Add a get_hash helper method to Content.

Code manipulating a Content object may want to access a hash
of configurable name; this method allows it to do that
without using getattr directly.

swh/model/model.py

@@ -11,6 +11,7 @@ import attr
 import dateutil.parser
 
 from .identifiers import normalize_timestamp
+from .hashutil import DEFAULT_ALGORITHMS
 
 SHA1_SIZE = 20
 
@@ -362,3 +363,8 @@ class Content(BaseModel):
             if content[field] is None:
                 del content[field]
         return content
+
+    def get_hash(self, hash_name):
+        if hash_name not in DEFAULT_ALGORITHMS:
+            raise ValueError('{} is not a valid hash name.'.format(hash_name))
+        return getattr(self, hash_name)

swh/model/tests/test_model.py

@@ -7,6 +7,7 @@ import copy
 
 from hypothesis import given
 
+from swh.model.model import Content
 from swh.model.hypothesis_strategies import objects
 
 
@@ -24,3 +25,11 @@ def test_todict_inverse_fromdict(objtype_and_obj):
 
     # Check the composition of from_dict and to_dict is the identity
     assert obj_as_dict == type(obj).from_dict(obj_as_dict).to_dict()
+
+
+def test_content_get_hash():
+    hashes = dict(
+        sha1=b'foo', sha1_git=b'bar', sha256=b'baz', blake2s256=b'qux')
+    c = Content(length=42, status='visible', **hashes)
+    for (hash_name, hash_) in hashes.items():
+        assert c.get_hash(hash_name) == hash_