Most common element (#29)

This is my suggestion for an implementation of the function most_common_element with pure Python commands, not using numpy. :-)
A test was provided.

src/listwiz/inspect.py

@@ -40,6 +40,28 @@ def longest_decreasing_streak(l):
     raise NotImplementedError
 
 
-def most_common_element(l):
-    # Find the most common element in the list
-    raise NotImplementedError
+def most_common_element(ll):
+    """Find the most common element in a list.
+
+    Parameters
+    ----------
+    ll : list
+
+    Returns
+    -------
+    most common element
+    """
+
+    if not isinstance(ll, list):
+        raise ValueError("Input must be a list")
+
+    counting ={}
+    for i in ll:
+        if i in counting:
+            counting[i] += 1
+        else:
+            counting[i] = 1
+
+    # Find the key with the maximum value
+    most_common = max(counting, key=counting.get)
+    return most_common  

tests/test_inspect.py

@@ -26,7 +26,10 @@ def test_longest_decreasing_streak():
     raise AssertionError("This test is missing.")
 
 
-@pytest.mark.skip(reason="Test is not implemented yet")
+
 def test_most_common_element():
-    raise AssertionError("This test is missing.")
+    ll = [1, 2, 3, 4, 1, 1, 2, 3, 3, 3, 3, 3]
+    res = lwi.most_common_element(ll)
+    assert res == 3
+