Fix bug: part optional value is not used when parent part is bumped

README.md

@@ -324,8 +324,8 @@ you may append a description between parens to the `file` keyword:
   `$`).
 
   Can be specified multiple times, bumpversion will try the serialization
-  formats beginning with the first and choose the last one where all values can
-  be represented like this:
+  formats beginning with the first (most complete) one and choose the last
+  one where all values can be represented like this:
 
 ```ini
 serialize =

bumpversion/version_part.py

@@ -61,7 +61,7 @@ def value(self):
         return self._value or self.config.optional_value
 
     def copy(self):
-        return VersionPart(self._value)
+        return VersionPart(self._value, self.config)
 
     def bump(self):
         return VersionPart(self.config.bump(self.value), self.config)
@@ -148,14 +148,10 @@ def __init__(self, parse, serialize, search, replace, part_configs=None):
 
         self.serialize_formats = serialize
 
-        if not part_configs:
-            part_configs = {}
-
-        self.part_configs = part_configs
+        self.part_configs = part_configs or {}
         self.search = search
         self.replace = replace
 
-
     def order(self):
         # currently, order depends on the first given serialization format
         # this seems like a good idea because this should be the most complete format
@@ -177,7 +173,6 @@ def parse(self, version_string):
 
         match = self.parse_regex.search(version_string)
 
-        _parsed = {}
         if not match:
             logger.warning(
                 "Evaluating 'parse' option: '%s' does not parse current version '%s'",
@@ -186,6 +181,7 @@ def parse(self, version_string):
             )
             return None
 
+        _parsed = {}
         for key, value in match.groupdict().items():
             _parsed[key] = VersionPart(value, self.part_configs.get(key))
 
@@ -277,7 +273,8 @@ def _choose_serialize_format(self, version, context):
 
         return chosen
 
-    def serialize(self, version, context):
+    def serialize(self, version, context=None):
+        context = context or {}
         serialized = self._serialize(
             version, self._choose_serialize_format(version, context), context
         )

tests/test_version_part.py

@@ -4,6 +4,7 @@
     ConfiguredVersionPartConfiguration,
     NumericVersionPartConfiguration,
     VersionPart,
+    VersionConfig,
 )
 
 
@@ -36,6 +37,29 @@ def test_version_part_bump(confvpc):
     assert vc.value == confvpc.bump(confvpc.first_value)
 
 
+def test_bump_resets_lower_parts_to_default_value():
+    # See bug 134.
+    release_part_config = ConfiguredVersionPartConfiguration(
+        first_value="dev",
+        optional_value="prod",
+        values=["dev", "prod"],
+    )
+    version_config = VersionConfig(
+        parse=r"(?P<major>\d+)(\-(?P<release>[a-z]+)(?P<build>\d+))?",
+        serialize=["{major}-{release}{build}", "{major}"],
+        part_configs={"release": release_part_config},
+        search=None,
+        replace=None,
+    )
+
+    # Start with version "0", mapping to "0-prod0".
+    # Bump `build` from 0 to 1.
+    # Part `release` should remain `prod` and be serialized as such.
+    version = version_config.parse("0")
+    new_version = version.bump("build", version_config.order())
+    assert version_config.serialize(new_version) == "0-prod1"
+
+
 def test_version_part_check_optional_false(confvpc):
     assert not VersionPart(confvpc.first_value, confvpc).bump().is_optional()