inspect.getframeinfo and traceback build every preceding co_positions() tuple #161799

Bug report

inspect.getframeinfo() and traceback read the position of the current instruction with:

next(itertools.islice(code.co_positions(), instruction_index // 2, None))

co_positions() yields one tuple per instruction. islice has to build and discard a tuple for every preceding instruction before it can return the requested one. The returned position is correct. The cost grows with the size of the code object.

inspect.stack(0) does this for every frame. context=0 only skips the source-line listing. It still computes the PEP 657 position. A caller that uses inspect.stack(0) on a hot path therefore pays for a full walk of each frame's line table.

On 3.11 and on current main, a short function and a function with about 2500 statements show the difference:

import inspect
import timeit

def make(n):
    src = "def f():\n" + "    x = 1\n" * n + "    return inspect.stack(0)\n"
    ns = {"inspect": inspect}
    exec(src, ns)
    return ns["f"]

small, large = make(10), make(2500)
print("small", timeit.timeit(small, number=2000) / 2000)
print("large", timeit.timeit(large, number=200) / 200)

The byte offset passed in is f_lasti or tb_lasti, the same value traceback.c already passes to PyCode_Addr2Location. One call of that function fills the same (start_line, end_line, start_column, end_column) tuple, with None where co_positions() yields None.

A direct lookup for real code objects removes the per-instruction tuple. Objects that only implement co_positions(), such as the stand-ins in test_traceback, can keep the iterator path.

CPython versions tested on:

3.11, CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-161799: Look up one bytecode position in inspect and traceback #161800