Programming with Subclasses Announcements for This Lecture - - PowerPoint PPT Presentation

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Programming with Subclasses Announcements for This Lecture - - PowerPoint PPT Presentation

Jul 10, 2023 270 likes •586 views

Lecture 21 Programming with Subclasses Announcements for This Lecture Assignments Prelim 2 Thursday, 5:15 or 7:30 A4 is now graded K Z at 5:15pm Mean : 90.4 Median : 93 A J at 7:30 pm Std Dev : 10.6 See website

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SLIDE 1

Programming with Subclasses

Lecture 21

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SLIDE 2

Announcements for This Lecture

Assignments Prelim 2

  • Thursday, 5:15 or 7:30

§ K – Z at 5:15pm § A – J at 7:30 pm § See website for room § Conflicts received e-mail

  • ANOTHER review Wed.

§ Run by the URMC § Open up to everyone

11/6/18 2 Programming with Subclasses

  • A4 is now graded

§ Mean: 90.4 Median: 93 § Std Dev: 10.6 § Mean: 9 hrs Median: 8 hrs § Std Dev: 4.1 hrs

  • A5 is also graded

§ Mean: 46.4 Median: 49

§ A: 47 (74%), B: 40 (19%)

§ Solutions posted in CMS

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SLIDE 3

class Fraction(object): """Instances are normal fractions n/d Instance attributes: numerator: top [int] denominator: bottom [int > 0] """ class BinaryFraction(Fraction): """Instances are fractions k/2n Instance attributes are same, BUT: numerator: top [int] denominator: bottom [= 2n, n ≥ 0] """ def __init__(self,k,n): """Make fraction k/2n """ assert type(n) == int and n >= 0 super().__init__(k,2 ** n)

>>> p = Fraction(1,2) >>> q = BinaryFraction(1,2) # 1/4 >>> r = p*q >>> r = p.__mul__(q) # ERROR

Python converts to

__mul__ has precondition type(q) == Fraction

11/6/18 Programming with Subclasses 3

A Problem with Subclasses

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SLIDE 4

The isinstance Function

  • isinstance(<obj>,<class>)

§ True if <obj>’s class is same as or a subclass of <class> § False otherwise

  • Example:

§ isinstance(e,Executive) is True § isinstance(e,Employee) is True § isinstance(e,object) is True § isinstance(e,str) is False

  • Generally preferable to type

§ Works with base types too!

11/6/18 Programming with Subclasses 4

e id4

id4

Executive

_salary 0.0 _start 2012 _name 'Fred' _bonus 0.0

  • bject

Employee Executive

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SLIDE 5

isinstance and Subclasses

>>> e = Employee('Bob',2011) >>> isinstance(e,Executive) ???

11/6/18 Programming with Subclasses 5

A: True B: False C: Error D: I don’t know

e id5

id5

Employee

_salary 50k _start 2012 _name 'Bob'

  • bject

Employee Executive

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SLIDE 6

isinstance and Subclasses

>>> e = Employee('Bob',2011) >>> isinstance(e,Executive) ???

11/6/18 Programming with Subclasses 6

A: True B: False C: Error D: I don’t know

  • bject

Executive Employee

→ means “extends”

  • r “is an instance of”

Correct

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SLIDE 7

Fixing Multiplication

class Fraction(object): """Instance attributes: numerator [int]: top denominator [int > 0]: bottom""" def __mul__(self,q): """Returns: Product of self, q Makes a new Fraction; does not modify contents of self or q Precondition: q a Fraction""" assert isinstance(q, Fraction) top = self.numerator*q.numerator bot = self.denominator*q.denominator return Fraction(top,bot)

>>> p = Fraction(1,2) >>> q = BinaryFraction(1,2) # 1/4 >>> r = p*q >>> r = p.__mul__(q) # OKAY

Python converts to

Can multiply so long as it has numerator, denominator

11/6/18 Programming with Subclasses 7

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SLIDE 8

Error Types in Python

def foo(): assert 1 == 2, 'My error' … >>> foo() AssertionError: My error def foo(): x = 5 / 0 … >>> foo() ZeroDivisionError: integer division or modulo by zero

11/6/18 Programming with Subclasses 8

Class Names

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SLIDE 9

Error Types in Python

def foo(): assert 1 == 2, 'My error' … >>> foo() AssertionError: My error def foo(): x = 5 / 0 … >>> foo() ZeroDivisionError: integer division or modulo by zero

11/6/18 Programming with Subclasses 9

Class Names Information about an error is stored inside an object. The error type is the class

  • f the error object.
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SLIDE 10

Error Types in Python

  • All errors are instances of class BaseException
  • This allows us to organize them in a hierarchy

11/6/18 Programming with Subclasses 10

BaseException AssertionError Exception

id4

AssertionError 'My error'

→ means “extends”

  • r “is an instance of”

__init__(msg) __str__() …

BaseException Exception(BE) AssError(SE)

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SLIDE 11

Error Types in Python

  • All errors are instances of class BaseException
  • This allows us to organize them in a hierarchy

11/6/18 Programming with Subclasses 11

id4

AssertionError 'My error'

→ means “extends”

  • r “is an instance of”

BaseException AssertionError Exception

__init__(msg) __str__() …

BaseException Exception(BE) AssError(SE)

All of these are actually empty! Why?

