title: drop_all() freezes in Flask with SQLAlchemy
body: <p>I am writing test cases for a Flask application.</p>
  
  <p>I have a setUp method which drops the tables in the db before re-creating them again. 
  It looks like this: </p>
  
  <pre><code>def setUp(self):
      # other stuff...
      myapp.db.drop_all()
      myapp.db.create_all()
      # db creation...
  </code></pre>
  
  <p>This works fine for the first test, but it freezes at <code>drop_all</code> before the second test is run. </p>
  
  <p><strong>EDIT:</strong> 
  The stack trace looks like this when interrupting the process</p>
  
  <pre><code>  File "populate.py", line 70, in create_test_db
      print (myapp.db.drop_all())
    File ".../flask_sqlalchemy/__init__.py", line 864, in drop_all
      self._execute_for_all_tables(app, bind, 'drop_all')
    File ".../flask_sqlalchemy/__init__.py", line 848, in _execute_for_all_tables
      op(bind=self.get_engine(app, bind), tables=tables)
    File ".../sqlalchemy/sql/schema.py", line 3335, in drop_all
    ....
    File "/Library/Python/2.7/site-packages/MySQLdb/cursors.py", line 190, in execute
      r = self._query(query)
  </code></pre>
  
  <p>Anybody has a clue how to fix this?</p>