Fix exception causes in lookup.py

Mako now performs exception chaining using ``raise from``, correctly
identifying underlying exception conditions when it raises its own
exceptions. Pull request courtesy Ram Rachum.

Additionally includes cleanup of the test suite to include
better exception fixtures.

Closes: #319
Pull-request: https://github.com/sqlalchemy/mako/pull/319
Pull-request-sha: d06526ac3f80ca9d24cd8143d8afde254f80b094

Additionally:
Fixes: #348

Change-Id: Ibb2864de822bf4b63adf22a6bb32cf0758d296bd

3 files changed, 12 insertions, 10 deletions

diff --git a/mako/lookup.py b/mako/lookup.py
index 17cef8e..7afe242 100644
--- a/mako/lookup.py
+++ b/mako/lookup.py
@@ -241,7 +241,7 @@ class TemplateLookup(TemplateCollection):
                 return self._check(uri, self._collection[uri])
             else:
                 return self._collection[uri]
-        except KeyError:
+        except KeyError as e:
             u = re.sub(r"^\/+", "", uri)
             for dir_ in self.directories:
                 # make sure the path seperators are posix - os.altsep is empty
@@ -252,8 +252,8 @@ class TemplateLookup(TemplateCollection):
                     return self._load(srcfile, uri)
             else:
                 raise exceptions.TopLevelLookupException(
-                    "Cant locate template for uri %r" % uri
-                )
+                    "Can't locate template for uri %r" % uri
+                ) from e
 
     def adjust_uri(self, uri, relativeto):
         """Adjust the given ``uri`` based on the given relative URI."""
@@ -337,11 +337,11 @@ class TemplateLookup(TemplateCollection):
                 return template
             self._collection.pop(uri, None)
             return self._load(template.filename, uri)
-        except OSError:
+        except OSError as e:
             self._collection.pop(uri, None)
             raise exceptions.TemplateLookupException(
-                "Cant locate template for uri %r" % uri
-            )
+                "Can't locate template for uri %r" % uri
+            ) from e
 
     def put_string(self, uri, text):
         """Place a new :class:`.Template` object into this
diff --git a/mako/pyparser.py b/mako/pyparser.py
index 2a1ba5f..5c55505 100644
--- a/mako/pyparser.py
+++ b/mako/pyparser.py
@@ -34,7 +34,7 @@ def parse(code, mode="exec", **exception_kwargs):
 
     try:
         return _ast_util.parse(code, "<unknown>", mode)
-    except Exception:
+    except Exception as e:
         raise exceptions.SyntaxException(
             "(%s) %s (%r)"
             % (
@@ -43,7 +43,7 @@ def parse(code, mode="exec", **exception_kwargs):
                 code[0:50],
             ),
             **exception_kwargs,
-        )
+        ) from e
 
 
 class FindIdentifiers(_ast_util.NodeVisitor):
diff --git a/mako/runtime.py b/mako/runtime.py
index f5dbe1a..6d7fa68 100644
--- a/mako/runtime.py
+++ b/mako/runtime.py
@@ -835,8 +835,10 @@ def _lookup_template(context, uri, relativeto):
     uri = lookup.adjust_uri(uri, relativeto)
     try:
         return lookup.get_template(uri)
-    except exceptions.TopLevelLookupException:
-        raise exceptions.TemplateLookupException(str(compat.exception_as()))
+    except exceptions.TopLevelLookupException as e:
+        raise exceptions.TemplateLookupException(
+            str(compat.exception_as())
+        ) from e
 
 
 def _populate_self_namespace(context, template, self_ns=None):