Why do python variables of same value point to the same memory address?

Question

I ran into an interesting case today wherein

a = 10
b = 10
print (a is b)

logged out True. I did some searching and came across the concept of interning. Now that explains why True is correct for the range [-5, 256]. However, I get the same results even while using floats. Please help me understand why.

Here is the part I don’t get-

a = 1000.00
b = 999.99 + 0.01

print (a is b)
# Output was 'True'

I expect the output to be False since a‘s value is assigned before running the program, whereas b‘s value is determined at run-time as a result of expression evaluation and hence should have a different memory address.

I’d also appreciate it if you could point out a case where a==b is True, but a is b evaluates to False where both a and b are of type float

Asked By: Senthooran B

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Answer 1

What you are looking at in your case is called "constant folding". That’s an implementation detail, not a language specification – meaning there is no guarantee that this behaviour will remain the same and you should not rely on it in your code. But, in general it comes down to the fact that things that can be calculated "statically" are optimised away to their simplest form before the code is actually being run.

Take a look at the output of the following snippet

import dis

def f():
    return 0.1 + 0.9
def g():
    a = 0.1
    b = 0.9
    return a+b

dis.dis(f)
dis.dis(g)

You will notice that there is no addition performed in f, even though looking at the code addition is clearly there.

As for two same-valued variables having different memory addresses, its not very difficult – any calculations that cannot be optimised away pre-runtime will yield you variables at different memory addresses if they are not within interning range.

a = 1
b = 1
for _ in range(1000):
    a += 1 
    b += 1

print(f"{id(a) = }")
print(f"{id(b) = }")
print(f"{a == b = }")
print(f"{a is b = }")

Answered By: matszwecja

Why do python variables of same value point to the same memory address?

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