diff --git a/data_juicer/ops/deduplicator/document_simhash_deduplicator.py b/data_juicer/ops/deduplicator/document_simhash_deduplicator.py
index 4e9ad1790..eca420b87 100644
--- a/data_juicer/ops/deduplicator/document_simhash_deduplicator.py
+++ b/data_juicer/ops/deduplicator/document_simhash_deduplicator.py
@@ -21,45 +21,45 @@
 with AvailabilityChecking(['simhash-py'], OP_NAME):
     import simhash
 
+    def local_num_differing_bits(hash_a, hash_b):
+        """
+        Local implementation of calculating the number of different bits
+        between two integers.
 
-def local_num_differing_bits(hash_a, hash_b):
-    """
-    Local implementation of calculating the number of different bits between
-    two integers.
-
-    :param hash_a: integer hash value a
-    :param hash_b: integer hash value b
-    :return: number of different bits between input hashes.
-    """
-    cnt = 0
-    n = hash_a ^ hash_b
-    while n != 0:
-        cnt += 1
-        n = n & (n - 1)
-    return cnt
-
-
-def num_differing_bits_selector():
-    """
-    Select a num_differing_bits method according to the Python version
-    installed.
-
-    When Python >= 3.9, the original simhash library cannot be compiled
-    correctly due to some changes in cython. After fixing this
-    incompatibility, RecursionError occurs sometimes when calling
-    simhash.num_differing_bits. So we use our implementation when Python
-    >= 3.9. Otherwise, we use implementation of simhash.
-
-    :return: an available num_differing_bits function.
-    """
-    import platform
-    a, b, _ = platform.python_version().split('.')
-    if a == '3' and int(b) >= 9:
-        # for >= 3.9, use local implementation
-        return local_num_differing_bits
-    else:
-        # for < 3.9, use simhash version
-        return simhash.num_differing_bits
+        :param hash_a: integer hash value a
+        :param hash_b: integer hash value b
+        :return: number of different bits between input hashes.
+        """
+        cnt = 0
+        n = hash_a ^ hash_b
+        while n != 0:
+            cnt += 1
+            n = n & (n - 1)
+        return cnt
+
+    def num_differing_bits_selector():
+        """
+        Select a num_differing_bits method according to the Python version
+        installed.
+
+        When Python >= 3.9, the original simhash library cannot be compiled
+        correctly due to some changes in cython. After fixing this
+        incompatibility, RecursionError occurs sometimes when calling
+        simhash.num_differing_bits. So we use our implementation when Python
+        >= 3.9. Otherwise, we use implementation of simhash.
+
+        :return: an available num_differing_bits function.
+        """
+        import platform
+        a, b, _ = platform.python_version().split('.')
+        if a == '3' and int(b) >= 9:
+            # for >= 3.9, use local implementation
+            return local_num_differing_bits
+        else:
+            # for < 3.9, use simhash version
+            return simhash.num_differing_bits
+
+    num_differing_bits = num_differing_bits_selector()
 
 
 @OPERATORS.register_module(OP_NAME)
@@ -114,8 +114,6 @@ def __init__(self,
         self.num_blocks = num_blocks
         self.hamming_distance = hamming_distance
 
-        self.num_differing_bits = num_differing_bits_selector()
-
     def compute_hash(self, sample):
         """
         Compute simhash values for the sample.
@@ -189,7 +187,7 @@ def process(self, dataset, show_num=0):
         dist = Counter()
         for x, y in matches:
             graph[x][y] = graph[y][x] = True
-            num_diff = self.num_differing_bits(x, y)
+            num_diff = num_differing_bits(x, y)
             dist[num_diff] += 1
         logger.info(f'Hash diff distribution: {dist}')