How does Python keep track of changes in class attributes?

userHEI01 0
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I bet there is an answer around, but unfortunately, I couldn't find it. When playing with classes, I found the following behaviour:

class C:
    i = 1
    
a = C()
b = C()
d = C()

b.i = 3
d.i = 4
print(C.i) # -> 1
print(a.i) # -> 1
print(b.i) # -> 3
print(d.i) # -> 4

d.i = 1
print(d.i) # -> 1

C.i = 5
print(C.i) # -> 5
print(a.i) # -> 5
print(b.i) # -> 3
print(d.i) # -> 1

Only the instances' class attributes, which weren't changed already, can be altered by changing the Class' class attribute. How is this behaviour internally checked? Is there a name for further reading?

Thanks!

python-class oop python
1 Answers

There's nothing specific to "keep track" of.

Simplifying things a bit, an attribute lookup a.i for class C's instance a looks for i in the instance's __dict__ first, then in the class's (and base classes') __dict__. (This is explained in more detail here in the context of descriptors.)

Writing an attribute a.i = 5 only writes into the instance's __dict__. Writing C.i = 10 writes into the class's dict (and thus any instance that didn't have an i of its own would see 10.)

>>> class C:
...     i = 1
...
>>> a = C()
>>> a.__dict__.get("i")
# (nothing)
>>> C.__dict__.get("i")
1
>>> a.i
1
>>> a.i = 5
>>> a.__dict__.get("i")
5
>>> C.i = 10  # Update i on class
>>> b = C()  # New instance with no i of its own
>>> b.i
10
>>> a.i  # No change here, we have our own i
5
>>> del a.i  # Scrub the instance-specific `i`
>>> a.i
10
AKX0
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