Structure using the value of attr.ib converter, if defined (#139)

* Implement support for attrib converters

* Update docs per PR feedback

* Update make_dict_structure_fn to take bool variable instead of reading _prefer_attrib_converters of converter argument

* Remove _passthru

* Add type_ attribute to StructureHandlerNotFoundError

* Update changelog

* Fix broken tests

* Fix linting errors

* Increase test coverage

Author: natemcmaster

Diff for: HISTORY.rst

@@ -7,6 +7,9 @@ History
 * Fix ``GenConverter`` mapping structuring for unannotated dicts on Python 3.8.
   (`#151 <https://github.com/Tinche/cattrs/issues/151>`_)
 * The source code for generated un/structuring functions is stored in the `linecache` cache, which enables more informative stack traces when un/structuring errors happen using the `GenConverter`. This behavior can optionally be disabled to save memory.
+* Support using the attr converter callback during structure. 
+  By default, this is a method of last resort, but it can be elevated to the default by setting `prefer_attrib_converters=True` on `Converter` or `GenConverter`. 
+  (`#138 <https://github.com/Tinche/cattrs/issues/138>`_)
 
 1.7.1 (2021-05-28)
 ------------------

Diff for: docs/structuring.rst

@@ -284,8 +284,8 @@ and their own converters work out of the box. Given a mapping ``d`` and class
 
     >>> @attr.s
     ... class A:
-    ...     a = attr.ib()
-    ...     b = attr.ib(converter=int)
+    ...     a: int = attr.ib()
+    ...     b: int = attr.ib()
     ...
     >>> cattr.structure({'a': 1, 'b': '2'}, A)
     A(a=1, b=2)
@@ -298,8 +298,8 @@ Classes like these deconstructed into tuples can be structured using
 
     >>> @attr.s
     ... class A:
-    ...     a = attr.ib()
-    ...     b = attr.ib(converter=int)
+    ...     a: str = attr.ib()
+    ...     b: int = attr.ib()
     ...
     >>> cattr.structure_attrs_fromtuple(['string', '2'], A)
     A(a='string', b=2)
@@ -312,15 +312,53 @@ Loading from tuples can be made the default by creating a new ``Converter`` with
     >>> converter = cattr.Converter(unstruct_strat=cattr.UnstructureStrategy.AS_TUPLE)
     >>> @attr.s
     ... class A:
-    ...     a = attr.ib()
-    ...     b = attr.ib(converter=int)
+    ...     a: str = attr.ib()
+    ...     b: int = attr.ib()
     ...
     >>> converter.structure(['string', '2'], A)
     A(a='string', b=2)
 
 Structuring from tuples can also be made the default for specific classes only;
 see registering custom structure hooks below.
 
+
+Using attribute types and converters
+------------------------------------
+
+By default, calling "structure" will use hooks registered using ``cattr.register_structure_hook``,
+to convert values to the attribute type, and fallback to invoking any converters registered on
+attributes with ``attrib``.
+
+.. doctest::
+
+    >>> from ipaddress import IPv4Address, ip_address
+    >>> converter = cattr.Converter()
+
+    # Note: register_structure_hook has not been called, so this will fallback to 'ip_address'
+    >>> @attr.s
+    ... class A:
+    ...     a: IPv4Address = attr.ib(converter=ip_address)
+
+    >>> converter.structure({'a': '127.0.0.1'}, A)
+    A(a=IPv4Address('127.0.0.1'))
+
+Priority is still given to hooks registered with ``cattr.register_structure_hook``, but this priority
+can be inverted by setting ``prefer_attrib_converters`` to ``True``.
+
+.. doctest::
+
+    >>> converter = cattr.Converter(prefer_attrib_converters=True)
+
+    >>> converter.register_structure_hook(int, lambda v, t: int(v))
+
+    >>> @attr.s
+    ... class A:
+    ...     a: int = attr.ib(converter=lambda v: int(v) + 5)
+
+    >>> converter.structure({'a': '10'}, A)
+    A(a=15)
+
+
 Complex ``attrs`` classes and dataclasses
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -352,7 +390,7 @@ attributes holding ``attrs`` classes and dataclasses.
     ...
     >>> @attr.s
     ... class B:
-    ...     b = attr.ib(type=A)  # Legacy syntax.
+    ...     b: A = attr.ib()
     ...
     >>> cattr.structure({'b': {'a': '1'}}, B)
     B(b=A(a=1))
@@ -376,7 +414,7 @@ Here's an example involving a simple, classic (i.e. non-``attrs``) Python class.
     >>> cattr.structure({'a': 1}, C)
     Traceback (most recent call last):
     ...
-    ValueError: Unsupported type: <class '__main__.C'>. Register a structure hook for it.
+    StructureHandlerNotFoundError: Unsupported type: <class '__main__.C'>. Register a structure hook for it.
     >>>
     >>> cattr.register_structure_hook(C, lambda d, t: C(**d))
     >>> cattr.structure({'a': 1}, C)

