Circular lazy import results in a crash

[ZeroIntensity]

Crash report

What happened?

Not even 3 hours since PEP 810 got merged, we already have a crash!

# a.py
lazy import b

def something():
 b.hello()

something()

# b.py
lazy import a

def hello():
 print(a)

Running a.py will result in the following crash:

Python/gc.c:533: visit_decref: Assertion "!_PyObject_IsFreed(op)" failed
Enable tracemalloc to get the memory block allocation traceback

object address : 0x7fcfb75797f0
object refcount : 2
object type : 0x55a7d0ea0140
object type name: dict
object repr : [1] 38419 segmentation fault (core dumped) /home/peter/develop/cpython/python a.py

Sometimes an ImportCycleError shows up before the crash as well:

Traceback (most recent call last):
 File "/home/peter/develop/cpython/b.py", line 1, in <module>
 lazy import a
ImportError: deferred import of 'a' raised an exception during resolution

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
 File "/home/peter/develop/cpython/a.py", line 6, in <module>
 something()
 ~~~~~~~~~^^
 File "/home/peter/develop/cpython/a.py", line 4, in something
 b.hello()
 ~~~~~~~^^
 File "/home/peter/develop/cpython/b.py", line 4, in hello
 print(a)
 ^
 File "/home/peter/develop/cpython/a.py", line 6, in <module>
 something()
 ~~~~~~~~~^^
 File "/home/peter/develop/cpython/a.py", line 4, in something
 b.hello()
 ~~~~~~~^^
 File "/home/peter/develop/cpython/b.py", line 4, in hello
 print(a)
 ^
ImportCycleError: cannot import name 'a' (most likely due to a circular import)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-144727: Add test for circular lazy import crash (#144727) #144838

[robsdedude]

Can no longer reproduce the crash since gh-144733 has been merged.

However, the same code works when doing the lazy import manually, i.e.,

# a.py
def something():
 import b
 b.hello()

something()

# b.py
def hello():
 import a
 print(a)

Reading the PEP, I'm not quite sure whether this is supposed to work with lazy imports or not.

[ZeroIntensity]

Thanks, I missed that PR.

@pablogsal No test case for #144733? This seems like a fairly common thing to try.

[pablogsal]

Thanks, I missed that PR.

@pablogsal No test case for #144733? This seems like a fairly common thing to try.

Do you want to open a PR adding it?

[pablogsal]

Opened #144838