Why is it possible to use more than 2 ^ 16 constants in a Python function?

Question

In internal modules like peephole, argument of LOAD_CONST is stored in the two bytes following the opcode.

For example, the macro it uses to get argument of an operation is implemented as:

#define GETARG(arr, i) ((int)((arr[i+2]<<8) + arr[i+1]))

The argument of LOAD_CONST is an index into the consts array.

So I guessed maybe we can only use at most 2 ^ 16 constants in a Python function.

But when I experiment with a function that use 66666 (> 65536) constants, it still runs normally.

What could be the reason?

Answer 1

From the dis docs:

EXTENDED_ARG(ext)

Prefixes any opcode which has an argument too big to fit into the default two bytes. ext holds two additional bytes which, taken together with the subsequent opcode’s argument, comprise a four-byte argument, ext being the two most-significant bytes.

If an opcode needs an argument longer than 2 bytes, an EXTENDED_ARG opcode provides 2 more bytes of argument.