diff --git a/python/ucf/ucf.py b/python/ucf/ucf.py
index 70d3754..e8208e4 100644
--- a/python/ucf/ucf.py
+++ b/python/ucf/ucf.py
@@ -64,11 +64,11 @@ class UCF:
             self.__timeStep[istep]      = self.__currentStepTime
             self.__posStep[istep]       = self.__fileID.tell()
             self.__numSetPerStep[istep] = self.__currentStepNumSet
+            istep = istep+1
             if self.__currentStepSize==-1:
               break
             else:
               self.__fileID.seek(self.__currentStepSize,1)
-            istep = istep+1
         nstep = istep
 
         # Truncate buffered arrays
@@ -83,6 +83,11 @@ class UCF:
         self.NumTimestep = nstep;
         self.__isFileHeaderWritten = True;
         self.__isStepHeaderWritten = True;
+
+    def close(self):
+        """Closes input file object"""
+        self.__fileID.close()
+        self.__init__
   
     def initBuffer(self,rank=0,rankijk=(0,0,0),ftype=1999):
         """Initialize a buffer to generate a new UCF file."""
@@ -244,6 +249,8 @@ class UCF:
 
     def readSet(self,step=1,dset=1,memmap=False):
         """Read a dataset from input file. If 'memmap' is activated, the file will only be read partially on demand."""
+        if not self.__inputAvailable:
+            raise IOError('No input file available')
         self.__fileID.seek(self.__findSet(step,dset),0)
         self.__readHeaderSet();
         params = self.__currentSetParams
@@ -269,14 +276,14 @@ class UCF:
             if currentMagic==self.__magicFile:
                 break
         if currentMagic!=self.__magicFile:
-            raise ValueError("Magic mismatch: invalid file header. %l" % currentMagic)
+            raise ValueError('Magic mismatch: invalid file header. {}'.format(currentMagic))
         self.Endian = fmt
         # Read header
         self.__fileID.seek(self.__fileBeg,0);
         buff = self.__fileID.read(self.__nHeaderFile*8)
         header = struct.unpack("%s%dq"%(self.Endian,8),buff)
         if self.Debug:
-            print("Read the following file header at %d bytes" % 0,file=sys.stderr)
+            print('Read the following file header at 0 bytes',file=sys.stderr)
             print(header,file=sys.stderr)
         # Parse version
         self.__versionMajor = np.floor(header[1]/self.__factorMajor)