Refactor to prefer rpy2's pandas2ri conversion system.

IPython/extensions/rmagic.py

@@ -46,8 +46,13 b' import numpy as np'
 
 import rpy2.rinterface as ri
 import rpy2.robjects as ro
-from rpy2.robjects.numpy2ri import numpy2ri
-ro.conversion.py2ri = numpy2ri
+try:
+    from rpy2.robjects import pandas2ri
+    pandas2ri.activate()
+except ImportError:
+    pandas2ri = None
+    from rpy2.robjects import numpy2ri
+    numpy2ri.activate()
 
 # IPython imports
 
@@ -58,6 +63,7 b' from IPython.testing.skipdoctest import skip_doctest'
 from IPython.core.magic_arguments import (
     argument, magic_arguments, parse_argstring
 )
+from IPython.external.simplegeneric import generic
 from IPython.utils.py3compat import str_to_unicode, unicode_to_str, PY3
 
 class RInterpreterError(ri.RRuntimeError):
@@ -114,17 +120,33 b' def Rconverter(Robj, dataframe=False):'
         Robj = np.rec.fromarrays(Robj, names = names)
     return np.asarray(Robj)
 
+@generic
 def pyconverter(pyobj):
-    """Convert Python objects to R objects."""
-    if 'pandas' in sys.modules:
-        # We only do this if pandas is already loaded
-        from pandas import DataFrame
-        if isinstance(pyobj, DataFrame):
-            from pandas.rpy.common import convert_to_r_dataframe
-            return convert_to_r_dataframe(pyobj, strings_as_factors=True)
+    """Convert Python objects to R objects. Add types using the decorator:
     
+    @pyconverter.when_type
+    """
+    return pyobj
+
+# The default conversion for lists seems to make them a nested list. That has
+# some advantages, but is rarely convenient, so for interactive use, we convert
+# lists to a numpy array, which becomes an R vector.
[e-mail]_type(list)
+def pyconverter_list(pyobj):
     return np.asarray(pyobj)
 
+if pandas2ri is None:
+    # pandas2ri was new in rpy2 2.3.3, so for now we'll fallback to pandas'
+    # conversion function.
+    try:
+        from pandas import DataFrame
+        from pandas.rpy.common import convert_to_r_dataframe
+        @pyconverter.when_type(DataFrame)
+        def pyconverter_dataframe(pyobj):
+            return convert_to_r_dataframe(pyobj, strings_as_factors=True)
+    except ImportError:
+        pass
+
 @magics_class
 class RMagics(Magics):
     """A set of magics useful for interactive work with R via rpy2.