How to create dict with 2 keys and one value?

Question

In python I can define dictionary as:

d = {}

and store data as:

d['a1'] = 1

How to store 2 keys?

d['a1']['b1'] = 1
d['a1']['b2'] = 2

d['a2']['b1'] = 3
d['a2']['b2'] = 4

and then print all keys and values for e.g. d['a1'] which would be:

b1 -> 1
b2 -> 2

Answer 1

You can use defaultdict from collections module (docs here):

from collections import defaultdict

d = defaultdict(dict)

d['a1']['b1'] = 1
d['a1']['b2'] = 2

d['a2']['b1'] = 3
d['a2']['b2'] = 4

print(d['a1'])

Prints:

{'b1': 1, 'b2': 2}

Answer 2

You can use collections.defaultdict but you cannot mix an int and dict assignment to d['a1'] as in your example.

from collections import defaultdict

d = defaultdict(dict)

d['a1']['b1'] = 1
d['a1']['b2'] = 2

d['a2']['b1'] = 3
d['a2']['b2'] = 4
d['a3'] = 1

print(d['a1'])
print(d['a3'])

>>> {'b1': 1, 'b2': 2}
>>> 1

If you really want to first assign 1 to d['a1'] then change it to a dictionary, you'll have to manually do that after the assignment with d['a1'] = {}.