An immutable collection cannot be changed after it is created. In Python, a tuple is immutable; a list is mutable. If you need different contents in a tuple, you create another tuple rather than change the original one. section 24.1 section 24.2
A new tuple, not a changed tuple
Start with `groceries = ("milk", "eggs")`. Later, `groceries = ("milk", "eggs", "bread")` gives the name `groceries` a new tuple. It does not add bread to the first tuple. By contrast, a list can add bread to the same object with `append()`. section 24.1 section 24.2
Figure 24.1. The variable named a is bound to an object with value 1 in one memory location. When the variable named a is bound to a different object with value 2, the original object with value 1 is still in memory; you just can’t access it anymore through a variable.
The diagram uses integers rather than collections, but it shows the same important distinction: assigning a new value to a name does not change the immutable object it previously named. section 24.1
Why does the distinction matter? A dictionary uses a key’s value to find its stored value again. Its keys must be immutable so that a key does not change between storing and looking up an item. The dictionary itself can still change, and its values may be mutable or immutable. Quick check: if you assign a new tuple to a name, which tuple changed? Neither. section 27.2 section 27.6
