Appending to a list inside an object also appends to that list in other objects

Question

Please help, not sure what I'm doing wrong here. I've initiated numbers as an instance variable, not a class variable. Somehow, when appending to x's numbers, it also gets appended to y's numbers. Instance variables should be independent of one another, no?

Code

class Foo:
        def __init__(self, numbers=[]):
                self.numbers = numbers

x = Foo()
y = Foo()

x.numbers.append(3)

print("X NUMBERS")
print(x.numbers)

print("Y NUMBERS")
print(y.numbers)

Output

X NUMBERS
[3]
Y NUMBERS
[3]

N.b - this problem only arises when a default parameter is provided for numbers in the constructor.