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SLIDE 12

Python Error Type Hierarchy

11/6/18 Programming with Subclasses 12

BaseException Exception SystemExit

AssertionError ArithmeticError AttributeError ValueError TypeError IOError … ZeroDivisionError OverflowError

… Argument has wrong type (e.g. float([1])) Argument has wrong value (e.g. float('a'))

Why so many error types?

http://docs.python.org/ library/exceptions.html

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SLIDE 13

Recall: Recovering from Errors

  • try-except blocks allow us to recover from errors

§ Do the code that is in the try-block § Once an error occurs, jump to the catch

  • Example:

try: val = input() # get number from user x = float(val) # convert string to float print('The next number is '+str(x+1)) except: print('Hey! That is not a number!')

might have an error executes if have an error

11/6/18 13 Programming with Subclasses

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SLIDE 14

Handling Errors by Type

  • try-except blocks can be restricted to specific errors

§ Doe except if error is an instance of that type § If error not an instance, do not recover

  • Example:

try: val = input() # get number from user x = float(val) # convert string to float print('The next number is '+str(x+1)) except ValueError: print('Hey! That is not a number!')

Only recovers ValueError. Other errors ignored.

11/6/18 14 Programming with Subclasses

May have ValueError May have IOError

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SLIDE 15

Handling Errors by Type

  • try-except blocks can be restricted to specific errors

§ Doe except if error is an instance of that type § If error not an instance, do not recover

  • Example:

try: val = input() # get number from user x = float(val) # convert string to float print('The next number is '+str(x+1)) except IOError: print('Check your keyboard!')

Only recovers IOError. Other errors ignored.

11/6/18 15 Programming with Subclasses

May have ValueError May have IOError

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SLIDE 16

Creating Errors in Python

def foo(x): assert x < 2, 'My error' … def foo(x): if x >= 2: m = 'My error' err = AssertionError(m) raise err

11/6/18 Programming with Subclasses 16

  • Create errors with raise

§ Usage: raise <exp> § exp evaluates to an object § An instance of Exception

  • Tailor your error types

§ ValueError: Bad value § TypeError: Bad type

  • Still prefer asserts for

preconditions, however

§ Compact and easy to read Identical

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SLIDE 17

Creating Errors in Python

def foo(x): assert x < 2, 'My error' … def foo(x): if x >= 2: m = 'My error' err = TypeError(m) raise err

11/6/18 Programming with Subclasses 17

  • Create errors with raise

§ Usage: raise <exp> § exp evaluates to an object § An instance of Exception

  • Tailor your error types

§ ValueError: Bad value § TypeError: Bad type

  • Still prefer asserts for

preconditions, however

§ Compact and easy to read Identical

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SLIDE 18

Raising and Try-Except

def foo(): x = 0 try: raise Exception() x = 2 except Exception: x = 3 return x

  • The value of foo()?

11/6/18 Programming with Subclasses 18

A: 0 B: 2 C: 3 D: No value. It stops! E: I don’t know

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SLIDE 19

Raising and Try-Except

def foo(): x = 0 try: raise Exception() x = 2 except Exception: x = 3 return x

  • The value of foo()?

11/6/18 Programming with Subclasses 19

A: 0 B: 2 C: 3 D: No value. It stops! E: I don’t know Correct

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SLIDE 20

Raising and Try-Except

def foo(): x = 0 try: raise Exception() x = 2 except BaseException: x = 3 return x

  • The value of foo()?

11/6/18 Programming with Subclasses 20

A: 0 B: 2 C: 3 D: No value. It stops! E: I don’t know

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SLIDE 21

Raising and Try-Except

def foo(): x = 0 try: raise Exception() x = 2 except BaseException: x = 3 return x

  • The value of foo()?

11/6/18 Programming with Subclasses 21

A: 0 B: 2 C: 3 D: No value. It stops! E: I don’t know Correct

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SLIDE 22

Raising and Try-Except

def foo(): x = 0 try: raise Exception() x = 2 except AssertionError: x = 3 return x

  • The value of foo()?

11/6/18 Programming with Subclasses 22

A: 0 B: 2 C: 3 D: No value. It stops! E: I don’t know

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SLIDE 23

Raising and Try-Except

def foo(): x = 0 try: raise Exception() x = 2 except AssertionError: x = 3 return x

  • The value of foo()?

11/6/18 Programming with Subclasses 23

A: 0 B: 2 C: 3 D: No value. It stops! E: I don’t know Correct

Python uses isinstance to match Error types

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SLIDE 24

Creating Your Own Exceptions

class CustomError(Exception): """An instance is a custom exception""" pass

This is all you need

§ No extra fields § No extra methods § No constructors

Inherit everything

11/6/18 24 Programming with Subclasses

Only issues is choice

  • f parent error class.

