Why Do Empty Tuples, Sets, and Lists Share the Same ID in CPython?

Why the ID of Empty Tuples, Sets, and Lists Stay the Same in CPython

In CPython, empty tuples, sets, and lists unexpectedly share the same ID. This is contrary to the behavior of mutable objects, where each instance has a unique ID.

The reason for this behavior lies in CPython's memory allocator. When the Python interpreter creates an empty tuple or list, it allocates a small block of memory. The id function then returns the address of this memory block, which is the same for all empty tuples or lists.

While dictionaries are also mutable, their behavior differs because they require more memory. When creating a new empty dictionary, a unique memory block is allocated, and the id function returns the address of this block.

To illustrate this concept, let's execute Python code that shows the consistency of IDs for empty tuples and lists:

<code class="python">tuple1 = ()
tuple2 = ()

id1 = id(tuple1)
id2 = id(tuple2)

print(id1 == id2)  # Output: True</code>

This demonstrates that multiple empty tuples have the same ID, even though they are separate objects.

Additionally, the ID of an empty set or list remains the same even after assigning it to a variable and then calling id(). This is because the initial empty object is still alive and has the same memory location.

In summary, the ID of an object in CPython is only unique for the lifetime of the object. Once the object is destroyed, the same memory block can be reused for a different object, resulting in duplicate IDs.

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