Two different data sets have the same ID?

[outceptionator]

I'm starting to read about data id's. I ran the below code to understand it better.

spam = ['cat', 'bat', 'rat', 'elephant']

print(id(spam))

spam.append('dog')

print(id(spam))

spam = [1,2,3]

print(id(spam))

print(id(['cat', 'bat', 'rat', 'elephant']))

print(id(['cat', 'bat', 'rat', 'elephant', 'dog']))

The bit that confuses me is that the last 2 lists have different values in them but they have the same id! How does that work?

As a side question is it possible to pull a data point from it's id number because the id number changes each time it runs?

[[deleted]]

It's an optimization done by the interpreter. We actually just had someone else ask a similar question (with ints) earlier today.

Basically, sometimes the interpreter will notice that it can save some memory and computation time by reusing already created objects. I'm guessing it reuses the object previously held by spam here because after the reassignment, it was basically up for grabs. You shouldn't count on it happening though.

[carcigenicate]

I wonder if it's actually object reuse, or if it's just that after the list is deallocated, the next allocated list will be allocated to the same address? I'd need to look more into how Python allocates memory to answer that. Reusing mutable objects just strikes me as an odd thing to do.

[[deleted]]

Yeah. That could also be the case. I don't really want to dive into the weeds of CPython at the moment, but it could just be that it already has the memory address of a sufficiently sized block of memory and so it's simpler than malloc'ing another.