when defaultdict is configured to instantiate a class, the behaviour is different if the class has default parameters.
As an example, this is the expected behaviour:
from typing import List
from collections import defaultdict
class Test:
def __init__(self):
self.values = []
tests = defaultdict(Test)
tests['a'].values.append(1)
tests['b'].values.append(2)
print([f"{id}: {t.values}" for id, t in tests.items()]) #--> ['a: [1]', 'b: [2]']
Now, when the class has a default value, all objects share the same reference:
class Test:
def __init__(self, values = []):
self.values = values
tests = defaultdict(Test)
tests['a'].values.append(1)
tests['b'].values.append(2)
print([f"{id}: {t.values}" for id, t in tests.items()]) #--> ['a: [1, 2]', 'b: [1, 2]']
This happens only if the parameter is a reference, so for example:
class Test:
def __init__(self, values = 0):
self.values = values
tests = defaultdict(Test)
tests['a'].values += 1
tests['b'].values += 1
print([f"{id}: {t.values}" for id, t in tests.items()]) #--> ['a: 1', 'b: 1']
Why is that?