Use Exception if you are unsure what.

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SLIDE 25

Handling Errors by Type

  • try-except can put the error in a variable
  • Example:

try: val = input() # get number from user x = float(val) # convert string to float print('The next number is '+str(x+1)) except ValueError as e: print(e.args[0]) print('Hey! That is not a number!')

11/6/18 25 Programming with Subclasses

Some Error subclasses have more attributes

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SLIDE 26

Accessing Attributes with Strings

  • hasattr(<obj>,<name>)

§ Checks if attribute exists

  • getattr(<obj>,<name>)

§ Reads contents of attribute

  • delattr(<obj>,<name>)

§ Deletes the given attribute

  • setattr(<obj>,<name>,<val>)

§ Sets the attribute value

  • <obj>.__dict__

§ List all attributes of object

11/6/18 Programming with Subclasses 26

id1 2.0 3.0 5.0

Point3

x y z id2 2.0 3.0 5.0

dict

'x' 'y' 'z'

Treat object like dictionary

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SLIDE 27

Typing Philosophy in Python

  • Duck Typing:

§ “Type” object is determined by its methods and properties § Not the same as type() value § Preferred by Python experts

  • Implement with hasattr()

§ hasattr(<object>,<string>) § Returns true if object has an attribute/method of that name

  • This has many problems

§ The name tells you nothing about its specification

11/6/18 Programming with Subclasses 27

class Fraction(object): """Instance attributes: numerator [int]: top denominator [int > 0]: bottom""" … def __eq__(self,q): """Returns: True if self, q equal, False if not, or q not a Fraction""" if type(q) != Fraction: return False left = self.numerator*q.denominator rght = self.denominator*q.numerator return left == rght

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SLIDE 28

Typing Philosophy in Python

  • Duck Typing:

§ “Type” object is determined by its methods and properties § Not the same as type() value § Preferred by Python experts

  • Implement with hasattr()

§ hasattr(<object>,<string>) § Returns true if object has an attribute/method of that name

  • This has many problems

§ The name tells you nothing about its specification

11/6/18 Programming with Subclasses 28

class Fraction(object): """Instance attributes: numerator [int]: top denominator [int > 0]: bottom""" … def __eq__(self,q): """Returns: True if self, q equal, False if not, or q not a Fraction""" if (not (hasattr(q,'numerator') and hasattr(q,'denomenator')): return False left = self.numerator*q.denominator rght = self.denominator*q.numerator return left == rght

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SLIDE 29

Typing Philosophy in Python

  • Duck Typing:

§ “Type” object is determined by its methods and properties § Not the same as type() value § Preferred by Python experts

  • Implement with hasattr()

§ hasattr(<object>,<string>) § Returns true if object has an attribute/method of that name

  • This has many problems

§ The name tells you nothing about its specification

11/6/18 Programming with Subclasses 29

class Fraction(object): """Instance attributes: numerator [int]: top denominator [int > 0]: bottom""" … def __eq__(self,q): """Returns: True if self, q equal, False if not, or q not a Fraction""" if (not (hasattr(q,'numerator') and hasattr(q,'denomenator')): return False left = self.numerator*q.denominator rght = self.denominator*q.numerator return left == rght

Compares anything with numerator & denominator

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SLIDE 30

Typing Philosophy in Python

  • Duck Typing:

§ “Type” object is determined by its methods and properties § Not the same as type() value § Preferred by Python experts

  • Implement with hasattr()

§ hasattr(<object>,<string>) § Returns true if object has an attribute/method of that name

  • This has many problems

§ The name tells you nothing about its specification

11/6/18 Programming with Subclasses 30

class Fraction(object): """Instance attributes: numerator [int] : top denominator [int > 0]: bottom""" … def __eq__(self,q): """Returns: True if self, q equal, False if not, or q not a Fraction""" if (not (hasattr(q,'numerator') and hasattr(q,'denomenator')): return False left = self.numerator*q.denominator rght = self.denominator*q.numerator return left == rght

How to properly implement/use typing is a major debate in language design

  • What we really care about is

specifications (and invariants)

  • Types are a “shorthand” for this

Different typing styles trade ease-of-use with overall program robustness/safety

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SLIDE 31

Typing Philosophy in Python

  • Duck Typing:

§ “Type” object is determined by its methods and properties § Not the same as type() value § Preferred by Python experts

  • Implement with hasattr()

§ hasattr(<object>,<string>) § Returns true if object has an attribute/method of that name

  • This has many problems

§ The name tells you nothing about its specification

class Employee(object): """An Employee with a salary"”" … def __eq__(self,other): if (not (hasattr(other,'name') and hasattr(other,'start') and hasattr(other,'salary')) return False return (self.name == other.name and self.start == other.start and self.salary == other.salary)

11/6/18 Programming with Subclasses 31