If you take a look at the python bytecode, it becomes quite obvious very quickly why unpacking is faster:
>>> import dis
>>> def unpack_or_index(t=(0, 1)):
... _, x = t
... x = t[1]
...
>>> dis.dis(unpack_or_index)
2 0 LOAD_FAST 0 (t)
3 UNPACK_SEQUENCE 2
6 STORE_FAST 1 (_)
9 STORE_FAST 2 (x)
3 12 LOAD_FAST 0 (t)
15 LOAD_CONST 1 (1)
18 BINARY_SUBSCR
19 STORE_FAST 2 (x)
22 LOAD_CONST 0 (None)
25 RETURN_VALUE
The tuple unpacking operation is a simple bytecode (UNPACK_SEQUENCE
), while the indexing operation has to call a method on the tuple (BINARY_SUBSCR
). The unpack operation can take place, inline, in the python evaluation loop, while the subscription call requires looking up of the function on the tuple object to retrieve the value, using PyObject_GetItem
.
The UNPACK_SEQUENCE
opcode source code special-cases a python tuple or list unpack where the the sequence length matches the argument length exactly:
if (PyTuple_CheckExact(v) &&
PyTuple_GET_SIZE(v) == oparg) {
PyObject **items =
((PyTupleObject *)v)->ob_item;
while (oparg--) {
w = items[oparg];
Py_INCREF(w);
PUSH(w);
}
Py_DECREF(v);
continue;
} // followed by an "else if" statement for a list with similar code
The above code reaches into the native structure of the tuple and retrieves the values directly; no need to use heavy calls such as PyObject_GetItem
which have to take into account that the object could be a custom python class.
The BINARY_SUBSCR
opcode is only optimized for python lists; anything that isn't a native python list requires a PyObject_GetItem
call.
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