Why does the `is` operator behave differently with large integers inside and outside functions in Python?

Is Operator's Unexpected Behavior with Large Integers in Functions

When experimenting with Python's interpreter, a curious discrepancy was discovered regarding the is operator.

When the evaluation is performed within a function, it returns True, but when it's done externally, the result is False.

<br>def func():</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a = 1000
b = 1000
return a is b

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

Since the is operator assesses the object's ids, this indicates that within the func function, a and b refer to the same integer instance, whereas outside the function, they refer to distinct objects.

Explanation

As the reference manual notes:

A block is a piece of Python program text that is executed as a unit.
The following are blocks: a module, a function body, and a class definition.
Each command typed interactively is a block.

Hence, in a function, a single code block holds a single numeric literal object, such as 1000, resulting in True for id(a) == id(b).

In the second instance, separate code objects exist, each with its numeric literal for 1000, causing id(a) != id(b).

Importantly, this behavior is not exclusive to integer literals; comparable results are observed with float literals (cf. here).

Remember to utilize the equality operator (==) for comparing objects, never the identity operator (is).

_{This knowledge pertains to CPython, Python's primary implementation. Alternative implementations may exhibit varying behavior.}

Code Analysis

For comprehension, let's verify this behavior using code object analysis.

Function func:

Function objects have a code attribute that reveals the compiled bytecode. dis.code_info presents this data succinctly:

<br>print(dis.code_info(func))<br>Name:              func<br>Filename:          <stdin><br>Argument count:    0<br>Kw-only arguments: 0<br>Number of locals:  2<br>Stack size:        2<br>Flags:             OPTIMIZED, NEWLOCALS, NOFREE<br>Constants:<br>   0: None<br>   1: 1000<br>Variable names:<br>   0: a<br>   1: b<br>

The Constants entry shows that the constants are None (always present) and 1000. Thus, there's one int instance representing 1000. a and b reference this one object.

Interactive commands:

Each command is a code block that is parsed, compiled, and evaluated independently:

<br>com1 = compile("a=1000", filename="", mode="single")<br>com2 = compile("b=1000", filename="", mode="single")<br>

The code object for each assignment looks similar, but crucially, com1 and com2 have separate int instances for 1000, leading to False for id(com1.co_consts[0]) == id(com2.co_consts[0]).

Different code objects, distinct contents.

Caveats

Chained Statements: Evaluating a = 1000; b = 1000 yields True identity, since these chained assignments compile into one code block, producing one instance of 1000.

Module Level: Execution at the module level (indicated by the reference manual) also yields True due to a single code object.

Mutable Objects: Identity checks fail for mutable objects unless explicitly initialized to the same object (e.g., a = b = []).

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