fix: tomlkit 0.12.5 : Encoder contract interferes with external TypeErrors raised in encoders

CHANGELOG.md

@@ -2,11 +2,16 @@
 
 ## [unreleased]
 
+### Changed
+
+- Expect a tomlkit-specific error instead of `TypeError` from a custom encoder. ([#355](https://github.com/python-poetry/tomlkit/issues/355))
+
 ### Fixed
 
 - Fix the incompatiblity with 3.13 because of the `datetime.replace()` change. ([#333](https://github.com/python-poetry/tomlkit/issues/333))
 - Revert the change of parsing out-of-order tables. ([#347](https://github.com/python-poetry/tomlkit/issues/347))
 
+
 ## [0.12.5] - 2024-05-08
 
 ### Fixed

tomlkit/exceptions.py

@@ -225,3 +225,10 @@ def __init__(self, value: str, invalid_sequences: Collection[str], delimiter: st
             f"Invalid string: {delimiter}{repr_}{delimiter}. "
             f"The character sequences {invalid_sequences} are invalid."
         )
+
+
+class ConvertError(TypeError, ValueError, TOMLKitError):
+    """Raised when item() fails to convert a value.
+    It should be a TypeError, but due to historical reasons
+    it needs to subclass ValueError as well.
+    """

tomlkit/items.py

@@ -33,6 +33,7 @@
 from tomlkit._types import wrap_method
 from tomlkit._utils import CONTROL_CHARS
 from tomlkit._utils import escape_string
+from tomlkit.exceptions import ConvertError
 from tomlkit.exceptions import InvalidStringError
 
 
@@ -46,13 +47,6 @@
 AT = TypeVar("AT", bound="AbstractTable")
 
 
-class _ConvertError(TypeError, ValueError):
-    """An internal error raised when item() fails to convert a value.
-    It should be a TypeError, but due to historical reasons
-    it needs to subclass ValueError as well.
-    """
-
-
 @overload
 def item(value: bool, _parent: Item | None = ..., _sort_keys: bool = ...) -> Bool: ...
 
@@ -206,16 +200,16 @@ def item(value: Any, _parent: Item | None = None, _sort_keys: bool = False) -> I
         for encoder in CUSTOM_ENCODERS:
             try:
                 rv = encoder(value)
-            except TypeError:
+            except ConvertError:
                 pass
             else:
                 if not isinstance(rv, Item):
-                    raise _ConvertError(
-                        f"Custom encoder returned {type(rv)}, not a subclass of Item"
+                    raise ConvertError(
+                        f"Custom encoder is expected to return an instance of Item, got {type(rv)}"
                     )
                 return rv
 
-    raise _ConvertError(f"Invalid type {type(value)}")
+    raise ConvertError(f"Unable to convert an object of {type(value)} to a TOML item")
 
 
 class StringType(Enum):