Python: Behaviour of float memory addressing

Question

All, When I execute the following code from inside Spyder IDE, I get the same ID of a, b, and the number 1000., yet when I execute the code from the Spyder console, I get different IDs (the ID of a is different from b). Floats are known to be immutable, yet they behave like mutable when executed by the spyder editor. Any idea why this is the case.

a=1000.
b=1000.
print('id of a='+str(id(a)))
print('id of b='+str(id(b)))
print('id of 1.'+str(id(1000.)))

Thanks

Answer 1

This has nothing to do with Spyder, this is purely a Python optimisation, as kindly explained by @Barmar.

Example 1: Line-based execution

When the values are assigned for line-execute (even if the lines are grouped when executed), the following output is provided:

>>> a = 1000.
>>> b = 1000.
>>> print(f'{id(a)=}\n{id(b)=}')

id(a)=140093502852336
id(b)=140093502851184

Different IDs.

Example 2: Function-based execution

However, when the assignments are wrapped in a function (as shown below), Python performs optimisations which basically say: "Hey, I've got a variable for that value already, so I'll just use it again."

def test():
    a = 1000.
    b = 1000.
    print(f'{id(a)=}\n{id(b)=}')

>>> test()
id(a)=140093502851472
id(b)=140093502851472

Same IDs.

---

Bytecode: Function-based execution

As shown in the bytecode for the test() function, the same ID has been assigned to the variables, due to compiler optimisations.

  2           0 LOAD_CONST               1 (1000.0)  # <-- Here (1)
              2 STORE_FAST               0 (a)

  3           4 LOAD_CONST               1 (1000.0)  # <-- Here (1)
              6 STORE_FAST               1 (b)

  4           8 LOAD_GLOBAL              0 (print)
             10 LOAD_CONST               2 ('id(a)=')
             12 LOAD_GLOBAL              1 (id)
             14 LOAD_FAST                0 (a)
             16 CALL_FUNCTION            1
             18 FORMAT_VALUE             2 (repr)
             20 LOAD_CONST               3 ('\nid(b)=')
             22 LOAD_GLOBAL              1 (id)
             24 LOAD_FAST                1 (b)
             26 CALL_FUNCTION            1
             28 FORMAT_VALUE             2 (repr)
             30 BUILD_STRING             4
             32 CALL_FUNCTION            1
             34 POP_TOP
             36 LOAD_CONST               0 (None)
             38 RETURN_VALUE