Small style fixes

IPython/nbformat/current.py

@@ -87,7 +87,7 b' def reads_json(nbjson, **kwargs):'
     nb = reader_reads(nbjson, **kwargs)
     nb_current = convert(nb, current_nbformat)
     errors = validate(nb_current)
-    if len(errors) > 0:
+    if errors:
         logger.error(
             "Notebook JSON is invalid (%d errors detected during read)",
             len(errors))
@@ -100,7 +100,7 b' def writes_json(nb, **kwargs):'
 
     """
     errors = validate(nb)
-    if len(errors) > 0:
+    if errors:
         logger.error(
             "Notebook JSON is invalid (%d errors detected during write)",
             len(errors))

IPython/nbformat/validator.py

@@ -23,7 +23,7 b' def isvalid(nbjson):'
     """
 
     errors = validate(nbjson)
-    return errors == 0
+    return errors == []
 
 
 def validate(nbjson):
@@ -42,7 +42,7 b' def validate(nbjson):'
 
     # count how many errors there are
     v = Draft3Validator(v3schema)
-    errors = [e for e in v.iter_errors(nbjson)]
+    errors = list(v.iter_errors(nbjson))
     return errors