title: Python- import ipdb fails
body: <p>I'm trying to use Python Debugger.</p>
  
  <p>I don't know what's going wrong here,</p>
  
  <pre><code>def main():
      print "over ipdb"
      import ipdb; ipdb.set_trace()
      low, high = 0, 1
      for i in xrange(10):
          print high
          low, high = high, low + high
  
  if __name__ == '__main__':
    main()
  </code></pre>
  
  <p>the terminal keep showing this:</p>
  
  <pre><code>over ipdb
  Before the function runs
  I am a stand alone function, don't you dare modify me
  After the function runs
  Traceback (most recent call last):
    File "pydebudder.py", line 12, in &lt;module&gt;
      main()
    File "pydebudder.py", line 5, in main
      import ipdb; ipdb.set_trace()
    File "/usr/local/lib/python2.7/dist-packages/ipdb/__init__.py", line 16, in &lt;module&gt;
      from ipdb.__main__ import set_trace, post_mortem, pm, run, runcall, runeval, launch_ipdb_on_exception
    File "/usr/local/lib/python2.7/dist-packages/ipdb/__main__.py", line 27, in &lt;module&gt;
      import IPython
    File "/usr/local/lib/python2.7/dist-packages/IPython/__init__.py", line 45, in &lt;module&gt;
      from .config.loader import Config
    File "/usr/local/lib/python2.7/dist-packages/IPython/config/__init__.py", line 6, in &lt;module&gt;
      from .application import *
    File "/usr/local/lib/python2.7/dist-packages/IPython/config/application.py", line 17, in &lt;module&gt;
      from IPython.external.decorator import decorator
  ImportError: cannot import name decorator
  </code></pre>
  
  <p>I upgraded ipython to 3.0.0</p>