TypeError: Object of type <class 'NoneType'> is not TOML serializable

[amotl]

Dear Taneli,

thanks a stack for conceiving and maintaining this excellent library. We just wanted to let you know about an issue we just discovered when serializing NoneType values [1]. We are aware that TOML does not know about any kinds of NULL values (toml-lang/toml#921).

Differently than #34, this issue is merely a proposal to handle NoneType values gracefully by just ignoring them on serialization. The toml package does just that [2]. We have exercised the situation with all of toml, tomlkit, and tomli-w on behalf of a unit test at [3] and also reported the same issue to the author of tomlkit at python-poetry/tomlkit#240.

With kind regards,
Andreas.

[1] isarengineering/SecPi#25
[2] isarengineering/SecPi#27
[3] https://gist.github.com/amotl/4d8bc928d781a5d0fbc7f404396f8417

[amotl]

We also found that, when using the access pattern to directly serialize to a file,

with open("testdrive.toml", "wb") as config_file:
    tomli_w.dump(data, config_file)

tomli-w raises an exception, but still writes the file up to the point where the error happens, when data contains any NoneType value. The test at [3] demonstrates it for easy reproduction.

[hukkin]

Hey there, thanks for the issue!

I think it's better to be strict than graceful when non serializable objects are found. If a consumer of Tomli-W needs to ignore Nones, it is trivial for them to write a wrapper function something like the following (completely untested):

def remove_nones(d):
    if isinstance(d, dict):
        return {k: remove_nones(v) for k, v in d.items() if v is not None}
    return d

def write_toml_ignoring_none(d):
    return tomli_w.dumps(remove_nones(d))

The function write_toml_ignoring_none should do what you want.

[amotl]

Dear Taneli,

thank you very much for taking the time to respond to my request. I think it is fine to defer ignoring NULL values to userspace.

However, you may want to revisit my comment at #39 (comment). I think it indicates that it may happen that tomli-w writes invalid/incomplete TOML.

With kind regards,
Andreas.