CODEWITHSUNDEEP
python-3.xdictionarydefaultdict
Harsh Patel
Harsh Patel

Reputation: 447

Erroneous behaviour while updating nested dictionary python3

While working on defaultdict class of collection package in python3.7, I see that new key is generated from the duplicate of last key, instead of initiating dictionary. Is there a way to initiate new element with given dictionary which is init_dict in below example code.

Example code to reproduce error:

from collections import defaultdict
init_dict = {'buy_qty': 0, 
             'sell_qty': 0}

pnl = defaultdict(lambda: init_dict)
pnl['a']['buy_qty'] += 1
pnl['a']['sell_qty'] += 1

Now when I do

pnl['b']

gives me

{'buy_qty': 1, 'sell_qty': 1}

I am looking for pnl['b'] to be initialized with init_dict. How can I achieve that?

Upvotes: 1

Views: 56

Answers (1)

RoadRunner
RoadRunner

Reputation: 26335

Your copying by reference, not by value. So whatever you do to one dictionary, the other will be affected.

You can check this with the id() function:

print(id(pnl['a']))
print(id(pnl['b']))

print(id(pnl['a']) == id(pnl['b']))

Which will give the same memory addresses:

1817103232768
1817103232768
True

verifying that they are the same objects. You can fix this by assigning a shallow copy of the dictionary using dict.copy(), as mentioned in the comments:

pnl = defaultdict(lambda: init_dict.copy())

Or casting dict():

pnl = defaultdict(lambda: dict(init_dict))

Or using ** from PEP 448 -- Additional Unpacking Generalizations :

pnl = defaultdict(lambda: {**init_dict})

Additionally, consider using a collections.Counter to do the counting, instead of initializing zero count dictionaries yourself:

from collections import defaultdict, Counter

pnl = defaultdict(Counter)

pnl['a']['buy_qty'] += 1
pnl['a']['sell_qty'] += 1

print(pnl)
# defaultdict(<class 'collections.Counter'>, {'a': Counter({'buy_qty': 1, 'sell_qty': 1})})

print(pnl['b']['buy_qty'])
# 0

print(pnl['b']['buy_qty'])
# 0

pnl['b']['buy_qty'] += 1
pnl['b']['sell_qty'] += 1

print(pnl)
# defaultdict(<class 'collections.Counter'>, {'a': Counter({'buy_qty': 1, 'sell_qty': 1}), 'b': Counter({'buy_qty': 1, 'sell_qty': 1})})

Counter is a subclass of dict, so they will work the same as normal dictionaries.

Upvotes: 2

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