Some small fixes and changes of nb version conversion

IPython/nbformat/current.py

@@ -73,7 +73,7 b' def writes_json(nb, **kwargs):'
 def reads_py(s, **kwargs):
     """Read a .py notebook from a string and return the NotebookNode object."""
     nbf, nbm, s = parse_py(s, **kwargs)
-    if nbf == 2 or nbf == 3:
+    if nbf in (2, 3):
         nb = versions[nbf].to_notebook_py(s, **kwargs)
     else:
         raise NBFormatError('Unsupported PY nbformat version: %i' % nbf)

IPython/nbformat/reader.py

@@ -62,8 +62,8 b' def get_version(nb):'
     return (major, minor)
 
 
-def reads(s, format='ipynb', **kwargs):
-    """Read a notebook from a 'ipynb' (json) string and return the 
+def reads(s, **kwargs):
+    """Read a notebook from a json string and return the 
     NotebookNode object.
 
     This function properly reads notebooks of any version.  No version 
@@ -103,4 +103,4 b' def read(fp, **kwargs):'
     nb : NotebookNode
         The notebook that was read.
     """
-    return reads(fp.read(), format, **kwargs)
+    return reads(fp.read(), **kwargs)