Dispatcher to try multiple applications until 404 not given

1 files changed, 50 insertions, 0 deletions

diff --git a/paste/cascade.py b/paste/cascade.py
new file mode 100644
index 0000000..300d787
--- /dev/null
+++ b/paste/cascade.py
@@ -0,0 +1,50 @@
+"""
+Cascades through several applications, so long as applications
+return ``404 Not Found``.
+"""
+import httpexceptions
+
+__all__ = ['Cascade']
+
+class Cascade(object):
+
+    """
+    Passed a list of applications, ``Cascade`` will try each of them
+    in turn.  If one returns a status code listed in ``catch`` (by
+    default just ``404 Not Found``) then the next application is
+    tried.
+
+    If all applications fail, then the last application's failure
+    response is used.
+    """
+
+    def __init__(self, applications, catch=(404,)):
+        self.apps = applications
+        self.catch_codes = {}
+        self.catch_exceptions = []
+        for error in catch:
+            if isinstance(error, str):
+                error = int(error.split(None, 1))
+            if isinstance(error, httpexceptions.HTTPException):
+                exc = error
+                code = error.code
+            else:
+                exc = httpexceptions.get_exception(error)
+                code = error
+            self.catch_codes[code] = exc
+            self.catch_exceptions.append(exc)
+        self.catch_exceptions = tuple(self.catch_exceptions)
+                
+    def __call__(self, environ, start_response):
+        def repl_start_response(status, headers, exc_info=None):
+            code = int(status.split(None, 1)[0])
+            if code in self.catch_codes:
+                raise self.catch_codes[code]
+            return start_response(status, headers, exc_info)
+
+        for app in self.apps[:-1]:
+            try:
+                return app(environ, repl_start_response)
+            except self.catch_exceptions:
+                pass
+        return self.apps[-1](environ, start_response)