Metadata API: Add default constructor arguments

This allows creating new metadata with less boilerplate:
    root = Metadata(Root())
    targets = Metadata(Targets())

Set reasonable default values for all the arguments -- version to
1, spec_version to current supported version, etc.

Expires does not have a good default value and my original plan was
to require expires argument to be set. That would mean an
incompatible API change though as arguments before expires would be
now optional... So expires now defaults to an arbitrary value of 1
day from moment of creation.

One noteworthy special case is consistent_snapshot where the default
value is True (since that's what we want people to use for new
metadata) but None is also used to imply that metadata does not contain
the field at all.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

tuf/api/metadata.py

@@ -32,7 +32,7 @@
 import io
 import logging
 import tempfile
-from datetime import datetime
+from datetime import datetime, timedelta
 from typing import (
     IO,
     Any,
@@ -120,11 +120,11 @@ class Metadata(Generic[T]):
     def __init__(
         self,
         signed: T,
-        signatures: Dict[str, Signature],
+        signatures: Optional[Dict[str, Signature]] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         self.signed: T = signed
-        self.signatures = signatures
+        self.signatures = signatures or {}
         self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}
 
     def __eq__(self, other: Any) -> bool:
@@ -480,11 +480,13 @@ def expires(self, value: datetime) -> None:
     # or "inner metadata")
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        unrecognized_fields: Optional[Mapping[str, Any]] = None,
+        version: Optional[int],
+        spec_version: Optional[str],
+        expires: Optional[datetime],
+        unrecognized_fields: Optional[Mapping[str, Any]],
     ):
+        if spec_version is None:
+            spec_version = ".".join(SPECIFICATION_VERSION)
         # Accept semver (X.Y.Z) but also X.Y for legacy compatibility
         spec_list = spec_version.split(".")
         if len(spec_list) not in [2, 3] or not all(
@@ -497,11 +499,15 @@ def __init__(
             raise ValueError(f"Unsupported spec_version {spec_version}")
 
         self.spec_version = spec_version
-        self.expires = expires
 
-        if version <= 0:
+        self.expires = expires or datetime.utcnow() + timedelta(days=1)
+
+        if version is None:
+            version = 1
+        elif version <= 0:
             raise ValueError(f"version must be > 0, got {version}")
         self.version = version
+
         self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}
 
     def __eq__(self, other: Any) -> bool:
@@ -838,20 +844,22 @@ class Root(Signed):
     # pylint: disable=too-many-arguments
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        keys: Dict[str, Key],
-        roles: Mapping[str, Role],
-        consistent_snapshot: Optional[bool] = None,
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        keys: Optional[Dict[str, Key]] = None,
+        roles: Optional[Mapping[str, Role]] = None,
+        consistent_snapshot: Optional[bool] = True,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         super().__init__(version, spec_version, expires, unrecognized_fields)
         self.consistent_snapshot = consistent_snapshot
-        self.keys = keys
-        if set(roles) != TOP_LEVEL_ROLE_NAMES:
-            raise ValueError("Role names must be the top-level metadata roles")
+        self.keys = keys or {}
 
+        if roles is None:
+            roles = {r: Role([], 1) for r in TOP_LEVEL_ROLE_NAMES}
+        elif set(roles) != TOP_LEVEL_ROLE_NAMES:
+            raise ValueError("Role names must be the top-level metadata roles")
         self.roles = roles
 
     def __eq__(self, other: Any) -> bool:
@@ -1129,14 +1137,14 @@ class Timestamp(Signed):
 
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        snapshot_meta: MetaFile,
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        snapshot_meta: Optional[MetaFile] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         super().__init__(version, spec_version, expires, unrecognized_fields)
-        self.snapshot_meta = snapshot_meta
+        self.snapshot_meta = snapshot_meta or MetaFile(1)
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, Timestamp):
@@ -1190,14 +1198,14 @@ class Snapshot(Signed):
 
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        meta: Dict[str, MetaFile],
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        meta: Optional[Dict[str, MetaFile]] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         super().__init__(version, spec_version, expires, unrecognized_fields)
-        self.meta = meta
+        self.meta = meta if meta is not None else {"targets.json": MetaFile(1)}
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, Snapshot):
@@ -1660,15 +1668,15 @@ class Targets(Signed):
     # pylint: disable=too-many-arguments
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        targets: Dict[str, TargetFile],
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        targets: Optional[Dict[str, TargetFile]] = None,
         delegations: Optional[Delegations] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
         super().__init__(version, spec_version, expires, unrecognized_fields)
-        self.targets = targets
+        self.targets = targets or {}
         self.delegations = delegations
 
     def __eq__(self, other: Any) -> bool: