Python Tuples

Python Tuples

TODO: Tuple basic info

Python Docs - Tuples

Creation:

An empty tuple is created using a pair of round brackets, ():

>>> empty_tuple = ()
>>> print(empty_tuple)
()
>>> type(empty_tuple)
<class 'tuple'>
>>> len(empty_tuple)
0

A tuple with elements is created by separating the elements with commas (surrounding round brackets, (), are optional with exceptions):

>>> tuple_1 = 1, 2, 3       # Create tuple without round brackets.
>>> print(tuple_1)
(1, 2, 3)
>>> type(tuple_1)
<class 'tuple'>
>>> len(tuple_1)
3
>>> tuple_2 = (1, 2, 3)     # Create tuple with round brackets.
>>> print(tuple_2)
(1, 2, 3)
>>> tuple_3 = 1, 2, 3,      # Trailing comma is optional.
>>> print(tuple_3)
(1, 2, 3)
>>> tuple_4 = (1, 2, 3,)    # Trailing comma in round brackets is also optional.
>>> print(tuple_4)
(1, 2, 3)

A tuple with a single element must have the trailing comma (with or without round brackets):

>>> not_tuple = (2)    # No trailing comma makes this not a tuple.
>>> print(not_tuple)
2
>>> type(not_tuple)
<class 'int'>
>>> a_tuple = (2,)     # Single element tuple. Requires trailing comma.
>>> print(a_tuple)
(2,)
>>> type(a_tuple)
<class 'tuple'>
>>> len(a_tuple)
1
>>> also_tuple = 2,    # Round brackets omitted. Requires trailing comma.
>>> print(also_tuple)
(2,)
>>> type(also_tuple)
<class 'tuple'>

Round brackets are required in cases of ambiguity (if the tuple is part of a larger expression):

Note that it is actually the comma which makes a tuple, not the parentheses. The parentheses are optional, except in the empty tuple case, or when they are needed to avoid syntactic ambiguity. For example, f(a, b, c) is a function call with three arguments, while f((a, b, c)) is a function call with a 3-tuple as the sole argument.

>>> print(1,2,3,4,)          # Calls print with 4 arguments: 1, 2, 3, and 4
1 2 3 4
>>> print((1,2,3,4,))        # Calls print with 1 argument: (1, 2, 3, 4,)
(1, 2, 3, 4)
>>> 1, 2, 3 == (1, 2, 3)     # Equivalent to 1, 2, (3 == (1, 2, 3))
(1, 2, False)
>>> (1, 2, 3) == (1, 2, 3)   # Use surrounding round brackets when ambiguous.
True

A tuple can also be created with the tuple constructor:

Python Docs - Tuple

>>> empty_tuple = tuple()
>>> print(empty_tuple)
()
>>> tuple_from_list = tuple([1,2,3,4])
>>> print(tuple_from_list)
(1, 2, 3, 4)
>>> tuple_from_string = tuple("Hello campers!")
>>> print(tuple_from_string)
('H', 'e', 'l', 'l', 'o', ' ', 'c', 'a', 'm', 'p', 'e', 'r', 's', '!')
>>> a_tuple = 1, 2, 3
>>> b_tuple = tuple(a_tuple)    # If the constructor is called with a tuple for
the iterable,
>>> a_tuple is b_tuple          # the tuple argument is returned.
True

Accessing elements of a tuple:

Elements of tuples are accessed and index the same way that lists are.

>>> my_tuple = 1, 2, 9, 16, 25
>>> print(my_tuple)
(1, 2, 9, 16, 25)

Zero indexed

>>> my_tuple[0]
1
>>> my_tuple[1]
2
>>> my_tuple[2]
9

Wrap around indexing

>>> my_tuple[-1]
25
>>> my_tuple[-2]
16

Packing and Unpacking:

The statement t = 12345, 54321, 'hello!' is an example of tuple packing: the values 12345, 54321 and 'hello!' are packed together in a tuple. The reverse operation is also possible:

>>> x, y, z = t

This is called, appropriately enough, sequence unpacking and works for any sequence on the right-hand side. Sequence unpacking requires that there are as many variables on the left side of the equals sign as there are elements in the sequence. Note that multiple assignment is really just a combination of tuple packing and sequence unpacking.

>>> t = 1, 2, 3    # Tuple packing.
>>> print(t)
(1, 2, 3)
>>> a, b, c = t    # Sequence unpacking.
>>> print(a)
1
>>> print(b)
2
>>> print(c)
3
>>> d, e, f = 4, 5, 6    # Multiple assignment combines packing and unpacking.
>>> print(d)
4
>>> print(e)
5
>>> print(f)
6
>>> a, b = 1, 2, 3       # Multiple assignment requires each variable (right)
have a matching element (left).
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 2)

Immutable:

tuples are immutable containers, guaranteeing which objects they contain will not change. It does not guarantee that the objects they contains will not change:

>>> a_list = []
>>> a_tuple = (a_list,)    # A tuple (immutable) with a list (mutable) element.
>>> print(a_tuple)
([],)

>>> a_list.append("Hello campers!")
>>> print(a_tuple)         # Element of the immutable is mutated.
(['Hello campers!'],)

Uses:

Functions can only return a single value, however, a heterogenuous tuple can be used to return multiple values from a function. One example is the built-in enumerate function that returns an iterable of heterogenuous tuples:

>>> greeting = ["Hello", "campers!"]
>>> enumerator = enumerate(greeting)
>>> enumerator.next()
>>> enumerator.__next__()
(0, 'Hello')
>>> enumerator.__next__()
(1, 'campers!')

Python Docs - Tuples and Sequences

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