New API: remove side effect when calling from_dict

Currently, when we are calling "from_dict" from one of the classes, we
are destroying the dict passed as argument.
The reason is that we are using dict.pop() function to access the dict
members, but that seems wasteful and unnecessary when we can access the
fields the traditional way using the [] operator.

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tuf/api/metadata.py

@@ -70,9 +70,6 @@ def from_dict(cls, metadata: Mapping[str, Any]) -> "Metadata":
             KeyError: The metadata dict format is invalid.
             ValueError: The metadata has an unrecognized signed._type field.
 
-        Side Effect:
-            Destroys the metadata Mapping passed by reference.
-
         Returns:
             A TUF Metadata object.
 
@@ -92,12 +89,12 @@ def from_dict(cls, metadata: Mapping[str, Any]) -> "Metadata":
             raise ValueError(f'unrecognized metadata type "{_type}"')
 
         signatures = []
-        for signature in metadata.pop("signatures"):
+        for signature in metadata["signatures"]:
             signature_obj = Signature.from_dict(signature)
             signatures.append(signature_obj)
 
         return cls(
-            signed=inner_cls.from_dict(metadata.pop("signed")),
+            signed=inner_cls.from_dict(metadata["signed"]),
             signatures=signatures,
         )
 
@@ -330,10 +327,10 @@ def _common_fields_from_dict(signed_dict: Mapping[str, Any]) -> list:
         See '{Root, Timestamp, Snapshot, Targets}.from_dict' methods for usage.
 
         """
-        _type = signed_dict.pop("_type")
-        version = signed_dict.pop("version")
-        spec_version = signed_dict.pop("spec_version")
-        expires_str = signed_dict.pop("expires")
+        _type = signed_dict["_type"]
+        version = signed_dict["version"]
+        spec_version = signed_dict["spec_version"]
+        expires_str = signed_dict["expires"]
         # Convert 'expires' TUF metadata string to a datetime object, which is
         # what the constructor expects and what we store. The inverse operation
         # is implemented in '_common_fields_to_dict'.
@@ -425,9 +422,9 @@ def __init__(
     def from_dict(cls, root_dict: Mapping[str, Any]) -> "Root":
         """Creates Root object from its dict representation. """
         common_args = cls._common_fields_from_dict(root_dict)
-        consistent_snapshot = root_dict.pop("consistent_snapshot")
-        keys = root_dict.pop("keys")
-        roles = root_dict.pop("roles")
+        consistent_snapshot = root_dict["consistent_snapshot"]
+        keys = root_dict["keys"]
+        roles = root_dict["roles"]
         return cls(*common_args, consistent_snapshot, keys, roles)
 
     def to_dict(self) -> Dict[str, Any]:
@@ -751,7 +748,7 @@ def from_dict(cls, targets_dict: Mapping[str, Any]) -> "Targets":
                 **targets_dict["targets"][target_path]
             )
 
-        delegations = targets_dict.pop("delegations")
+        delegations = targets_dict["delegations"]
         return cls(*common_args, targets, delegations)
 
     def to_dict(self) -> Dict[str, Any]: