Minor update to how the secret key is generated in the production environment.

Large-app-how-to.md

@@ -97,7 +97,7 @@ We'll create 4 modules, a user module (manage user's registration, login, lost p
     DEBUG = False
 
     ADMINS = frozenset(['youremail@yourdomain.com'])
-    SECRET_KEY = 'SecretKeyForSessionSigning'
+    SECRET_KEY = 'This string will be replaced with a proper key in production.'
 
     SQLALCHEMY_DATABASE_URI = 'sqlite:///' + os.path.join(_basedir, 'app.db')
     DATABASE_CONNECT_OPTIONS = {}
@@ -402,6 +402,9 @@ Finally, a simple `/app/templates/users/profile.html`:
 Here is the `/app/__init__.py` :
 
 ```python
+    import os
+    import sys
+    
     from flask import Flask, render_template
     from flask.ext.sqlalchemy import SQLAlchemy
 
@@ -410,6 +413,32 @@ Here is the `/app/__init__.py` :
 
     db = SQLAlchemy(app)
 
+    ########################
+    # Configure Secret Key #
+    ########################
+    def install_secret_key(app, filename='secret_key'):
+        """Configure the SECRET_KEY from a file
+        in the instance directory.
+
+        If the file does not exist, print instructions
+        to create it from a shell with a random key,
+        then exit.
+        """
+        filename = os.path.join(app.instance_path, filename)
+        
+        try:
+            app.config['SECRET_KEY'] = open(filename, 'rb').read()
+        except IOError:
+            print('Error: No secret key. Create it with:')
+            full_path = os.path.dirname(filename)
+            if not os.path.isdir(full_path):
+                print('mkdir -p {filename}'.format(filename=full_path))
+            print('head -c 24 /dev/urandom > {filename}'.format(filename=full_path))
+            sys.exit(1)
+
+    if not app.config['DEBUG']:
+        install_secret_key(app)
+
     @app.errorhandler(404)
     def not_found(error):
         return render_template('404.html'), 404