Minor pep8 changes. Most salient is that membership test should be 'x not in y' whereas 'not x in y' was being used.

chain.py

@@ -5,7 +5,7 @@
 
 
 class Handler:
-    
+
     def __init__(self):
         self._successor = None
 
@@ -15,6 +15,7 @@ class Handler:
     def handle(self, request):
         raise NotImplementedError('Must provide implementation in subclass.')
 
+
 class ConcreteHandler1(Handler):
 
     def handle(self, request):
@@ -41,6 +42,7 @@ class ConcreteHandler3(Handler):
         elif self._successor:
             self._successor.handle(request)
 
+
 class DefaultHandler(Handler):
 
     def handle(self, request):

graph_search.py

@@ -18,7 +18,7 @@ class GraphSearch:
         self.path += [self.start]
         if self.start == self.end:
             return self.path
-        if not self.start in self.graph:
+        if self.start not in self.graph:
             return None
         for node in self.graph[self.start]:
             if node not in self.path:
@@ -34,7 +34,7 @@ class GraphSearch:
         self.path += [self.start]
         if self.start == self.end:
             return [self.path]
-        if not self.start in self.graph:
+        if self.start not in self.graph:
             return []
         paths = []
         for node in self.graph[self.start]:
@@ -52,7 +52,7 @@ class GraphSearch:
         self.path += [self.start]
         if self.start == self.end:
             return self.path
-        if not self.start in self.graph:
+        if self.start not in self.graph:
             return None
         shortest = None
         for node in self.graph[self.start]:

observer.py

@@ -10,7 +10,7 @@ class Subject(object):
         self._observers = []
 
     def attach(self, observer):
-        if not observer in self._observers:
+        if observer not in self._observers:
             self._observers.append(observer)
 
     def detach(self, observer):

publish_subscribe.py

@@ -17,7 +17,7 @@ class Provider:
         self.msg_queue.append(msg)
 
     def subscribe(self, msg, subscriber):
-        if not msg in self.subscribers:
+        if msg not in self.subscribers:
             self.subscribers[msg] = []
             self.subscribers[msg].append(subscriber)  # unfair
         else: