diff --git a/IPython/nbformat/current.py b/IPython/nbformat/current.py
index ab1f6a9..889d6d5 100644
--- a/IPython/nbformat/current.py
+++ b/IPython/nbformat/current.py
@@ -73,7 +73,7 @@ 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)
diff --git a/IPython/nbformat/reader.py b/IPython/nbformat/reader.py
index 7993cd5..1526301 100644
--- a/IPython/nbformat/reader.py
+++ b/IPython/nbformat/reader.py
@@ -62,8 +62,8 @@ 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 @@ def read(fp, **kwargs):
     nb : NotebookNode
         The notebook that was read.
     """
-    return reads(fp.read(), format, **kwargs)
+    return reads(fp.read(), **kwargs)