Cookiecutter fails to get config if the .cookiecutterrc is an empty file

[apirogov]

• Cookiecutter version: 2.5.0
• Python version: 3.8
• Operating System: Linux

Description:

When using get_user_config(), the method fails with an exception instead of returning default values, if the .cookiecutterrc file exists, but is empty.

Stacktrace:

>>> get_user_config()
Traceback (most recent call last):
  File "/local/home/a.pirogov/.cache/pypoetry/virtualenvs/fair-python-cookiecutter-Y_dY9N1_-py3.8/lib/python3.8/site-packages/cookiecutter/config.py", line 115, in get_user_config
    env_config_file = os.environ['COOKIECUTTER_CONFIG']
  File "/local/home/a.pirogov/.pyenv/versions/3.8.17/lib/python3.8/os.py", line 675, in __getitem__
    raise KeyError(key) from None
KeyError: 'COOKIECUTTER_CONFIG'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/local/home/a.pirogov/.cache/pypoetry/virtualenvs/fair-python-cookiecutter-Y_dY9N1_-py3.8/lib/python3.8/site-packages/cookiecutter/config.py", line 121, in get_user_config
    return get_config(USER_CONFIG_PATH)
  File "/local/home/a.pirogov/.cache/pypoetry/virtualenvs/fair-python-cookiecutter-Y_dY9N1_-py3.8/lib/python3.8/site-packages/cookiecutter/config.py", line 69, in get_config
    config_dict = merge_configs(DEFAULT_CONFIG, yaml_dict)
  File "/local/home/a.pirogov/.cache/pypoetry/virtualenvs/fair-python-cookiecutter-Y_dY9N1_-py3.8/lib/python3.8/site-packages/cookiecutter/config.py", line 44, in merge_configs
    for k, v in overwrite.items():
AttributeError: 'NoneType' object has no attribute 'items'

What I've run:

Just have an empty .cookiecutterrc and run get_user_config().

Expected behavior:

This exception should not happen, a missing .cookiecutterrc should be treated the same as an empty one.

[alanverresen]

I've been able to reproduce this error, and have observed the following:

1. pyyaml.safe_load(stream) returns None if the YAML stream is empty [1]; an empty YAML stream is a stream that contains no YAML documents [2], and an empty file contains no YAML documents
2. the implementation of cookiecutter.config.get_config(config_path) does not check if None is returned, and instead, passes the result along to cookiecutter.config.merge_configs(default, overwrite) without checking
3. in addition, the current implementation assumes that the top level of the YAML config file is an object (which would be transformed to a dict), but it could also be an array (which would be transformed to a list), which would also result in an Exception

pyyaml works as described in (1), and the mistake has been made in the cookiecutter package. To address (2) and (3), I would like to propose to add the following checks to cookiecutter.config.get_config(config_path):

• if the result returned by pyyaml.safe_load(stream) is None, then the result is replaced by an empty dictionary
• if the result returned by pyyaml.safe_load(stream) is not a dict (or None), then raise an InvalidConfiguration exception to inform the user that something is wrong with their configuration file, even though it's a valid YAML file

I'll make a pull request for a fix and the necessary tests.

[1] https://pyyaml.org/wiki/PyYAMLDocumentation#reference
[2] https://pyyaml.org/wiki/PyYAMLDocumentation#documents