Dumping an object with no attribute crashes

[lwinkler]

This is a weird bug with the more recent version of python. Dumping an object that has no attribute raises an error.

It happens with Python 3.11.0 and pyyaml 6.0 on Windows but did not happen for older versions of Python (3.9.13) and pyyaml 6.0

This can be reproduced with:

import yaml

class MyClass(yaml.YAMLObject):
    yaml_loader = yaml.SafeLoader
    yaml_tag = "!MyClass" # Commenting this line fixes the issue


my_object0 = MyClass()
# my_object0.aaa = 1 # Uncommenting this line fixes the issue

print("engine0 is none ?", my_object0 is None, type(my_object0))

print(yaml.dump(my_object0))

Output:

$ python aaa.py 
engine0 is none ? False <class '__main__.MyClass'>
Traceback (most recent call last):
  File "D:\prog\requisite\aaa.py", line 14, in <module>
    print(yaml.dump(my_object0))
          ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\__init__.py", line 253, in dump
    return dump_all([data], stream, Dumper=Dumper, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\__init__.py", line 241, in dump_all
    dumper.represent(data)
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\representer.py", line 27, in represent
    node = self.represent_data(data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\representer.py", line 48, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\__init__.py", line 388, in to_yaml
    return dumper.represent_yaml_object(cls.yaml_tag, data, cls,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\representer.py", line 228, in represent_yaml_object
    return self.represent_mapping(tag, state, flow_style=flow_style)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\lwink\AppData\Roaming\Python\Python311\site-packages\yaml\representer.py", line 116, in represent_mapping
    for item_key, item_value in mapping:
TypeError: 'NoneType' object is not iterable

[lwinkler]

This happened with an unstable version of python. It is probably linked to that.

[lwinkler]

I re-opened the issue because it happens again with python 3.11.4 and PyYAML 6.0.1

import yaml

class MyClass(yaml.YAMLObject):
    yaml_loader = yaml.SafeLoader
    yaml_tag = "!MyClass" # Commenting this line fixes the issue


my_object0 = MyClass()
# my_object0.aaa = 1 # Uncommenting this line fixes the issue

print("my_object is none ?", my_object0 is None, type(my_object0))

print(yaml.dump(my_object0))

gives the error:

my_object is none ? False <class '__main__.MyClass'>
Traceback (most recent call last):
  File "/home/lwinkler/prog/python/modus/ignore/bug_pyyaml.py", line 13, in <module>
    print(yaml.dump(my_object0))
          ^^^^^^^^^^^^^^^^^^^^^
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/__init__.py", line 253, in dump
    return dump_all([data], stream, Dumper=Dumper, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/__init__.py", line 241, in dump_all
    dumper.represent(data)
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/representer.py", line 27, in represent
    node = self.represent_data(data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/representer.py", line 48, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/__init__.py", line 388, in to_yaml
    return dumper.represent_yaml_object(cls.yaml_tag, data, cls,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/representer.py", line 228, in represent_yaml_object
    return self.represent_mapping(tag, state, flow_style=flow_style)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lwinkler/venv/modus/lib/python3.11/site-packages/yaml/representer.py", line 116, in represent_mapping
    for item_key, item_value in mapping:
TypeError: 'NoneType' object is not iterable

[lwinkler]

It seems that this change python/cpython#70766 causes objects to have a getstate method when none existed before. For an empty object this new getstate method returns None. This causes the serialization to crash.

[nitzmahone]

@lwinkler thanks for the investigation and link to the underlying issue- that'll save us a lot of time!

[lwinkler]

You are most welcome. Glad to be of any help.