PEP 695 changed how class decorators are traced

[nedbat]

Commit 24d8b88 ("gh-103763: Implement PEP 695 (#103764)") changed how nested class decorators are traced.

Example file nightly-20230518.py:

def decorator(arg):
    def _dec(c):
        return c
    return _dec

@decorator(6)
@decorator(
    len([8]),
)
class MyObject(
    object
):
    X = 13
a = 14

Tracing with 3.12.0a7:

% python3.12 -m trace --trace nightly-20230518.py
 --- modulename: nightly-20230518, funcname: <module>
nightly-20230518.py(1): def decorator(arg):
nightly-20230518.py(6): @decorator(6)
 --- modulename: nightly-20230518, funcname: decorator
nightly-20230518.py(2):     def _dec(c):
nightly-20230518.py(4):     return _dec
nightly-20230518.py(7): @decorator(
nightly-20230518.py(8):     len([8]),
nightly-20230518.py(7): @decorator(
 --- modulename: nightly-20230518, funcname: decorator
nightly-20230518.py(2):     def _dec(c):
nightly-20230518.py(4):     return _dec
nightly-20230518.py(10): class MyObject(            <*****
nightly-20230518.py(11):     object
nightly-20230518.py(10): class MyObject(
 --- modulename: nightly-20230518, funcname: MyObject
nightly-20230518.py(6): @decorator(6)
nightly-20230518.py(13):     X = 13
nightly-20230518.py(7): @decorator(
 --- modulename: nightly-20230518, funcname: _dec
nightly-20230518.py(3):         return c
nightly-20230518.py(6): @decorator(6)
 --- modulename: nightly-20230518, funcname: _dec
nightly-20230518.py(3):         return c
nightly-20230518.py(10): class MyObject(
nightly-20230518.py(14): a = 14

% python3.12 -c "import sys; print(sys.version)"
3.12.0a7 (main, Apr  5 2023, 05:51:58) [Clang 14.0.3 (clang-1403.0.22.14.1)]

Running with newer code:

% /usr/local/cpython/bin/python3 -m trace --trace nightly-20230518.py
 --- modulename: nightly-20230518, funcname: <module>
nightly-20230518.py(1): def decorator(arg):
nightly-20230518.py(6): @decorator(6)
 --- modulename: nightly-20230518, funcname: decorator
nightly-20230518.py(2):     def _dec(c):
nightly-20230518.py(4):     return _dec
nightly-20230518.py(7): @decorator(
nightly-20230518.py(8):     len([8]),
nightly-20230518.py(7): @decorator(
 --- modulename: nightly-20230518, funcname: decorator
nightly-20230518.py(2):     def _dec(c):
nightly-20230518.py(4):     return _dec
nightly-20230518.py(6): @decorator(6)               <*****
nightly-20230518.py(11):     object
nightly-20230518.py(10): class MyObject(
 --- modulename: nightly-20230518, funcname: MyObject
nightly-20230518.py(6): @decorator(6)
nightly-20230518.py(13):     X = 13
nightly-20230518.py(7): @decorator(
 --- modulename: nightly-20230518, funcname: _dec
nightly-20230518.py(3):         return c
nightly-20230518.py(6): @decorator(6)
 --- modulename: nightly-20230518, funcname: _dec
nightly-20230518.py(3):         return c
nightly-20230518.py(10): class MyObject(
nightly-20230518.py(14): a = 14

% /usr/local/cpython/bin/python3 -c "import sys; print(sys.version)"
3.12.0a7+ (tags/v3.12.0a7-548-g24d8b88420:24d8b88420, May 20 2023, 06:39:38) [Clang 14.0.3 (clang-1403.0.22.14.1)]

The different lines are marked with <*****.

cc: @JelleZijlstra @markshannon

Linked PRs

• gh-104686: Fix tracing for decorated classes #104708

[JelleZijlstra]

In the disassembly I see that the instructions for creating the class are on a different (wrong) line:

main:

  8          42 CALL                     1

  7          50 PUSH_NULL
             52 LOAD_BUILD_CLASS
             54 LOAD_CONST               3 (<code object MyObject at 0x104ea5e40, file "<dis>", line 7>)
             56 MAKE_FUNCTION            0
             58 LOAD_CONST               4 ('MyObject')

 12          60 LOAD_NAME                2 (object)

3.11:

  8          54 PRECALL                  1
             58 CALL                     1

 11          68 PUSH_NULL
             70 LOAD_BUILD_CLASS
             72 LOAD_CONST               3 (<code object MyObject at 0x10536ef50, file "<dis>", line 7>)
             74 MAKE_FUNCTION            0
             76 LOAD_CONST               4 ('MyObject')

 12          78 LOAD_NAME                2 (object)

So presumably I put the wrong location somewhere in the compiler. I'll take a look.

[JelleZijlstra]

Submitted a fix in #104708, thanks @nedbat for the early report!