Data Structures

Tuples

Tuples are effectively an immutable list. Once a tuple has been defined, you cannot add, update, or delete items from it. A tuple begins and ends with parentheses () and items are separated by a comma ,

Spaces don't matter here.

a = ( 1, 2, 3 )

Indexing

Like other iterable types in Python, you can index and slice tuples

a[0]
a[1]
a[::2]

Unpacking

Often used in for loops, you can unpack a tuple into individual variables

a, b, c = ( 1, 2, 3 )

In this case a = 1, b = 2, and c = 3. Remember this pattern as it often appears in code and documentation.

Immutability

The immutability property of a tuple is desirable under some circumstances. For example, only immutable types in Python can be hashed and only hashable types can be used as dictionary keys. For example, this is perfectly legal code

a = {
    ( 'StudyA', 'Subject1' ): [ 'Session1', 'Session2' ],
    ( 'StudyA', 'Subject2' ): [ 'Session3', 'Session4' ],
    ( 'StudyB', 'Subject1' ): [ 'Session5', 'Session6' ]
}

Note that you cannot hash a tuple if it contains a mutable type.

and subsequently you can retrieve an item from this dictionary using ('StudyA','Subject2')

a[( 'StudyA', 'Subject2' )]