Using ``key in list`` to check if key is contained in list
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Using ``key in list`` to iterate through a list can potentially take ``n`` iterations to complete, where ``n`` is the number of items in the list. If possible, you should change the list to a set or dictionary instead, because Python can search for items in a set or dictionary by attempting to directly accessing them without iterations, which is much more efficient.

Anti-pattern
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The code below defines a list ``l`` and then calls ``if 3 in l`` to check if the number 3 exists in the list. This is inefficient. Behind the scenes, Python iterates through the list until it finds the number or reaches the end of the list.

.. code:: python

    l = [1, 2, 3, 4]

    # iterates over three elements in the list
    if 3 in l:
        print("The number 3 is in the list.")
    else:
        print("The number 3 is NOT in the list.")

Best practice
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Use a set or dictionary instead of a list
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In the modified code below, the list has been changed to a set. This is much more efficient behind the scenes, as Python can attempt to directly access the target number in the set, rather than iterate through every item in the list and compare every item to the target number.

.. code:: python

    s = set([1, 2, 3, 4])

    if 3 in s:
        print("The number 3 is in the list.")
    else:
        print("The number 3 is NOT in the list.")