title: rpy2 object not found error
body: <p>I am trying to use rpy2 to let me use some r functionality in python. Here is a simple regression I want to do. I create a data frame, convert it to R data frame and then try using R's lm. But the R data frame cannot be found (see below). Where should I look to troubleshoot?</p>
  
  <p>FYI I am using python 2.7.3, rpy2-2.3.2, pandas version '0.10.1' and R2.15.3</p>
  
  <pre><code>&gt;&gt;&gt; import rpy2
  &gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; import pandas.rpy.common as com
  &gt;&gt;&gt; datframe = pd.DataFrame({'a' : [1, 2, 3], 'b' : [3, 4, 5]})
  &gt;&gt;&gt; r_df = com.convert_to_r_dataframe(datframe)
  &gt;&gt;&gt; r_df     
  (DataFrame - Python:0x32547e8 / R:0x345d640)
  [IntVector, IntVector]
    a: (class 'rpy2.robjects.vectors.IntVector')
    (IntVector - Python:0x3254e18 / R:0x345d608)
  [       1,        2,        3]
    b: (class 'rpy2.robjects.vectors.IntVector')
    (IntVector - Python:0x3254e60 / R:0x345d5d0)
  [       3,        4,        5]
  &gt;&gt;&gt; print type(r_df)
  (class 'rpy2.robjects.vectors.DataFrame')
  &gt;&gt;&gt; from rpy2.robjects import r
  &gt;&gt;&gt; r('lmout &lt;- lm(r_df$a ~ r_df$b)')
  
  Error in eval(expr, envir, enclos) : object 'r_df' not found
  Traceback (most recent call last):
    File "&lt;pyshell#8&gt;", line 1, in &lt;module&gt;
      r('lmout &lt;- lm(r_df$a ~ r_df$b)')
    File "/usr/local/lib/python2.7/dist-packages/rpy2/robjects/__init__.py", line 236, in __call__
      res = self.eval(p)
    File "/usr/local/lib/python2.7/dist-packages/rpy2/robjects/functions.py", line 86, in __call__
      return super(SignatureTranslatedFunction, self).__call__(*args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/rpy2/robjects/functions.py", line 35, in __call__
      res = super(Function, self).__call__(*new_args, **new_kwargs)
  RRuntimeError: Error in eval(expr, envir, enclos) : object 'r_df' not found
  </code></pre>