Question

I’m pretty new to programming and am having some trouble understanding the output of this code.

#testclass.py

class TestCount:
    count = 0
    def __init__(self):
        self.attr1 = TestCount.count
        self.attr2 = TestCount.count + 1
        TestCount.count += 2

x = TestCount()
y = TestCount()

print(x.attr1, x.attr2)
print(y.attr1, y.attr2)

This is a rework of a larger example out of a book I’m currently learning out of. When this code is ran it gives:

0 1
2 3

When I expect it to be:

0 1
0 1

What is the fundamental that I’m missing here? I see y as a new instance but it seems to be picking up where x left off. Sorry if I’m not explaining myself clearly but I’m new at this.

Answer 1

count is a property of the class, not the instance. That means this value is shared by all instances.

You are initializing attr1 with TestCount.count and later increasing it by 2. So the second instance will start with attr1 being set to 2, for the third instance 4, etc.

If you want both properties (attr1 and attr2) to be initialized with 0 and 1, you should do so:

class TestCount:
    def __init__(self):
        self.attr1 = 0
        self.attr2 = 1

For ore information, I suggest to read the Classes section in the official Python tutorial.