Initialization of list of empty dictionaries in python

Question

I was expecting below ways of initialization to result in same output:

a = [{}, {}]
b = [{}] * 2
a[0]["assigned"] = 1
b[0]["assigned"] = 1
print(a)
print(b)

but the output is as follows:

[{'assigned': 1}, {}]
[{'assigned': 1}, {'assigned': 1}]

Looks like in case of b, a single dictionary instance is being created and referenced in all the list elements.

Can someone help me with reason behind this implementation?

Thanks.

Answer 1

Multiplying a list evidently copies references to objects. As it can be seen here:

>>> obj = object()
>>> id(obj)
140589987770544
>>> 
>>> objects = [obj] * 5
>>> print(*map(id, objects), sep="\n")
140589987770544
140589987770544
140589987770544
140589987770544
140589987770544

The fix for this is to create a new object in a list comprehension:

>>> objects = [object() for _ in range(5)]
>>> print(*map(id, objects), sep="\n")
140589987770576
140589987770528
140589987770560
140589987770592
140589987770608