`tomlkit` 0.12.5 : Encoder contract interferes with external `TypeError`s raised in encoders

[sanmai-NL]

Problem

register_encoder expects encoder functions to raise a TypeError when encoding fails. TypeErrors are an error class not specific to tomlkit, however, and this causes interference and unclear tracebacks.

For example:

> /Users/user/bla/__main__.py(50)encoder()
-> if any(
(Pdb) n
TypeError: any() takes exactly one argument (6 given)
(Pdb) c
...
  File "/Users/user/bla/.venv/lib/python3.12/site-packages/tomlkit/items.py", line 218, in item
    raise _ConvertError(f"Invalid type {type(value)}")
tomlkit.items._ConvertError: Invalid type <class 'Legitimateclass'>

Solution

Change the API to use a tomlkit-specific and fine-grained, encoder-specific exception class.

[frostming]

Can you show me how the TypeError occurs? It seems like the encoder is not well implemented.

Library and frameworks should be loose about input and strict about output.

[sanmai-NL]

@frostming I think the OP's backtrace is sufficient to answer your question. Not well implemented is besides the point here: non-standard libraries should define their own exception type(s) if they assume they're the only one to raise them. The example is to show that.

[frostming]

json's default= argument also expects a TypeError to skip the encoder, the error can also be raised on wrong argument, we just follow the same convention.

In [1]: def my_encoder(o):
   ...:     if any(1,2,3,4):
   ...:         return str(o)
   ...:     return None
   ...: 

In [2]: import json

In [3]: json.dumps({"a": object()}, default=my_encoder)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[12], line 1
----> 1 json.dumps({"a": object()}, default=my_encoder)

File /opt/homebrew/Cellar/python@3.11/3.11.8/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/__init__.py:238, in dumps(obj, skipkeys, ensure_ascii, check_circular, allow_nan, cls, indent, separators, default, sort_keys, **kw)
    232 if cls is None:
    233     cls = JSONEncoder
    234 return cls(
    235     skipkeys=skipkeys, ensure_ascii=ensure_ascii,
    236     check_circular=check_circular, allow_nan=allow_nan, indent=indent,
    237     separators=separators, default=default, sort_keys=sort_keys,
--> 238     **kw).encode(obj)

File /opt/homebrew/Cellar/python@3.11/3.11.8/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py:200, in JSONEncoder.encode(self, o)
    196         return encode_basestring(o)
    197 # This doesn't pass the iterator directly to ''.join() because the
    198 # exceptions aren't as detailed.  The list call should be roughly
    199 # equivalent to the PySequence_Fast that ''.join() would do.
--> 200 chunks = self.iterencode(o, _one_shot=True)
    201 if not isinstance(chunks, (list, tuple)):
    202     chunks = list(chunks)

File /opt/homebrew/Cellar/python@3.11/3.11.8/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py:258, in JSONEncoder.iterencode(self, o, _one_shot)
    253 else:
    254     _iterencode = _make_iterencode(
    255         markers, self.default, _encoder, self.indent, floatstr,
    256         self.key_separator, self.item_separator, self.sort_keys,
    257         self.skipkeys, _one_shot)
--> 258 return _iterencode(o, 0)

Cell In[1], line 2, in my_encoder(o)
      1 def my_encoder(o):
----> 2     if any(1,2,3,4):
      3         return str(o)
      4     return None

TypeError: any() takes exactly one argument (4 given)

[sanmai-NL]

It could be a suboptimal design, more of Python‘s standard library suffers from that. (But json is part of that, and tomlkit isn't, so TypeError is it‘s own.)

You could enforce using a subclass of TypeError for the custom encoder to improve the design.

To illustrate your example, can you show how the custom encoder gets invoked? I don't see that in the example which makes it harder to comment on the appropriateness of the exception message.

[frostming]

Okay, I am convinced.

To illustrate your example, can you show how the custom encoder gets invoked?

The example was wrong, I've updated it