link variable to exc blocks garbage collection

[Astrobert]

I found this strange behavior. Why is the error variable not garbage collected? If error is another object, this wouldn't happen.

import gc
import weakref

class MyException(Exception):
    pass

def func():
    try:
        raise MyException()
    except MyException as exc:
        error = exc
    return error

# Only for inspection
import dis
dis.dis(func)
        
# Only keep a weak reference to the exception
err = func()
ref = weakref.ref(err)
del err

# Check if anything refers to the exception
refs = gc.get_referrers(ref())
for ref_ in refs:
    if ref_.__class__.__name__ == "frame":
        print(ref_)

The output with Python 3.12 is:

Output:
  7           0 RESUME                   0

  8           2 NOP

  9           4 LOAD_GLOBAL              1 (NULL + MyException)
             14 CALL                     0
             22 RAISE_VARARGS            1
        >>   24 PUSH_EXC_INFO

 10          26 LOAD_GLOBAL              0 (MyException)
             36 CHECK_EXC_MATCH
             38 POP_JUMP_IF_FALSE       13 (to 66)
             40 STORE_FAST               0 (exc)

 11          42 LOAD_FAST                0 (exc)
             44 STORE_FAST               1 (error)
             46 POP_EXCEPT
             48 LOAD_CONST               0 (None)
             50 STORE_FAST               0 (exc)
             52 DELETE_FAST              0 (exc)

 12          54 LOAD_FAST                1 (error)
             56 RETURN_VALUE
        >>   58 LOAD_CONST               0 (None)
             60 STORE_FAST               0 (exc)
             62 DELETE_FAST              0 (exc)
             64 RERAISE                  1

 10     >>   66 RERAISE                  0
        >>   68 COPY                     3
             70 POP_EXCEPT
             72 RERAISE                  1
ExceptionTable:
  4 to 22 -> 24 [0]
  24 to 40 -> 68 [1] lasti
  42 to 44 -> 58 [1] lasti
  58 to 66 -> 68 [1] lasti
<frame at 0x7f37f9f4d3c0, file '/home/XXX/test.py', line 12, code func>

[Gribouillis]

When I add this at line 22

import gc
gc.collect()

the error does not happen.

It means that err has not yet been collected when you print the results.