Diff for: src/cattr/converters.py

@@ -1,9 +1,11 @@
 from collections import Counter
 from collections.abc import MutableSet as AbcMutableSet
+from dataclasses import Field
 from enum import Enum
 from functools import lru_cache
-from typing import Any, Callable, Dict, Optional, Tuple, Type, TypeVar
+from typing import Any, Callable, Dict, Optional, Tuple, Type, TypeVar, Union
 
+from attr import Attribute
 from attr import has as attrs_has
 from attr import resolve_types
 
@@ -34,6 +36,7 @@
 )
 from .disambiguators import create_uniq_field_dis_func
 from .dispatch import MultiStrategyDispatch
+from .errors import StructureHandlerNotFoundError
 from .gen import (
     AttributeOverride,
     make_dict_structure_fn,
@@ -72,14 +75,17 @@ class Converter(object):
         "_dict_factory",
         "_union_struct_registry",
         "_structure_func",
+        "_prefer_attrib_converters",
     )
 
     def __init__(
         self,
         dict_factory: Callable[[], Any] = dict,
         unstruct_strat: UnstructureStrategy = UnstructureStrategy.AS_DICT,
+        prefer_attrib_converters: bool = False,
     ) -> None:
         unstruct_strat = UnstructureStrategy(unstruct_strat)
+        self._prefer_attrib_converters = prefer_attrib_converters
 
         # Create a per-instance cache.
         if unstruct_strat is UnstructureStrategy.AS_DICT:
@@ -292,7 +298,11 @@ def _structure_default(self, obj, cl):
             return obj
 
         if is_generic(cl):
-            fn = make_dict_structure_fn(cl, self)
+            fn = make_dict_structure_fn(
+                cl,
+                self,
+                _cattrs_prefer_attrib_converters=self._prefer_attrib_converters,
+            )
             self.register_structure_hook(cl, fn)
             return fn(obj)
 
@@ -301,7 +311,7 @@ def _structure_default(self, obj, cl):
             "Unsupported type: {0}. Register a structure hook for "
             "it.".format(cl)
         )
-        raise ValueError(msg)
+        raise StructureHandlerNotFoundError(msg, type_=cl)
 
     @staticmethod
     def _structure_call(obj, cl):
@@ -328,18 +338,34 @@ def structure_attrs_fromtuple(
         conv_obj = []  # A list of converter parameters.
         for a, value in zip(fields(cl), obj):  # type: ignore
             # We detect the type by the metadata.
-            converted = self._structure_attr_from_tuple(a, a.name, value)
+            converted = self._structure_attribute(a, value)
             conv_obj.append(converted)
 
         return cl(*conv_obj)  # type: ignore
 
-    def _structure_attr_from_tuple(self, a, _, value):
+    def _structure_attribute(
+        self, a: Union[Attribute, Field], value: Any
+    ) -> Any:
         """Handle an individual attrs attribute."""
         type_ = a.type
+        attrib_converter = getattr(a, "converter", None)
+        if self._prefer_attrib_converters and attrib_converter:
+            # A attrib converter is defined on this attribute, and prefer_attrib_converters is set
+            # to give these priority over registered structure hooks. So, pass through the raw
+            # value, which attrs will flow into the converter
+            return value
         if type_ is None:
             # No type metadata.
             return value
-        return self._structure_func.dispatch(type_)(value, type_)
+
+        try:
+            return self._structure_func.dispatch(type_)(value, type_)
+        except StructureHandlerNotFoundError:
+            if attrib_converter:
+                # Return the original value and fallback to using an attrib converter.
+                return value
+            else:
+                raise
 
     def structure_attrs_fromdict(
         self, obj: Mapping[str, Any], cl: Type[T]
@@ -348,10 +374,7 @@ def structure_attrs_fromdict(
         # For public use.
 
         conv_obj = {}  # Start with a fresh dict, to ignore extra keys.
-        dispatch = self._structure_func.dispatch
         for a in fields(cl):  # type: ignore
-            # We detect the type by metadata.
-            type_ = a.type
             name = a.name
 
             try:
@@ -362,9 +385,7 @@ def structure_attrs_fromdict(
             if name[0] == "_":
                 name = name[1:]
 
-            conv_obj[name] = (
-                dispatch(type_)(val, type_) if type_ is not None else val
-            )
+            conv_obj[name] = self._structure_attribute(a, val)
 
         return cl(**conv_obj)  # type: ignore
 
@@ -484,9 +505,10 @@ def _get_dis_func(union):
             )
 
         if not all(has(get_origin(e) or e) for e in union_types):
-            raise ValueError(
+            raise StructureHandlerNotFoundError(
                 "Only unions of attr classes supported "
-                "currently. Register a loads hook manually."
+                "currently. Register a loads hook manually.",
+                type_=union,
             )
         return create_uniq_field_dis_func(*union_types)
 
