61A Lecture 12 - University of California, Berkeleycs61a/fa18/assets/slides/... · 2019-01-11 · 61A Lecture 12. Announcements. Objects (Demo) Objects 4. Objects ... •A type of

61A Lecture 12

Announcements

Objects

(Demo)

Objects

4

Objects

•Objects represent information

4

Objects


• They consist of data and behavior, bundled together to create abstractions

4

Objects



• Objects can represent things, but also properties, interactions, & processes

4

Objects




• A type of object is called a class; classes are first-class values in Python

4

Objects





• Object-oriented programming:

4

Objects






•A metaphor for organizing large programs

4

Objects






•A metaphor for organizing large programs•Special syntax that can improve the composition of programs

4

Objects







• In Python, every value is an object

4

Objects








• All objects have attributes

4

Objects








• All objects have attributes• A lot of data manipulation happens through object methods

4

Objects








• All objects have attributes• A lot of data manipulation happens through object methods• Functions do one thing; objects do many related things

4

Example: Strings

(Demo)

Representing Strings: the ASCII Standard

American Standard Code for Information Interchange

6


American Standard Code for Information Interchange8

rows

: 3

bits

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rows

: 3

bits

16 columns: 4 bits

6



rows

: 3

bits

16 columns: 4 bits

• Layout was chosen to support sorting by character code

6



rows

: 3

bits

16 columns: 4 bits


• Rows indexed 2-5 are a useful 6-bit (64 element) subset

6



rows

: 3

bits

16 columns: 4 bits



• Control characters were designed for transmission

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rows

: 3

bits

16 columns: 4 bits




"Line feed" (\n)

6



rows

: 3

bits

16 columns: 4 bits




"Line feed" (\n)"Bell" (\a)

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rows

: 3

bits

16 columns: 4 bits




"Line feed" (\n)"Bell" (\a)

6

(Demo)

Representing Strings: the Unicode Standard

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http://ian-albert.com/unicode_chart/unichart-chinese.jpg

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•109,000 characters

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•109,000 characters•93 scripts (organized)

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•109,000 characters•93 scripts (organized)•Enumeration of character properties, such as case

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•Supports bidirectional display order

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•Supports bidirectional display order•A canonical name for every character

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U+0058 LATIN CAPITAL LETTER X

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U+263a WHITE SMILING FACE

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U+2639 WHITE FROWNING FACE

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'☺'7








'☺' '☹'7








'☺' '☹'7

(Demo)

Mutation Operations

Some Objects Can Change

9

[Demo]


First example in the course of an object changing state

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[Demo]



The same object can change in value throughout the course of computation

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[Demo]




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[Demo]

👶same_person




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[Demo]

👶BABY

same_person




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[Demo]

👶BABY

same_person

Unicode character

name




9

[Demo]

👧GIRL

same_person

Unicode character

name




9

[Demo]

👧GIRL

jessicasame_person

Unicode character

name




9

[Demo]

👩WOMAN

jessicasame_person

Unicode character

name




9

[Demo]

👵OLDERWOMAN

jessicasame_person

Unicode character

name




9

[Demo]

👵OLDERWOMAN

jessicasame_person

Unicode character

name

All names that refer to the same object are affected by a mutation




9

[Demo]

👵OLDERWOMAN

jessicasame_person

Unicode character

name


Only objects of mutable types can change: lists & dictionaries




9

[Demo]

👵OLDERWOMAN

jessicasame_person

Unicode character

name


Only objects of mutable types can change: lists & dictionaries

{Demo}

Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope

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10

>>> four = [1, 2, 3, 4]



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>>> four = [1, 2, 3, 4]>>> len(four)4



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>>> four = [1, 2, 3, 4]>>> len(four)4>>> mystery(four)



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>>> four = [1, 2, 3, 4]>>> len(four)4>>> mystery(four)>>> len(four)2



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def mystery(s): s.pop() s.pop()

Interactive Diagram



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Interactive Diagram

def mystery(s): s[2:] = []

or



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>>> four = [1, 2, 3, 4]


Interactive Diagram


or



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>>> four = [1, 2, 3, 4]>>> len(four)4


Interactive Diagram


or



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>>> four = [1, 2, 3, 4]>>> len(four)4>>> another_mystery() # No arguments!


Interactive Diagram


or



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>>> four = [1, 2, 3, 4]>>> len(four)4>>> another_mystery() # No arguments!>>> len(four)2


Interactive Diagram


or



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>>> four = [1, 2, 3, 4]>>> len(four)4>>> another_mystery() # No arguments!>>> len(four)2


def another_mystery(): four.pop() four.pop()

Interactive Diagram


or

Tuples

(Demo)

Tuples are Immutable Sequences

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Immutable values are protected from mutation

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12

>>> turtle = (1, 2, 3)



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>>> turtle = (1, 2, 3)>>> ooze()



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>>> turtle = (1, 2, 3)>>> ooze()>>> turtle



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>>> turtle = (1, 2, 3)>>> ooze()>>> turtle(1, 2, 3)



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>>> turtle = [1, 2, 3]



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>>> turtle = [1, 2, 3]>>> ooze()



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>>> turtle = [1, 2, 3]>>> ooze()>>> turtle



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>>> turtle = [1, 2, 3]>>> ooze()>>> turtle['Anything could be inside!']



