title: python mock default init argument of class
body: <p>I want to mock the default argument in a class constructor:</p>
  
  <pre><code>class A (object):
      def __init__(self, connection=DefaultConnection()):
          self.connection = connection
  </code></pre>
  
  <p>I want to mock <code>DefaultConnection</code> in my unittests, but it doesn't work when passed in as a default value.</p>