9]: models_count = dict.fromkeys(router_models, 0)

In [10]: models_count
Out[10]: {'ISR2811': 0, 'ISR2911': 0, 'ISR2921': 0, 'ASR9002': 0}

This option of creating a dictionary is not suitable for all cases. For example, if you use a mutable data type in value, a reference to the same object will be created:

In [10]: router_models = ['ISR2811', 'ISR2911', 'ISR2921', 'ASR9002']

In [11]: routers = dict.fromkeys(router_models, [])
    ...:

In [12]: routers
Out[12]: {'ISR2811': [], 'ISR2911': [], 'ISR2921': [], 'ASR9002': []}

In [13]: routers['ASR9002'].append('london_r1')

In [14]: routers
Out[14]:
{'ISR2811': ['london_r1'],
 'ISR2911': ['london_r1'],
 'ISR2921': ['london_r1'],
 'ASR9002': ['london_r1']}

In this case, each key refers to the same list. Therefore, when a value is added to one of lists, others are updated.

Note

A dictionary comprehension is better for this task. See section List, dict, set comprehensions