Why does the \'is\' operator behave differently with non-cached integers inside and outside a function in Python?

The 'is' Operator's Unexpected Behavior with Non-Cached Integers

Inquiry

While experimenting with Python's interpreter, a paradox was noticed regarding the 'is' operator. Specifically, 'is' returns True when evaluated within a function but False when evaluated outside of it:

>>> def func():
...     a = 1000
...     b = 1000
...     return a is b
...

>>> a = 1000
>>> b = 1000
>>> a is b, func()
(False, True)

Since 'is' evaluates the object's 'id', this implies that within the 'func' function, 'a' and 'b' refer to the same int instance. However, outside of the function, they refer to different objects. Why is this the case?

Explanation

The Python reference manual offers an insightful clarification:

"A block is a piece of Python program text that is executed as a unit. Each command typed interactively is a block."

Within a function, a single code block exists, containing only one object for the number 1000. Thus, 'id(a)' and 'id(b)' return the same value, resulting in a True evaluation.

Outside of the function, we have two separate code objects, each with its object for 1000. Therefore, 'id(a)' and 'id(b)' are different, leading to a False evaluation.

This quirk is not exclusive to integers. Similar results are observed with, for instance, float literals. Remember, comparing objects for identity (using 'is') is generally discouraged; instead, the equality operator ('==') should be employed.

Code Demonstration

To gain a clearer understanding, we can delve into the code objects for both cases:

Within the 'func' Function:

>>> print(dis.code_info(func))
...
Constants:
   0: None
   1: 1000

We have a single 'int' instance for 1000 that is assigned to both 'a' and 'b'.

Outside the 'func' Function:

>>> com1 = compile("a=1000", filename="", mode="single")
>>> com2 = compile("b=1000", filename="", mode="single")
>>> id(com1.co_consts[0]) == id(com2.co_consts[0])
False

We see that each code object has its instance of 1000, leading to the False evaluation.

Notes

• This observation pertains to CPython, the most widely used Python implementation.
• Chained statements evaluate to True for 'is' because they are treated as a single code block.
• Execution at the module level also yields True.
• Identity checks using 'is' are not recommended for mutable objects, as they will always evaluate to False.

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