How Does Python Implement the \'is\' Keyword?

How is the 'is' Keyword Implemented in Python?

In Python, the 'is' keyword is used to test for object identity, rather than equality. This behavior differs from the __eq__() method, which compares the values of two objects. When applied to strings, however, the 'is' keyword exhibits unique characteristics.

Strings that are interned in Python are stored in a shared memory pool, and thus have the same memory address. Consequently, comparing interned strings with 'is' returns True, as they are the same object in memory. This interning process typically occurs for string literals.

When comparing non-intered strings with 'is', the result is False. Non-intered strings occupy different memory locations, even if their values are identical. For instance:

<code class="python">s = "Test String"
s2 = "Test String"

print(s is s2)  # Output: False</code>

In this example, both 's' and 's2' have the same value, but they are not the same object in memory. Therefore, 'is' returns False.

Custom classes cannot override the behavior of the 'is' keyword. Overriding __is__() will not affect the result of comparing objects with 'is'. The 'is' keyword always tests for object identity, regardless of the class definition or any defined methods.

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