Follow comments.

error_rate.py

 # -*- coding: utf-8 -*-
-"""
-    This module provides functions to calculate error rate in different level.
-    e.g. wer for word-level, cer for char-level.
+"""This module provides functions to calculate error rate in different level.
+e.g. wer for word-level, cer for char-level.
 """
 
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
 import numpy as np
 
 
@@ -42,8 +44,7 @@ def levenshtein_distance(ref, hyp):
 
 
 def wer(reference, hypothesis, ignore_case=False, delimiter=' '):
-    """
-    Calculate word error rate (WER). WER compares reference text and 
+    """Calculate word error rate (WER). WER compares reference text and 
     hypothesis text in word-level. WER is defined as:
 
     .. math::
@@ -71,6 +72,7 @@ def wer(reference, hypothesis, ignore_case=False, delimiter=' '):
     :type delimiter: char
     :return: Word error rate.
     :rtype: float
+    :raises ValueError: If reference length is zero.
     """
     if ignore_case == True:
         reference = reference.lower()
@@ -88,8 +90,7 @@ def wer(reference, hypothesis, ignore_case=False, delimiter=' '):
 
 
 def cer(reference, hypothesis, ignore_case=False):
-    """
-    Calculate charactor error rate (CER). CER compares reference text and
+    """Calculate charactor error rate (CER). CER compares reference text and
     hypothesis text in char-level. CER is defined as:
 
     .. math::
@@ -117,6 +118,7 @@ def cer(reference, hypothesis, ignore_case=False):
     :type ignore_case: bool
     :return: Character error rate.
     :rtype: float
+    :raises ValueError: If reference length is zero.
     """
     if ignore_case == True:
         reference = reference.lower()

tests/test_error_rate.py

 # -*- coding: utf-8 -*-
+"""Test error rate."""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
 import unittest
-import sys
-sys.path.append('..')
 import error_rate
 
 
 class TestParse(unittest.TestCase):
-    def test_wer(self):
+    def test_wer_1(self):
         ref = 'i UM the PHONE IS i LEFT THE portable PHONE UPSTAIRS last night'
         hyp = 'i GOT IT TO the FULLEST i LOVE TO portable FROM OF STORES last night'
         word_error_rate = error_rate.wer(ref, hyp)
         self.assertTrue(abs(word_error_rate - 0.769230769231) < 1e-6)
 
-    def test_cer_en(self):
+    def test_wer_2(self):
+        ref = 'i UM the PHONE IS i LEFT THE portable PHONE UPSTAIRS last night'
+        word_error_rate = error_rate.wer(ref, ref)
+        self.assertEqual(word_error_rate, 0.0)
+
+    def test_wer_3(self):
+        ref = ' '
+        hyp = 'Hypothesis sentence'
+        try:
+            word_error_rate = error_rate.wer(ref, hyp)
+        except Exception as e:
+            self.assertTrue(isinstance(e, ValueError))
+
+    def test_cer_1(self):
         ref = 'werewolf'
         hyp = 'weae  wolf'
         char_error_rate = error_rate.cer(ref, hyp)
         self.assertTrue(abs(char_error_rate - 0.25) < 1e-6)
 
-    def test_cer_zh(self):
+    def test_cer_2(self):
+        ref = 'werewolf'
+        char_error_rate = error_rate.cer(ref, ref)
+        self.assertEqual(char_error_rate, 0.0)
+
+    def test_cer_3(self):
         ref = u'我是中国人'
         hyp = u'我是 美洲人'
         char_error_rate = error_rate.cer(ref, hyp)
         self.assertTrue(abs(char_error_rate - 0.6) < 1e-6)
 
+    def test_cer_4(self):
+        ref = u'我是中国人'
+        char_error_rate = error_rate.cer(ref, ref)
+        self.assertFalse(char_error_rate, 0.0)
+
+    def test_cer_5(self):
+        ref = ''
+        hyp = 'Hypothesis'
+        try:
+            char_error_rate = error_rate.cer(ref, hyp)
+        except Exception as e:
+            self.assertTrue(isinstance(e, ValueError))
+
 
 if __name__ == '__main__':
     unittest.main()