tuf.api: drop use of dateutil

All of the functionality we need is available from the standard library
which reduces our dependency footprint. Having minimal dependencies is
especially important for update clients which often have to vendor their
dependencies.

However, dateutil.relativedelta is richer than timedelta and helps to
provide a clearer API. For example, with relativedelta it's possible
to specify a delta in years *and* dateutil will do the right thing for
leap years.

Signed-off-by: Joshua Lock <jlock@vmware.com>

tests/test_tuf_api.py

@@ -35,11 +35,11 @@
 import sys
 import errno
 import os
+from datetime import timedelta
 
 from tuf.api import metadata
 from tuf.api import keys
 
-from dateutil.relativedelta import relativedelta
 import iso8601
 import six
 
@@ -108,7 +108,7 @@ def test_metadata_base(self):
     md.bump_expiration()
     self.assertEqual(md.expiration,
                      iso8601.parse_date("2030-01-02").replace(tzinfo=None))
-    md.bump_expiration(relativedelta(years=1))
+    md.bump_expiration(timedelta(days=365))
     self.assertEqual(md.expiration,
                      iso8601.parse_date("2031-01-02").replace(tzinfo=None))
 
@@ -157,7 +157,7 @@ def test_metadata_timestamp(self):
     timestamp.bump_expiration()
     self.assertEqual(timestamp.expiration,
                      iso8601.parse_date("2030-01-02").replace(tzinfo=None))
-    timestamp.bump_expiration(relativedelta(years=1))
+    timestamp.bump_expiration(timedelta(days=365))
     self.assertEqual(timestamp.expiration,
                      iso8601.parse_date("2031-01-02").replace(tzinfo=None))
 

tuf/api/metadata.py

@@ -4,14 +4,14 @@
 from tuf.api.keys import KeyRing
 
 # 2nd-party.
-from datetime import datetime
+from datetime import datetime, timedelta
 from typing import Any, Dict, List, Optional
 
 import json
 import tempfile
 
 # 3rd-party.
-from dateutil.relativedelta import relativedelta
+import iso8601
 from securesystemslib.formats import encode_canonical
 from securesystemslib.util import load_json_file, persist_temp_file
 from securesystemslib.storage import StorageBackendInterface
@@ -24,8 +24,6 @@
     generate_timestamp_metadata,
 )
 
-import iso8601
-
 # Types.
 
 JsonDict = Dict[str, Any]
@@ -34,7 +32,7 @@
 
 class Metadata:
     # By default, a Metadata would be a rather empty one.
-    def __init__(self, consistent_snapshot: bool = True, expiration: relativedelta = relativedelta(), keyring: Optional[KeyRing] = None, version: int = 1) -> None:
+    def __init__(self, consistent_snapshot: bool = True, expiration: datetime = datetime.today(), keyring: Optional[KeyRing] = None, version: int = 1) -> None:
         self.consistent_snapshot = consistent_snapshot
 
         self.keyring = keyring
@@ -56,6 +54,8 @@ def read_from_json(cls, filename: str, storage_backend: StorageBackendInterface
         signed = signable['signed']
 
         # We always intend times to be UTC
+        # NOTE: we could do this with datetime.fromisoformat() but that is not
+        # available in Python 2.7's datetime
         expiration = iso8601.parse_date(signed['expires']).replace(tzinfo=None)
         version = signed['version']
 
@@ -109,7 +109,7 @@ def expiration(self, datetime) -> None:
     def bump_version(self) -> None:
         self.version = self.version + 1
 
-    def bump_expiration(self, delta: relativedelta = relativedelta(days=1)) -> None:
+    def bump_expiration(self, delta: timedelta = timedelta(days=1)) -> None:
         self._expiration = self._expiration + delta
 
     def sign(self) -> JsonDict:
@@ -161,7 +161,7 @@ def write_to_json(self, filename: str, storage_backend: StorageBackendInterface
             persist_temp_file(f, filename, storage_backend)
 
 class Timestamp(Metadata):
-    def __init__(self, consistent_snapshot: bool = True, expiration: relativedelta = relativedelta(days=1), keyring: KeyRing = None, version: int = 1):
+    def __init__(self, consistent_snapshot: bool = True, expiration: datetime = datetime.today(), keyring: KeyRing = None, version: int = 1):
         super().__init__(consistent_snapshot, expiration, keyring, version)
         self.snapshot_fileinfo = {}
 
@@ -189,7 +189,7 @@ def update(self, version: int, length: int, hashes: JsonDict):
         self.snapshot_fileinfo['snapshot.json'] = fileinfo
 
 class Snapshot(Metadata):
-    def __init__(self, consistent_snapshot: bool = True, expiration: relativedelta = relativedelta(days=1), keyring: KeyRing = None, version: int = 1):
+    def __init__(self, consistent_snapshot: bool = True, expiration: datetime = datetime.today(), keyring: KeyRing = None, version: int = 1):
         super().__init__(consistent_snapshot, expiration, keyring, version)
         self.targets_fileinfo = {}
 
@@ -218,7 +218,7 @@ def update(self, rolename: str, version: int, length: Optional[int] = None, hash
         self.targets_fileinfo[f'{rolename}.json'] = tuf.formats.make_metadata_fileinfo(version, length, hashes)
 
 class Targets(Metadata):
-    def __init__(self, consistent_snapshot: bool = True, expiration: relativedelta = relativedelta(days=1), keyring: KeyRing = None, version: int = 1):
+    def __init__(self, consistent_snapshot: bool = True, expiration: datetime = datetime.today(), keyring: KeyRing = None, version: int = 1):
         super().__init__(consistent_snapshot, expiration, keyring, version)
         self.targets = {}
         self.delegations = {}