When answering a question on StackOverflow, I noticed that a function that only loads a constant tuple to a local variable still has a large co_stacksize as if it was built with BUILD_TUPLE.

e.g.

    >>> def foo():
    ...     a = (1,2,3,4,5,6,7,8,9,10)
    ...
    >>> foo.__code__.co_stacksize
    10
    >>> dis.dis(foo)
      2           0 LOAD_CONST              11 ((1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
                  3 STORE_FAST               0 (a)
                  6 LOAD_CONST               0 (None)
                  9 RETURN_VALUE

I suspect it is because in the makecode the stack usage is calculated from the unoptimized assembler output instead of the actual optimized bytecode. I do not know if there is any optimization that would increase the stack usage, but perhaps it should be calculated from the resulting output.

I also suspect you're right, Antti, that the stack size is calculated prior to the bytecode being passed to through the peepholer which would have made the built tuple a value in the const array.

Off the top of my head I don't remember where the stack size calculation is made, but my suspicion is it's in the AT -> bytecode step, which would mean making it work from bytecode would mean re-implementing that calculation to work from the bytecode itself (assuming I'm right).

This also affects co_consts, which includes constants that are no longer used by the optimized code:

In [8]: def f():
return (1, 2, 3, 4, 5)
...:

In [9]: f.func_code.co_consts
Out[9]: (None, 1, 2, 3, 4, 5, (1, 2, 3, 4, 5))

In [12]: dis.dis(f)
2 0 LOAD_CONST 6 ((1, 2, 3, 4, 5))
3 RETURN_VALUE

Strictly speaking, the stack size is calculated *after* the peephole optimizer is run, but the code that computes the stack depth operates on the basic block graph instead of the assembled and optimized byte code.

Anyway, the conclusion is the same as Brett noted -- the stack depth analysis would need to be re-written to operate on the optimized bytecode array.

See also bpo-24340

Seems moving constant folding to the AST level (bpo-29469) have solved this issue.

>>> def foo():
...     a = (1,2,3,4,5,6,7,8,9,10)
... 
>>> foo.__code__.co_stacksize
1

Seems moving constant folding to the AST level (bpo-29469) have solved this issue.

Wow, it's cool to see this bug finally fixed!