source.py & element.py: Fixing SourceError() and ElementError() constructors.

Recently I added the `reason` member which can be used to set
machine readable error reason strings for the purpose of testing.

Forgot to add the necessary `*` argument, forcing `reason` to be
a keyword-only argument.

2 files changed, 12 insertions, 6 deletions

diff --git a/buildstream/element.py b/buildstream/element.py
index ee056f39c..195365390 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -80,11 +80,14 @@ class Scope(Enum):
 
 
 class ElementError(BstError):
-    """Raised by Element implementations.
+    """This exception should be raised by :class:`.Element` implementations
+    to report errors to the user.
 
-    This exception is raised when an :class:`.Element` encounters an error.
+    Args:
+       message (str): The error message to report to the user
+       reason (str): An optional machine readable reason string, used for test cases
     """
-    def __init__(self, message, reason=None):
+    def __init__(self, message, *, reason=None):
         super().__init__(message, domain=ErrorDomain.ELEMENT, reason=reason)
 
 
diff --git a/buildstream/source.py b/buildstream/source.py
index bfea143ae..5f41fa1a1 100644
--- a/buildstream/source.py
+++ b/buildstream/source.py
@@ -54,11 +54,14 @@ class Consistency():
 
 
 class SourceError(BstError):
-    """Raised by Source implementations.
+    """This exception should be raised by :class:`.Source` implementations
+    to report errors to the user.
 
-    This exception is raised when a :class:`.Source` encounters an error.
+    Args:
+       message (str): The error message to report to the user
+       reason (str): An optional machine readable reason string, used for test cases
     """
-    def __init__(self, message, reason=None):
+    def __init__(self, message, *, reason=None):
         super().__init__(message, domain=ErrorDomain.SOURCE, reason=reason)