Nested defaultdict raises AttributeError

Question

I am using a nested defaultdict to keep code clean and reduce the redundant code.

I am constructing a dictionary such as:

{"Store1": {"Product1": 1}, "Store2": {"Product1": 2, "Product2": 1}}

I tried to implement the answer to this question Nested dictionary with defaults, which would raise the exception:

AttributeError: 'int' object has no attribute 'items'

from collections import defaultdict, Counter

d = defaultdict(lambda: defaultdict(lambda: Counter()))
d["Store1"]["Product1"] += 1
print(d)

is there anyway I can for example:

d["Store1"]["Product1"] += 1

This works! :). Could you you please explain why it works? I could also mark your comment as a solution if you add it as an answer. Thank you — coredumped0x, Dec 06 '21 at 20:57

score 1 · Accepted Answer · answered Dec 06 '21 at 21:06

When you use the following

d = defaultdict(lambda: defaultdict(lambda: Counter()))
d["Store1"]["Product1"] += 1

then, d["Store1"] will create a new element of "type" defaultdict(lambda: Counter()) and thus d["Store1"]["Product1"] will create a new element of type Counter. Hence when you do += 1, it attempts to augment the Counter object by 1. This, however, is not defined, as the types are not compatible:

>>> c = Counter()
>>> c += 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/---/lib/python3.7/collections/__init__.py", line 832, in __iadd__
    for elem, count in other.items():
AttributeError: 'int' object has no attribute 'items'

As you can observe, it assumes that the right hand side follows the Mapping protocol and it attempts to add the r.h.s. counts to its own. I.e. something like:

>>> c = Counter(a=1)
>>> c += Counter(a=2, b=3)
>>> c
Counter({'a': 3, 'b': 3})

When you use defaultdict(lambda: defaultdict(int)) on the other hand, then d["Store1"]["Product1"] creates a new int object, which can be incremented by += 1 and written back to the dict.

Thank you for both the answer and explanation! ;) – coredumped0x Dec 06 '21 at 21:10 — coredumped0x, Dec 06 '21 at 21:10

score 0 · Answer 2 · answered Dec 06 '21 at 21:00

0

You need to write something like that

from collections import defaultdict, Counter

d = defaultdict(lambda: defaultdict(lambda: Counter()))
d["A"]["B"]["Store1"] += 1
print(d)

answered Dec 06 '21 at 21:00

Beemen Sameh

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Nested defaultdict raises AttributeError

2 Answers2