@@ -509,9 +531,12 @@ def __init__(
         forbid_extra_keys: bool = False,
         type_overrides: Mapping[Type, AttributeOverride] = {},
         unstruct_collection_overrides: Mapping[Type, Callable] = {},
+        prefer_attrib_converters: bool = False,
     ):
         super().__init__(
-            dict_factory=dict_factory, unstruct_strat=unstruct_strat
+            dict_factory=dict_factory,
+            unstruct_strat=unstruct_strat,
+            prefer_attrib_converters=prefer_attrib_converters,
         )
         self.omit_if_default = omit_if_default
         self.forbid_extra_keys = forbid_extra_keys
@@ -662,6 +687,7 @@ def gen_structure_attrs_fromdict(self, cl: Type[T]) -> T:
             cl,
             self,
             _cattrs_forbid_extra_keys=self.forbid_extra_keys,
+            _cattrs_prefer_attrib_converters=self._prefer_attrib_converters,
             **attrib_overrides,
         )
         self._structure_func.register_cls_list([(cl, h)], direct=True)

Diff for: src/cattr/dispatch.py

@@ -3,6 +3,8 @@
 
 import attr
 
+from .errors import StructureHandlerNotFoundError
+
 
 @attr.s
 class _DispatchNotFound:
@@ -121,4 +123,6 @@ def dispatch(self, typ):
                     return handler(typ)
                 else:
                     return handler
-        raise KeyError("unable to find handler for {0}".format(typ))
+        raise StructureHandlerNotFoundError(
+            f"unable to find handler for {typ}", type_=typ
+        )

Diff for: src/cattr/errors.py

@@ -0,0 +1,9 @@
+from typing import Type
+
+
+class StructureHandlerNotFoundError(Exception):
+    """Error raised when structuring cannot find a handler for converting inputs into :attr:`type_`."""
+
+    def __init__(self, message: str, type_: Type) -> None:
+        super().__init__(message)
+        self.type_ = type_

Diff for: src/cattr/gen.py

@@ -1,13 +1,18 @@
+import functools
 import linecache
 import re
 import uuid
 from dataclasses import is_dataclass
-from typing import Any, Optional, Type, TypeVar
+from typing import Any, Optional, TYPE_CHECKING, Type, TypeVar
 
 import attr
 from attr import NOTHING, resolve_types
 
 from ._compat import adapted_fields, get_args, get_origin, is_bare, is_generic
+from .errors import StructureHandlerNotFoundError
+
+if TYPE_CHECKING:
+    from cattr.converters import Converter
 
 
 @attr.s(slots=True, frozen=True)
@@ -130,9 +135,10 @@ def generate_mapping(cl: Type, old_mapping):
 
 def make_dict_structure_fn(
     cl: Type,
-    converter,
+    converter: "Converter",
     _cattrs_forbid_extra_keys: bool = False,
     _cattrs_use_linecache: bool = True,
+    _cattrs_prefer_attrib_converters: bool = False,
     **kwargs,
 ):
     """Generate a specialized dict structuring function for an attrs class."""
@@ -185,26 +191,40 @@ def make_dict_structure_fn(
         # For each attribute, we try resolving the type here and now.
         # If a type is manually overwritten, this function should be
         # regenerated.
-        if type is not None:
+        if _cattrs_prefer_attrib_converters and a.converter is not None:
+            # The attribute has defined its own conversion, so pass
+            # the original value through without invoking cattr hooks
+            handler = None
+        elif type is not None:
             handler = converter._structure_func.dispatch(type)
         else:
             handler = converter.structure
 
+        if not _cattrs_prefer_attrib_converters and a.converter is not None:
+            handler = _fallback_to_passthru(handler)
+
         struct_handler_name = f"structure_{an}"
         globs[struct_handler_name] = handler
 
         ian = an if (is_dc or an[0] != "_") else an[1:]
         kn = an if override.rename is None else override.rename
         globs[f"type_{an}"] = type
         if a.default is NOTHING:
-            lines.append(
-                f"    '{ian}': {struct_handler_name}(o['{kn}'], type_{an}),"
-            )
+            if handler:
+                lines.append(
+                    f"    '{ian}': {struct_handler_name}(o['{kn}'], type_{an}),"
+                )
+            else:
+                lines.append(f"    '{ian}': o['{kn}'],")
         else:
             post_lines.append(f"  if '{kn}' in o:")
-            post_lines.append(
-                f"    res['{ian}'] = {struct_handler_name}(o['{kn}'], type_{an})"
-            )
+            if handler:
+                post_lines.append(
+                    f"    res['{ian}'] = {struct_handler_name}(o['{kn}'], type_{an})"
+                )
+            else:
+                post_lines.append(f"    res['{ian}'] = o['{kn}']")
+
     lines.append("    }")
     if _cattrs_forbid_extra_keys:
         allowed_fields = {a.name for a in attrs}
@@ -237,6 +257,17 @@ def make_dict_structure_fn(
     return globs[fn_name]
 
 
+def _fallback_to_passthru(func):
+    @functools.wraps(func)
+    def invoke(obj, type_):
+        try:
+            return func(obj, type_)
+        except StructureHandlerNotFoundError:
+            return obj
+
+    return invoke
+
+
 def make_iterable_unstructure_fn(cl: Any, converter, unstructure_to=None):
     """Generate a specialized unstructure function for an iterable."""
     handler = converter.unstructure