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TUPLES

S.Madhumalar

Assistant Professor

C.P.A College, Bodinayakanur

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Tuples

Same as lists but

Immutable
Enclosed in parentheses
A tuple with a single element must have a comma inside the parentheses:

a = (11,)

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Examples

>>> mytuple = (11, 22, 33)
>>> mytuple[0]�11
>>> mytuple[-1]�33
>>> mytuple[0:1]�(11,)
The comma is required!

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Why?

No confusion possible between [11] and 11

(11) is a perfectly acceptable expression

(11) without the comma is the integer 11
(11, ) with the comma is a list containing the integer 11

Sole dirty trick played on us by tuples!

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Tuples are immutable

>>> mytuple = (11, 22, 33)
>>> saved = mytuple
>>> mytuple += (44,)
>>> mytuple�(11, 22, 33, 44)
>>> saved�(11, 22, 33)

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Things that do not work

mytuple += 55�Traceback (most recent call last):Z�…�TypeError:� can only concatenate tuple (not "int") to tuple

Can understand that!

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Sorting tuples

>>> atuple = (33, 22, 11)
>>> atuple.sort()�Traceback (most recent call last):�…�AttributeError:�'tuple' object has no attribute 'sort'
>>> atuple = sorted(atuple)
>>> atuple�[11, 22, 33]

Tuples are immutable!

sorted( ) returns a list!

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Most other things work!

>>> atuple = (11, 22, 33)
>>> len(atuple)�3
>>> 44 in atuple�False
>>> [ i for [i for i in atuple]�[11, 22, 33]

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The reverse does not work

>>> alist = [11, 22, 33]
>>> (i for i in alist)�<generator object <genexpr> at 0x02855DA0>

Does not work!

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Converting sequences into tuples

>>> alist = [11, 22, 33]
>>> atuple = tuple(alist)
>>> atuple�(11, 22, 33)
>>> newtuple = tuple('Hello World!')
>>> newtuple�('H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!')

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Sets

Indentified by curly braces

{'Alice', 'Bob', 'Carol'}
{'Dean'} is a singleton

Can only contain unique elements

Duplicates are eliminated

Immutable like tuples and strings

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Sets do not contain duplicates

>>> cset = {11, 11, 22}
>>> cset�{11, 22}

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Sets are immutable

>>> aset = {11, 22, 33}
>>> bset = aset
>>> aset = aset | {55}
>>> aset�{33, 11, 22, 55}
>>> bset�{33, 11, 22}

Union of two sets

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Sets have no order

>>> {1, 2, 3, 4, 5, 6, 7}�{1, 2, 3, 4, 5, 6, 7}
>>> {11, 22, 33}�{33, 11, 22}

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Sets do not support indexing

>>> myset = {'Apples', 'Bananas', 'Oranges'}
>>> myset�{'Bananas', 'Oranges', 'Apples'}
>>> myset[0]�Traceback (most recent call last):� File "<pyshell#2>", line 1, in <module>� myset[0]�TypeError: 'set' object does not support indexing

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Examples

>>> alist = [11, 22, 33, 22, 44]
>>> aset = set(alist)
>>> aset�{33, 11, 44, 22}
>>> aset = aset + {55}�SyntaxError: invalid syntax

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Boolean operations on sets (I)

Union of two sets

Contains all elements that are in set A or in set B

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Boolean operations on sets (II)

Intersection of two sets

Contains all elements that are in both sets A and B

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Boolean operations on sets (III)

Difference of two sets

Contains all elements that are in A but not in B

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Boolean operations on sets (IV)

Symmetric difference of two sets

Contains all elements that are either

in set A but not in set B or
in set B but not in set A

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Boolean operations on sets (V)

>>> aset = {11, 22, 33}
>>> bset = {12, 23, 33}
Union of two sets

>>> aset | bset �{33, 22, 23, 11, 12}

Intersection of two sets:

>>> aset & bset�{33}

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Boolean operations on sets (VI)

>>> aset = {11, 22, 33}
>>> bset = {12, 23, 33}
Difference:

>>> aset – bset�{11, 22}

Symmetric difference:

>>> aset ^ bset�{11, 12, 22, 23}

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DICTIONARIES

Very useful!

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Dictionaries (I)

Store pairs of entries called items�{ 'CS' : '743-713-3350', 'UHPD' : '713-743-3333'}
Each pair of entries contains

A key
A value

Key and values are separated by a colon
Paris of entries are separated by commas
Dictionary is enclosed within curly braces

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Usage

Keys must be unique within a dictionary

No duplicates

If we have� age = {'Alice' : 25, 'Bob' :28}�then� age['Alice'] is 25�and� age[Bob'] is 28

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Dictionaries are mutable

>>> age = {'Alice' : 25, 'Bob' : 28}
>>> saved = age
>>> age['Bob'] = 29
>>> age�{'Bob': 29, 'Alice': 25}
>>> saved�{'Bob': 29, 'Alice': 25}

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Keys must be unique

>>> age = {'Alice' : 25, 'Bob' : 28, 'Alice' : 26}
>>> age�{'Bob': 28, 'Alice': 26}

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Displaying contents

>>> age = {'Alice' : 25, 'Carol': 'twenty-two'}
>>> age.items()�dict_items([ ('Alice', 25), ('Carol', 'twenty-two')])
>>> age.keys()�dict_keys([ 'Alice', 'Carol'])
age.values()�dict_values([28, 25, 'twenty-two'])

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Updating directories

>>> age = {'Alice': 26 , 'Carol' : 22}
>>> age.update({'Bob' : 29})
>>> age�{'Bob': 29, 'Carol': 22, 'Alice': 26}
>>> age.update({'Carol' : 23})
>>> age�{'Bob': 29, 'Carol': 23, 'Alice': 26}

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Returning a value

>>> age = {'Bob': 29, 'Carol': 23, 'Alice': 26}
>>> age.get('Bob')�29
>>> age['Bob']�29

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Removing a specific item (I)

>>> a = {'Alice' : 26, 'Carol' : 'twenty-two'}
>>> a�{'Carol': 'twenty-two', 'Alice': 26}
>>> a.pop('Carol’)�'twenty-two'
>>> a�{'Alice': 26}

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Removing a specific item (II)

>>> a.pop('Alice')�26
>>> a�{}
>>>

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Remove a random item

>>> age = {'Bob': 29, 'Carol': 23, 'Alice': 26}
>>> age.popitem()�('Bob', 29)
>>> age
{'Carol': 23, 'Alice': 26}
>>> age.popitem()�('Carol', 23)
>>> age�{'Alice': 26}

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Summary

Strings, lists, tuples, sets and dictionaries all deal with aggregates
Two big differences

Lists and dictionaries are mutable

Unlike strings, tuples and sets

Strings, lists and tuples are ordered

Unlike sets and dictionaries

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Mutable aggregates

Can modify individual items

x = [11, 22, 33]�x[0] = 44�will work

Cannot save current value

x = [11,22, 33]�y = x�will not work

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Immutable aggregates

Cannot modify individual items

s = 'hello!'�s[0] = 'H'�is an ERROR

Can save current value

s= 'hello!'�t = s�will work

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Ordered aggregates

Entities in the collection can be accessed through a numerical index

s= 'Hello!'�s[0]
x = ['Alice', 'Bob', 'Carol']�x[-1]
t = (11, 22)�t[1]

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Other aggregates

Cannot index sets

myset = {'Apples', 'Bananas', 'Oranges'} myset[0] is WRONG

Can only index dictionaries through their keys

age = {'Bob': 29, 'Carol': 23, 'Alice': 26}�age['Alice'] works�age[0] is WRONG