diff --git a/README.md b/README.md
index 4c5d873..e3f0f59 100644
--- a/README.md
+++ b/README.md
@@ -35,15 +35,18 @@ Credits and links can be found in AUTHORS.md.
     # instantiate BloomFilter with custom settings,
     # max_elements is how many elements you expect the filter to hold.
     # error_rate defines accuracy; You can use defaults with
+    # key is a mapping from input objects to something the filter can take as input
     # `BloomFilter()` without any arguments. Following example
     # is same as defaults:
     bloom = BloomFilter(max_elements=10000, error_rate=0.1)
 
-    # Test whether the bloom-filter has seen a key:
-    assert "test-key" in bloom is False
+    # Test whether the bloom-filter has seen a object:
+    assert "test-obj" in bloom is False
 
-    # Mark the key as seen
-    bloom.add("test-key")
+    # Mark the object as seen
+    bloom.add("test-obj")
 
     # Now check again
-    assert "test-key" in bloom is True
+    assert "test-obj" in bloom is True
+
+    # If you are using a key make sure it's a one to one mapping
diff --git a/src/bloom_filter/bloom_filter.py b/src/bloom_filter/bloom_filter.py
index e16c952..7056c59 100644
--- a/src/bloom_filter/bloom_filter.py
+++ b/src/bloom_filter/bloom_filter.py
@@ -368,7 +368,6 @@ def __ior__(self, other):
                 self.set(bitno)
             else:
                 self.clear(bitno)
-
         return self
 
     def close(self):
@@ -510,7 +509,8 @@ def __init__(self,
                  error_rate=0.1,
                  probe_bitnoer=get_bitno_lin_comb,
                  filename=None,
-                 start_fresh=False):
+                 start_fresh=False,
+                 key=None):
         # pylint: disable=R0913
         # R0913: We want a few arguments
         if max_elements <= 0:
@@ -547,16 +547,20 @@ def __init__(self,
         real_num_probes_k = (self.num_bits_m / self.ideal_num_elements_n) * math.log(2)
         self.num_probes_k = int(math.ceil(real_num_probes_k))
         self.probe_bitnoer = probe_bitnoer
+        self.key = key
 
     def __repr__(self):
-        return 'BloomFilter(ideal_num_elements_n=%d, error_rate_p=%f, num_bits_m=%d)' % (
+        return 'BloomFilter(ideal_num_elements_n=%d, error_rate_p=%f, num_bits_m=%d, key=%s)' % (
             self.ideal_num_elements_n,
             self.error_rate_p,
             self.num_bits_m,
+            str(self.key),
         )
 
     def add(self, key):
         """Add an element to the filter"""
+        if self.key:
+            key = self.key(key)
         for bitno in self.probe_bitnoer(self, key):
             self.backend.set(bitno)
 
@@ -587,6 +591,8 @@ def __iand__(self, bloom_filter):
         return self
 
     def __contains__(self, key):
+        if self.key:
+            key=self.key(key)
         for bitno in self.probe_bitnoer(self, key):
             if not self.backend.is_set(bitno):
                 return False
diff --git a/tests/test_bloom_filter.py b/tests/test_bloom_filter.py
index a94508c..f526cec 100755
--- a/tests/test_bloom_filter.py
+++ b/tests/test_bloom_filter.py
@@ -18,6 +18,7 @@
 
 import random
 
+# if you are doing tests this imports from your pip, so it might not be the updated version
 import bloom_filter
 
 CHARACTERS = 'abcdefghijklmnopqrstuvwxyz1234567890'
@@ -105,7 +106,6 @@ def _test(description, values, trials, error_rate, probe_bitnoer=None, filename=
 
     return all_good
 
-
 class States(object):
     """Generate the USA's state names"""
 
@@ -200,6 +200,42 @@ def length(self):
         """How many members?"""
         return int(math.ceil(self.maximum / 2.0))
 
+def key_test():
+    """ Test that we can do everything with a key too """
+    class Dummy():
+        def __init__(self, biscuit):
+            self.biscuit = biscuit
+        def get(self):
+            return self.biscuit
+    def dummy_key(dummy):
+        return dummy.get()
+
+    dummy_filter = bloom_filter.BloomFilter(key=dummy_key)
+
+    dummies = [Dummy(3), Dummy('a'), Dummy('b')]
+    for dummy in dummies:
+        dummy_filter += dummy
+    for dummy in dummies:
+        if dummy not in dummy_filter:
+            return False # failed to insert
+
+    other_dummy_filter = bloom_filter.BloomFilter(key=dummy_key)
+    other_dummy_filter += Dummy('hello world')
+
+    dummy_filter |= other_dummy_filter
+    for dummy in dummies + [Dummy('hello world')]:
+        if dummy not in dummy_filter:
+            return False
+
+    dummy_filter &= other_dummy_filter
+    for dummy in dummies:
+        if dummy in dummy_filter:
+            return False
+    if Dummy('hello world') not in dummy_filter:
+        return False
+
+    return True
+
 
 def and_test():
     """Test the & operator"""
@@ -304,6 +340,8 @@ def test_bloom_filter():
 
     all_good &= or_test()
 
+    all_good &= key_test()
+
     if performance_test:
         sqrt_of_10 = math.sqrt(10)
         # for exponent in range(5): # this is a lot, but probably not unreasonable
@@ -346,7 +384,5 @@ def test_bloom_filter():
     if not all_good:
         sys.stderr.write('%s: One or more tests failed\n' % sys.argv[0])
         sys.exit(1)
-
-
 if __name__ == '__main__':
     test_bloom_filter()