signup form with validation

main.py

@@ -1,5 +1,6 @@
 import cgi
 
+import re
 import webapp2
 
 rot13_form = """
@@ -13,10 +14,10 @@
 signup_form = """
 <h1>Signup form</h1 required="True">
 <form method="post" action="/signup">
-    <div>Name: <input title="Please, provide your name" type="text" name="username" placeholder="John Doe"><span style="color: red">%(username_error)s</span></div>
-    <div>Password: <input title="Please, provide a strong password" type="password" name="password"><span style="color: red">%(password_error)s</span></div>
-    <div>Verify passowrd: <input title="Please, verify your password" type="password" name="verify"><span style="color: red">%(verify_error)s</span></div>
-    <div>Your email: <input title="Please, provide your email address" type="email" name="email" placeholder="[email protected]"><span style="color: red">%(email_error)s</span></div>
+    <div>Name: <input title="Please, provide your name" type="text" name="username" value="%(username)s" placeholder="John Doe"><span style="color: red">%(username_error)s</span></div>
+    <div>Password: <input title="Please, provide a strong password" type="password" name="password" value=""><span style="color: red">%(password_error)s</span></div>
+    <div>Verify password: <input title="Please, verify your password" type="password" name="verify" value=""><span style="color: red">%(verify_error)s</span></div>
+    <div>Your email: <input title="Please, provide your email address" type="text" name="email" value="%(email)s" placeholder="[email protected]"><span style="color: red">%(email_error)s</span></div>
     <button type="submit">Sign up</button>
 </form>
 """
@@ -55,15 +56,77 @@ def rot13(self, text):
 
 
 class Signup(webapp2.RequestHandler):
+
     def get(self):
         return self.write_form()
 
     def post(self):
-        self.redirect("/welcome")
-
-    def write_form(self, username_error="", password_error="", verify_pass_error="", email_error=""):
+        username = self.request.get("username")
+        password = self.request.get("password")
+        password_verify = self.request.get("verify")
+        email = self.request.get("email")
+
+        username_error = ""
+        password_error = ""
+        verify_pass_error = ""
+        email_error = ""
+
+        have_error = False
+
+        is_username_valid = self.is_username_valid(username=username)
+        is_password_valid = self.is_password_valid(password=password)
+        is_password_verify_valid = self.is_password_verify_valid(password=password, password_verify=password_verify)
+        is_email_valid = self.is_email_valid(email=email)
+
+        if not is_username_valid:
+            username_error = "Invalid username"
+            have_error = True
+
+        if not is_password_valid:
+            password_error = "Invalid password"
+            have_error = True
+
+        if not is_password_verify_valid:
+            verify_pass_error = "Passwords don't match"
+            have_error = True
+
+        if not is_email_valid:
+            email_error = "Invalid email"
+            have_error = True
+
+        if have_error:
+            self.write_form(
+                username=username,
+                email=email,
+                username_error=username_error,
+                password_error=password_error,
+                verify_pass_error=verify_pass_error,
+                email_error=email_error
+            )
+        else:
+            self.redirect("/welcome?username=" + username)
+
+
+    USER_RE = re.compile(r"^[a-zA-Z0-9_-]{3,20}$")
+    def is_username_valid(self, username):
+        return username and self.USER_RE.match(username)
+
+    PASS_RE = re.compile(r"^.{3,20}$")
+    def is_password_valid(self, password):
+        return password and self.PASS_RE.match(password)
+
+    def is_password_verify_valid(self, password, password_verify):
+        return password == password_verify
+
+    EMAIL_RE = re.compile(r"^[\S]+@[\S]+.[\S]+$")
+    def is_email_valid(self, email):
+        return not email or self.EMAIL_RE.match(email)
+
+    def write_form(self, username="", email="", username_error="", password_error="", verify_pass_error="", email_error=""):
         self.response.headers['Content-Type'] = 'text/html'
         self.response.out.write(signup_form % {
+            "username": username,
+            "email": email,
             "username_error": username_error,
             "password_error": password_error,
             "verify_error": verify_pass_error,
@@ -73,7 +136,8 @@ def write_form(self, username_error="", password_error="", verify_pass_error="",
 
 class Welcome(webapp2.RequestHandler):
     def get(self):
-        return self.response.out.write("Welcome")
+        username = self.request.get('username')
+        return self.response.out.write("Welcome, " + username)
 
 
 app = webapp2.WSGIApplication([