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?
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.