Why Equal Integers Behaves Differently To Equal Lists?

This question is more a curiosity than anything else. I've been reading the details of the implementation of the int object in Python (1 and 2) and, as far as I can see, a Python

Solution 1:

You are not modifying the original integer, you are creating a new one and assigning the variable to it, and so the id is different.

a = 5
b = a
b += 1             # created a new intprintid(a), id(b) # different

is the same as

a = 5
b = a
b = b + 1          # created a new intprintid(a), id(b) # different

The list equivalent would not be to use append, but to use +:

a = [5]
b = a
b = b + [6]        # created a new listprintid(a), id(b) # different

The is no equivalent append for ints, since ints cannot be modified but lists can.

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The only potentially confusing thing is

a = [5]
b = a
b += [1]
printid(a), id(b) # same

The reason is that the += operator (unfortunately, IMO) modifies the original list, so b += [1] and b = b + [1] are not equivalent statements. (See Why does += behave unexpectedly on lists?)