diff --git a/Chapter3-Deferreds/ex4.py b/Chapter3-Deferreds/ex4.py
index d9482e8..b6b7b53 100644
--- a/Chapter3-Deferreds/ex4.py
+++ b/Chapter3-Deferreds/ex4.py
@@ -1,10 +1,17 @@
+# Errata note: when TNPE 2ed was first printed, Twisted 12.0.0 was the latest
+# version of Twisted. Since then, passing bare strings to the errback method
+# was deprecated in Twisted 12.3.0. A failure or Exception must now be
+# passed instead.
+
 from twisted.internet import reactor, defer
 
+
 class HeadlineRetriever(object):
+
     def processHeadline(self, headline):
         if len(headline) > 50:
             self.d.errback(
-                "The headline ``%s'' is too long!" % (headline,))
+                ValueError("The headline ``%s'' is too long!" % (headline,)))
         else:
             self.d.callback(headline)
 
@@ -17,10 +24,12 @@ def getHeadline(self, input):
         self.d.addCallback(self._toHTML)
         return self.d
 
+
 def printData(result):
     print result
     reactor.stop()
 
+
 def printError(failure):
     print failure
     reactor.stop()
diff --git a/Chapter4-Web-Servers/ex2-requesthandler.py b/Chapter4-Web-Servers/ex2-requesthandler.py
index 504580c..a49663c 100644
--- a/Chapter4-Web-Servers/ex2-requesthandler.py
+++ b/Chapter4-Web-Servers/ex2-requesthandler.py
@@ -9,7 +9,7 @@ class MyRequestHandler(http.Request):
 
     def process(self):
         self.setHeader('Content-Type', 'text/html')
-        if self.resources.has_key(self.path):
+        if self.path in self.resources:
             self.write(self.resources[self.path])
         else:
             self.setResponseCode(http.NOT_FOUND)
diff --git a/Chapter5-Web-Clients/ex1-print_resource.py b/Chapter5-Web-Clients/ex1-print_resource.py
index 039a102..2e92f0a 100644
--- a/Chapter5-Web-Clients/ex1-print_resource.py
+++ b/Chapter5-Web-Clients/ex1-print_resource.py
@@ -12,7 +12,7 @@ def stop(result):
     reactor.stop()
 
 if len(sys.argv) != 2:
-    print >>sys.stderr, "Usage: python print_resource.py <URL>"
+    printError("Usage: python print_resource.py <URL>")
     exit(1)
 
 d = getPage(sys.argv[1])
diff --git a/Chapter5-Web-Clients/ex2-download_resource.py b/Chapter5-Web-Clients/ex2-download_resource.py
index 77a200e..a05eeac 100644
--- a/Chapter5-Web-Clients/ex2-download_resource.py
+++ b/Chapter5-Web-Clients/ex2-download_resource.py
@@ -9,7 +9,7 @@ def stop(result):
     reactor.stop()
 
 if len(sys.argv) != 3:
-    print >>sys.stderr, "Usage: python download_resource.py <URL> <output file>"
+    printError("Usage: python download_resource.py <URL> <output file>")
     exit(1)
 
 d = downloadPage(sys.argv[1], sys.argv[2])
diff --git a/Chapter5-Web-Clients/ex3-agent_print_resource.py b/Chapter5-Web-Clients/ex3-agent_print_resource.py
index 02b046b..d340ade 100644
--- a/Chapter5-Web-Clients/ex3-agent_print_resource.py
+++ b/Chapter5-Web-Clients/ex3-agent_print_resource.py
@@ -27,7 +27,7 @@ def stop(result):
     reactor.stop()
 
 if len(sys.argv) != 2:
-    print >>sys.stderr, "Usage: python agent_print_resource.py URL"
+    printError("Usage: python agent_print_resource.py URL")
     exit(1)
 
 agent = Agent(reactor)
diff --git a/Chapter5-Web-Clients/ex4-print_metadata.py b/Chapter5-Web-Clients/ex4-print_metadata.py
index 1900fbf..6319f39 100644
--- a/Chapter5-Web-Clients/ex4-print_metadata.py
+++ b/Chapter5-Web-Clients/ex4-print_metadata.py
@@ -19,7 +19,7 @@ def stop(result):
     reactor.stop()
 
 if len(sys.argv) != 2:
-    print >>sys.stderr, "Usage: python print_metadata.py URL"
+    printError("Usage: python print_metadata.py URL")
     exit(1)
 
 agent = Agent(reactor)
diff --git a/Chapter5-Web-Clients/ex5-post_data.py b/Chapter5-Web-Clients/ex5-post_data.py
index fe8b498..d5edd4b 100644
--- a/Chapter5-Web-Clients/ex5-post_data.py
+++ b/Chapter5-Web-Clients/ex5-post_data.py
@@ -46,7 +46,7 @@ def stop(result):
     reactor.stop()
 
 if len(sys.argv) != 3:
-    print >>sys.stderr, "Usage: python post_resource.py URL 'POST DATA'"
+    printError("Usage: python post_resource.py URL 'POST DATA'")
     exit(1)
 
 agent = Agent(reactor)