MongoValueViews and better wrapping...

[thorwhalen]

Had the problem that the special values(), returning a MongoValuesView, wasn't being carried over when wrapped (by kv_wrap: https://github.com/i2mint/mongodol/blob/de97a7e36e591bbc9d82885b8c1626e2cd9e048a/mongodol/base.py#L90

To hack it through, did this: https://github.com/i2mint/py2store/commit/3dbdb04f590e701790154c14fa9808b3a0958f8f

Which worked:

from mongodol.tests.util import clear_all_and_populate, get_test_collection_persister, mk_dflt_mgc
s = get_test_collection_persister()
assert type(s.values()).__name__ == 'MongoValuesView'

from py2store import wrap_kvs

ss = wrap_kvs(s)
assert type(ss.values()).__name__ == 'MongoValuesView'

But...

Resolved by taking distinct out of MongoValuesView. It seems it's better that way, but it also sheds light on the limitations of the wrapping mechanism.

[thorwhalen]

Use factory methods to create the view classes instead (Gang of Four design patterns, not suprisingly, calls this the : Factory method pattern):

class BaseMapping:
    # factories for mapping view objects
    # that subclasses can replace
    KeysView = BaseKeysView
    ValuesView = BaseValuesView
    ItemsView = BaseItemsView

    def keys(self):
        # each of these methods use the factory method on self,
        # here that's self.KeysView(), and expect it to take specific arguments.
        return self.KeysView(self)

    def values(self):
        return self.ValuesView(self)

    def items(self):
        return self.ItemsView(self)

class SpecialKeysView(BaseMapping.KeysView):   # or SpecialKeysView(BaseKeysView)
    def extra_method(self):
        # ...

class SpecialMapping:
    KeysView = SpecialKeysView
    # ...

sm = SpecialMapping()
type(sm.keys())  # will be SpecialKeysView

If your subclassed factories need to accept extra arguments, you could use a partial(), or have the __init__ method pull in more information from the standard context that is passed in (in the above examples, BaseMapping.keys() passes in the mapping object, self, to each factory call).

[thorwhalen]

Tests in mongodol/tests/not_working.py:test_mongo_values_view_when_wrapping

[thorwhalen]

Attempt at the above (still not working):

from collections import Mapping
from py2store import KvReader
from collections import (
    KeysView as BaseKeysView,
    ValuesView as BaseValuesView,
    ItemsView as BaseItemsView,
)

class BaseMapping(dict):
    # factories for mapping view objects
    # that subclasses can replace
    KeysView = BaseKeysView
    ValuesView = BaseValuesView
    ItemsView = BaseItemsView

    def keys(self):
        # each of these methods use the factory method on self,
        # here that's self.KeysView(), and expect it to take specific arguments.
        return self.KeysView(self)

    def values(self):
        return self.ValuesView(self)

    def items(self):
        return self.ItemsView(self)

class SpecialValuesView(BaseKeysView):   # or SpecialKeysView(BaseKeysView)
    def distinct(self):
        return set(v for k, v in self._mapping.items())  # TODO: How to use super values() to get values?

class SpecialMapping(BaseMapping):
    KeysView = SpecialKeysView
    ValuesView = SpecialValuesView

from py2store import wrap_kvs

t = SpecialMapping(a=1, b=2, c=1)
assert str(type(sm.keys())) == "<class '__main__.SpecialKeysView'>"
t.values().distinct() == {1, 2}

# But...
tt = wrap_kvs(t)
assert str(type(tt.values())) == "<class 'collections.abc.ValuesView'>"  # TODO: Needs to be SpecialKeysView!!!

# And same for type wrapping:
TTT = wrap_kvs(SpecialMapping)
ttt = TTT(a=1, b=2, c=1)
assert str(type(ttt.values())) == "<class 'collections.abc.ValuesView'>"  # TODO: Needs to be SpecialKeysView!!!