Data Type - 'tuple' for Immutable List

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∟Data Type - 'tuple' for Immutable List

This section describes the 'tuple' data type, which stores a sequence of references to data objects of any data types. 'tuple' data objects are immutable.

What Is the "tuple" Data Type? "tuple" is the immutable list data type in Python. Each element in a "tuple" object stores a reference to another data object.

"tuple" data type has the following main Features.

1. "tuple" data objects can be created in several ways:

Using listed objects within parentheses like ('apple', 'orange'). For singleton, an extra trailing comma is needed: ('apple',).
Using listed objects without parentheses like: 'apple', 'orange'. For singleton, an extra trailing comma is needed: 'apple',.
Using the cast function, tuple(object), like tuple(['apple', 'orange']), etc.

Here are some examples on how to create "tuple" objects:

>>> (9, 1, 1)
(9, 1, 1)

>>> 9, 1, 1
(9, 1, 1)

# a singleton tuple within parentheses
>>> (9,)
(9,)

# a singleton tuple without parentheses
>>> 9,
(9,)

# this is not a tuple. it's an integer in parentheses.
>>> (9)
9

>>> ('nine', 'one', 'one')
('nine', 'one', 'one')

# elements are of different data types
>>> ('nine', b'one', 1)
('nine', b'one', 1)

2. "tuple" data type provides an array-style expression to access the element of a given position.

>>> x = ('nine', b'one', 1)
>>> type(x)
<class 'tuple'>

>>> e = x[0]
>>> e
'nine'
>>> type(e)
<class 'str'>

3. "tuple" data type is an immutable data type. Once a "tuple" data object is created to store a sequence of references, this sequence will never change.

>>> x = ('nine', b'one', 1)
>>> e = x[0]
>>> e
'nine'

>>> x[0] = 'ten'
TypeError: 'tuple' object does not support item assignment

Don't get confused about "tuple" immutability and "tuple" variable re-assignment capability. A "tuple" variable can be re-assigned with different "tuple" objects many times.

# new object is created for (9, 1, 1) and assigned to x
>>> x = (9, 1, 1)
>>> id(x)
4459103168

# assign the same object to y
>>> y = x
>>> id(y)
4459103168

# new object is created for ('nine', 'one', 'one') and assigned to x
>>> x = ('nine', 'one', 'one')
>>> id(x)
4458939456

# the first object for (9, 1, 1) is still there in memory
>>> y
(9, 1, 1)
>>> id(y)
4459103168

3. Because "tuple" data type is immutable, an object created for a given object sequence is not allowed to change its value. This allow "tuple" objects to be cached in memory and reused later whenever are needed again. Reusing objects will reduce execution time and memory consumption.

However on my macOS computer, "tuple" objects are not cached and reused at all. The code example below shows you that there are 4 "tuple" objects created for 4 requests of (9,1,1).

>>> x = (9, 1, 1)
>>> y = (9, 1, 1)
>>> (id(x), id(y), id((9, 1, 1)), id(tuple([9, 1, 1])))
(4458939136, 4458939456, 4459103168, 4458811648)

5. "tuple" objects support the following operations:

    Syntax   Operation       Note
----------   -------------   ------------------
     x + y   Concatenation
     x * n   Repetition      n is an "int"
     n * x   Repetition      Same as x * n
    s in x   Look up         s is an object
s not in x   Look up         Same as !(s in x)

6. Some built-in functions are provided for "tuple" objects.

len(x) - Returns the number of elements in this "tuple" object.

7. Some instance methods are provided for "tuple" objects.

tuple.count(object) - Instance method returns the number of occurrences of the given object in this tuple.
tuple.find(object) - Instance method returns the lowest index in this tuple where the given object is found. Returns -1, if the given object is not found.
tuple.index(object) - Instance method returns the lowest index in this tuple where the given object is found. Raises ValueError, if the given object is not found.