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Next lecture: ooze can change turtle's binding



12

The value of an expression can change because of changes in names or objects






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Name change:




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>>> x + x

>>> x + x

Name change:




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>>> x + x

>>> x + x

Name change:

>>> x = 2




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>>> x + x

>>> x + x

Name change:

>>> x = 2

4




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>>> x + x

>>> x + x

Name change:

>>> x = 2

4>>> x = 3




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>>> x + x

>>> x + x

Name change:

>>> x = 2

4>>> x = 3

6




12




>>> x + x

>>> x + x

Name change: Object mutation:

>>> x = 2

4>>> x = 3

6




12




>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x + x

>>> x + x




12




>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]>>> x + x

>>> x + x




12




>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x + x

>>> x + x




12




>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

>>> x + x

>>> x + x




12




>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> x + x

>>> x + x




12


An immutable sequence may still change if it contains a mutable value as an element



>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)>>> s[0] = 4

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)>>> s[0] = 4ERROR

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)>>> s[0] = 4ERROR

>>> s = ([1, 2], 3)

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)>>> s[0] = 4ERROR

>>> s = ([1, 2], 3)>>> s[0][0] = 4

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)>>> s[0] = 4ERROR

>>> s = ([1, 2], 3)>>> s[0][0] = 4>>> s

>>> x + x

>>> x + x




12





>>> x + x

>>> x + x


>>> x = 2

4>>> x = 3

6

>>> x = [1, 2]

[1, 2, 1, 2]>>> x.append(3)

[1, 2, 3, 1, 2, 3]

>>> s = ([1, 2], 3)>>> s[0] = 4ERROR

>>> s = ([1, 2], 3)>>> s[0][0] = 4>>> s([4, 2], 3)

>>> x + x

>>> x + x


Mutation

Sameness and Change

14

Sameness and Change

• As long as we never modify objects, a compound object is just the totality of its pieces

14

Sameness and Change


• A rational number is just its numerator and denominator

14

Sameness and Change



• This view is no longer valid in the presence of change

14

Sameness and Change




• A compound data object has an "identity" in addition to the pieces of which it is composed

14

Sameness and Change





• A list is still "the same" list even if we change its contents

14

Sameness and Change






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>>> a = [10]

Sameness and Change






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>>> a = [10]>>> b = a

Sameness and Change






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>>> a = [10]>>> b = a>>> a == bTrue

Sameness and Change






14

>>> a = [10]>>> b = a>>> a == bTrue>>> a.append(20)

Sameness and Change






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>>> a = [10]>>> b = a>>> a == bTrue>>> a.append(20)>>> a == bTrue

Sameness and Change






14

>>> a = [10]>>> b = a>>> a == bTrue>>> a.append(20)>>> a == bTrue>>> a[10, 20]

Sameness and Change






14

>>> a = [10]>>> b = a>>> a == bTrue>>> a.append(20)>>> a == bTrue>>> a[10, 20]>>> b[10, 20]

Sameness and Change






• Conversely, we could have two lists that happen to have the same contents, but are different

14


Sameness and Change







14

>>> a = [10]>>> a = [10]>>> b = a>>> a == bTrue>>> a.append(20)>>> a == bTrue>>> a[10, 20]>>> b[10, 20]

Sameness and Change







14

>>> a = [10]>>> b = [10]


Sameness and Change







14

>>> a = [10]>>> b = [10]>>> a == bTrue


Sameness and Change







14

>>> a = [10]>>> b = [10]>>> a == bTrue>>> b.append(20)


Sameness and Change







14

>>> a = [10]>>> b = [10]>>> a == bTrue>>> b.append(20)>>> a[10]


Sameness and Change







14

>>> a = [10]>>> b = [10]>>> a == bTrue>>> b.append(20)>>> a[10]>>> b[10, 20]


Sameness and Change







14

>>> a = [10]>>> b = [10]>>> a == bTrue>>> b.append(20)>>> a[10]>>> b[10, 20]>>> a == bFalse


Identity Operators

15

Identity Operators

Identity

<exp0> is <exp1>

evaluates to True if both <exp0> and <exp1> evaluate to the same object

15

Identity Operators

Identity

<exp0> is <exp1>


Equality

<exp0> == <exp1>

evaluates to True if both <exp0> and <exp1> evaluate to equal values

15

Identity Operators

Identity

<exp0> is <exp1>


Equality

<exp0> == <exp1>


Identical objects are always equal values

15

Identity Operators

Identity

<exp0> is <exp1>


Equality

<exp0> == <exp1>


Identical objects are always equal values

15

(Demo)

Mutable Default Arguments are Dangerous

16


A default argument value is part of a function value, not generated by a call

16



16

>>> def f(s=[]):... s.append(3)... return len(s)...



16

>>> def f(s=[]):... s.append(3)... return len(s)... >>> f()1



16

>>> def f(s=[]):... s.append(3)... return len(s)... >>> f()1>>> f()2



16

>>> def f(s=[]):... s.append(3)... return len(s)... >>> f()1>>> f()2>>> f()3



16Interactive Diagram




16Interactive Diagram


Each time the function is called, s is bound to the same value!

Documents

61A Lecture 12 - University of California, Berkeleycs61a/fa18/assets/slides/... · 2019-01-11 · 61A Lecture 12. Announcements. Objects (Demo) Objects 4. Objects ... •A type of