Never Use Lists Inside Tuples

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List and Tuple are really common and frequently used data-types of Python. Although both of these store values in quite similar fashion there's one main difference between these two data types. The list data-type is mutable, which means the values inside a list object can be changed. However, the tuple data-type is immutable, meaning the values inside a tuple object cannot be changed once its been initialized. To modify the values of a tuple Python Interpreter must create another instance of tuple with the modified values.

Once we create a tuple we want the values inside the tuples to never change. That is one of the main reasons we use tuples instead of lists. But when we add a list as an element of a tuple it creates a possibility that the tuple we declare might get modified(even if it's immutable).

Let's look at this example:

a = (['a', 'b'], 'c')
print(a)
# (['a', 'b'], 'c')
a[0].append('d')
print(a)
# (['a', 'b', 'd'], 'c')

We can clearly see that even though tuple is immutable we can modify the list element inside of the tuple. This clearly violates the pythonic data structure. That's why tuples should never contain list as its element.