Make model dump/load symmetric for aliased fields (#160)

* Make dump/load symmetric for aliased fields

* Only populate model fields by alias if enabled

* Document Config.allow_population_by_alias

* Update history

* Fix doc formatting

* Fix doc errors

HISTORY.rst

@@ -3,6 +3,10 @@
 History
 -------
 
+v0.9.2 (2018-XX-XX)
+...................
+* add ``Config.allow_population_by_alias`` #160, thanks @bendemaree
+
 v0.9.1 (2018-05-10)
 ...................
 * allow custom ``get_field_config`` on config classes #159

docs/index.rst

@@ -262,6 +262,19 @@ Options:
     rather than the raw enum - useful if you want to serialise ``model.dict()`` later (default: ``False``)
 :fields: extra information on each field, currently just "alias" is allowed (default: ``None``)
 :validate_assignment: whether to perform validation on assignment to attributes or not (default: ``False``)
+:allow_population_by_alias: whether or not an aliased field may be populated by its name as given by the model
+    attribute, rather than strictly the alias; please be sure to read the warning below before enabling this (default:
+    ``False``)
+
+.. warning::
+
+   Think twice before enabling ``allow_population_by_alias``! Enabling it could cause previously correct code to become
+   subtly incorrect. As an example, say you have a field named ``card_number`` with the alias ``cardNumber``. With
+   population by alias disabled (the default), trying to parse an object with only the key ``card_number`` will fail.
+   However, if you enable population by alias, the ``card_number`` field can now be populated from ``cardNumber``
+   **or** ``card_number``, and the previously-invalid example object would now be valid. This may be desired for some
+   use cases, but in others (like the one given here, perhaps!), relaxing strictness with respect to aliases could
+   introduce bugs.
 
 .. literalinclude:: examples/config.py
 

pydantic/main.py

@@ -24,6 +24,7 @@ class BaseConfig:
     ignore_extra = True
     allow_extra = False
     allow_mutation = True
+    allow_population_by_alias = False
     use_enum_values = False
     fields = {}
     validate_assignment = False
@@ -265,6 +266,9 @@ class BaseModel(metaclass=MetaModel):
 
         for name, field in self.__fields__.items():
             value = input_data.get(field.alias, MISSING)
+            if value is MISSING and self.__config__.allow_population_by_alias and field.alt_alias:
+                value = input_data.get(field.name, MISSING)
+
             if value is MISSING:
                 if self.__config__.validate_all or field.validate_always:
                     value = field.default

tests/test_main.py

@@ -292,6 +292,20 @@ def test_alias():
     assert Model(_a='different').dict() == {'a': 'different'}
 
 
+def test_population_by_alias():
+    class Model(BaseModel):
+        a: str
+
+        class Config:
+            allow_population_by_alias = True
+            fields = {
+                'a': {'alias': '_a'}
+            }
+
+    assert Model(a='different').a == 'different'
+    assert Model(a='different').dict() == {'a': 'different'}
+
+
 def test_field_order():
     class Model(BaseModel):
         c: float