Flow exporting/importing (#187)

* stages/*: Add SerializerModel as base model, implement serializer property

* flows: add initial flow exporter and importer

* policies/*: implement .serializer for all policies

* root: fix missing dacite requirement

passbook/flows/transfer/common.py

@@ -0,0 +1,59 @@
+"""transfer common classes"""
+from dataclasses import asdict, dataclass, field, is_dataclass
+from json.encoder import JSONEncoder
+from typing import Any, Dict, List
+from uuid import UUID
+
+from passbook.lib.models import SerializerModel
+from passbook.lib.sentry import SentryIgnoredException
+
+
+def get_attrs(obj: SerializerModel) -> Dict[str, Any]:
+    """Get object's attributes via their serializer, and covert it to a normal dict"""
+    data = dict(obj.serializer(obj).data)
+    if "policies" in data:
+        data.pop("policies")
+    if "stages" in data:
+        data.pop("stages")
+    return data
+
+
+@dataclass
+class FlowBundleEntry:
+    """Single entry of a bundle"""
+
+    identifier: str
+    model: str
+    attrs: Dict[str, Any]
+
+    @staticmethod
+    def from_model(model: SerializerModel) -> "FlowBundleEntry":
+        """Convert a SerializerModel instance to a Bundle Entry"""
+        return FlowBundleEntry(
+            identifier=model.pk,
+            model=f"{model._meta.app_label}.{model._meta.model_name}",
+            attrs=get_attrs(model),
+        )
+
+
+@dataclass
+class FlowBundle:
+    """Dataclass used for a full export"""
+
+    version: int = field(default=1)
+    entries: List[FlowBundleEntry] = field(default_factory=list)
+
+
+class DataclassEncoder(JSONEncoder):
+    """Convert FlowBundleEntry to json"""
+
+    def default(self, o):
+        if is_dataclass(o):
+            return asdict(o)
+        if isinstance(o, UUID):
+            return str(o)
+        return super().default(o)
+
+
+class EntryInvalidError(SentryIgnoredException):
+    """Error raised when an entry is invalid"""