Add support for field processors.

docs/advanced/fields.rst

@@ -151,6 +151,34 @@ function when accessing the fields:
 Now any field can be converted from sync to async, or the other way around,
 and the code would keep working.
 
+Field processors
+----------------
+
+It's often needed to clean or process field values using reusable functions.
+``@field`` takes an optional ``out`` argument with a list of such functions.
+They will be applied to the field value before returning it:
+
+.. code-block:: python
+
+    from web_poet import ItemPage, HttpResponse, field
+
+    def clean_tabs(s):
+        return s.replace('\t', ' ')
+
+    class MyPage(ItemPage):
+        response: HttpResponse
+
+        @field(out=[clean_tabs, str.strip])
+        def name(self):
+            return self.response.css(".name ::text").get()
+
+Note that while processors can be applied to async fields, they need to be
+normal functions themselves.
+
+It's also possible to implement field cleaning and processing in ``to_item``
+but in that case accessing a field directly will return the value without
+processing, so it's preferable to use field processors instead.
+
 Item classes
 ------------
 
@@ -198,9 +226,6 @@ but item classes are of a great help when
 * you need to extract data in the same format from multiple websites, or
 * if you want to define the schema upfront.
 
-Item classes can also be used to hold common attribute
-pre-processing and validation logic.
-
 Error prevention
 ~~~~~~~~~~~~~~~~
 

tests/test_fields.py

@@ -368,3 +368,32 @@ def field_foo_cached(self):
     assert page.field_foo == "foo"
     assert page.field_foo_meta == "foo"
     assert page.field_foo_cached == "foo"
+
+
+def test_field_processors_sync() -> None:
+    def proc1(s):
+        return s + "x"
+
+    @attrs.define
+    class Page(ItemPage):
+        @field(out=[str.strip, proc1])
+        def name(self):  # noqa: D102
+            return "  name\t "
+
+    page = Page()
+    assert page.name == "namex"
+
+
+@pytest.mark.asyncio
+async def test_field_processors_async() -> None:
+    def proc1(s):
+        return s + "x"
+
+    @attrs.define
+    class Page(ItemPage):
+        @field(out=[str.strip, proc1])
+        async def name(self):  # noqa: D102
+            return "  name\t "
+
+    page = Page()
+    assert await page.name == "namex"

web_poet/fields.py

@@ -2,8 +2,9 @@
 ``web_poet.fields`` is a module with helpers for putting extraction logic
 into separate Page Object methods / properties.
 """
-from functools import update_wrapper
-from typing import Dict, List, Optional, Type, TypeVar
+import inspect
+from functools import update_wrapper, wraps
+from typing import Callable, Dict, List, Optional, Type, TypeVar
 
 import attrs
 from itemadapter import ItemAdapter
@@ -24,6 +25,9 @@ class FieldInfo:
     #: field metadata
     meta: Optional[dict] = None
 
+    #: field processors
+    out: Optional[List[Callable]] = None
+
 
 class FieldsMixin:
     """A mixin which is required for a class to support fields"""
@@ -43,7 +47,13 @@ def __init_subclass__(cls, **kwargs):
                 delattr(cls, _FIELDS_INFO_ATTRIBUTE_WRITE)
 
 
-def field(method=None, *, cached: bool = False, meta: Optional[dict] = None):
+def field(
+    method=None,
+    *,
+    cached: bool = False,
+    meta: Optional[dict] = None,
+    out: Optional[List[Callable]] = None,
+):
     """
     Page Object method decorated with ``@field`` decorator becomes a property,
     which is then used by :class:`~.ItemPage`'s to_item() method to populate
@@ -55,6 +65,9 @@ def field(method=None, *, cached: bool = False, meta: Optional[dict] = None):
     The ``meta`` parameter allows to store arbitrary information for the field,
     e.g. ``@field(meta={"expensive": True})``. This information can be later
     retrieved for all fields using the :func:`get_fields_dict` function.
+
+    The ``out`` parameter is an optional list of field processors, which are
+    functions applied to the value of the field before returning it.
     """
 
     class _field:
@@ -63,6 +76,9 @@ def __init__(self, method):
                 raise TypeError(
                     f"@field decorator must be used on methods, {method!r} is decorated instead"
                 )
+            if out:
+                method = self._processed(method)
+
             if cached:
                 self.unbound_method = cached_method(method)
             else:
@@ -72,12 +88,32 @@ def __set_name__(self, owner, name):
             if not hasattr(owner, _FIELDS_INFO_ATTRIBUTE_WRITE):
                 setattr(owner, _FIELDS_INFO_ATTRIBUTE_WRITE, {})
 
-            field_info = FieldInfo(name=name, meta=meta)
+            field_info = FieldInfo(name=name, meta=meta, out=out)
             getattr(owner, _FIELDS_INFO_ATTRIBUTE_WRITE)[name] = field_info
 
         def __get__(self, instance, owner=None):
             return self.unbound_method(instance)
 
+        @staticmethod
+        def _process(value):
+            for processor in out:
+                value = processor(value)
+            return value
+
+        def _processed(self, method):
+            """Returns a wrapper for method that calls processors on its result"""
+            if inspect.iscoroutinefunction(method):
+
+                async def processed(*args, **kwargs):
+                    return self._process(await method(*args, **kwargs))
+
+            else:
+
+                def processed(*args, **kwargs):
+                    return self._process(method(*args, **kwargs))
+
+            return wraps(method)(processed)
+
     if method is not None:
         # @field syntax
         res = _field(method)