Lecture 18 Methods and Operations
Announcements for This Lecture Assignments Lab this Week • Simple class exercise • A4 Due Thursday at midnight § Fill in predefined methods § Hopefully you are on Task 4 § Setting you up for A6… § Extra consultants available • Will post A5 on Thursday Exams § Written assignment like A2 § Needs material from next Tues • Moved to handback room § Located in Gates 216 • Will also post A6 as well § Open 12-4:30 daily § Not due until November 19 • Regrades still open this week § Want to avoid exam crunch 10/27/15 Methods and Operations 2
Important! YES NO class Point3(object): class Point3: """Instances are 3D points """Instances are 3D points Attributes: Attributes: x: x-coord [float] x: x-coord [float] y: y-coord [float] y: y-coord [float] z: z-coord [float]""" z: z-coord [float]""" … … 3.0-Style Classes “Old-Style” Classes Well-Designed Very, Very Bad 10/27/15 Methods and Operations 3
Case Study: Fractions class Fraction(object): • Want to add a new type """Instance is a fraction n/d § Values are fractions: ½ , ¾ Attributes: § Operations are standard numerator: top [int] multiply, divide, etc. denominator: bottom [int > 0] § Example : ½ * ¾ = ⅜ """ • Can do this with a class def __init__(self,n=0,d=1): § Values are fraction objects """Init: makes a Fraction""" § Operations are methods self.numerator = n self.denominator = d • Example : simplefrac.py 10/27/15 Methods and Operations 4
Problem: Doing Math is Unwieldy What We Want What We Get 1 2 + 1 3 + 1 4 ∗ 5 >>> p = Fraction(1,2) >>> q = Fraction(1,3) 4 >>> r = Fraction(1,4) >>> s = Fraction(5,4) >>> (p.add(q.add(r))).mult(s) This is confusing! 10/27/15 Methods and Operations 5
Problem: Doing Math is Unwieldy What We Want What We Get 1 2 + 1 3 + 1 4 ∗ 5 >>> p = Fraction(1,2) >>> q = Fraction(1,3) 4 >>> r = Fraction(1,4) >>> s = Fraction(5,4) Why not use the >>> (p.add(q.add(r))).mult(s) standard Python math operations? This is confusing! 10/27/15 Methods and Operations 6
Recall: The __init__ Method two underscores w = Worker('Obama', 1234, None) Called by the constructor def __init__(self, n, s, b): ""”Initializer: creates a Worker id8 Worker Has last name n, SSN s, and boss b lname 'Obama' Precondition: n a string, s an int in ssn 1234 range 0..999999999, and b either a Worker or None. boss None self.lname = n self.ssn = s self.boss = b 10/27/15 Methods and Operations 7
Recall: The __init__ Method two underscores w = Worker('Obama', 1234, None) def __init__(self, n, s, b): ""”Initializer: creates a Worker Has last name n, SSN s, and boss b Are there other special methods Precondition: n a string, s an int in range 0..999999999, and b either that we can use? a Worker or None. self.lname = n self.ssn = s self.boss = b 10/27/15 Methods and Operations 8
Example: Converting Values to Strings str() Function Backquotes • Usage : str( <expression> ) • Usage : ` <expression> ` § Evaluates the expression § Evaluates the expression § Converts it into a string § Converts it into a string • How does it convert? • How does it convert? § str(2) → '2' § `2` → '2' § str(True) → 'True' § `True` → 'True' § str('True') → 'True' § `'True'` → "'True'" § str(Point3()) → '(0.0,0.0,0.0)' § `Point3()` → "<class 'Point3'> (0.0,0.0,0.0)" 10/27/15 Methods and Operations 9
Example: Converting Values to Strings str() Function Backquotes • Usage : str( <expression> ) • Usage : ` <expression> ` Backquotes are § Evaluates the expression § Evaluates the expression for unambigious § Converts it into a string § Converts it into a string representation • How does it convert? • How does it convert? What type is The value’s § str(2) → '2' § `2` → '2' this value? type is clear § str(True) → 'True' § `True` → 'True' § str('True') → 'True' § `'True'` → "'True'" § str(Point3()) → '(0.0,0.0,0.0)' § `Point3()` → "<class 'Point3'> (0.0,0.0,0.0)" 10/27/15 Methods and Operations 10
What Does str() Do On Objects? class Point3(object): • Does NOT display contents """Instances are points in 3d space""" >>> p = Point3(1,2,3) … >>> str(p) def __str__(self): '<Point3 object at 0x1007a90>' """Returns: string with contents""" • Must add a special method return '('+self.x + ',' + § __str__ for str() self.y + ',' + self.z + ')' § __repr__ for backquotes • Could get away with just one def __repr__(self): § Backquotes require __repr__ """Returns: unambiguous string""" § str() can use __repr__ return str(self.__class__)+ (if __str__ is not there) str(self) 10/27/15 Methods and Operations 11
What Does str() Do On Objects? class Point3(object): • Does NOT display contents """Instances are points in 3d space""" >>> p = Point3(1,2,3) … >>> str(p) def __str__(self): '<Point3 object at 0x1007a90>' """Returns: string with contents""" • Must add a special method return '('+self.x + ',' + § __str__ for str() self.y + ',' + self.z + ')' § __repr__ for backquotes Gives the • Could get away with just one def __repr__(self): class name § Backquotes require __repr__ """Returns: unambiguous string""" § str() can use __repr__ return str(self.__class__)+ (if __str__ is not there) str(self) __repr__ using __str__ as helper 10/27/15 Methods and Operations 12
Special Methods in Python class Point3(object): • Have seen three so far """Instances are points in 3D space""" § __init__ for initializer … § __str__ for str() def __init__(self,x=0,y=0,z=0): § __repr__ for backquotes """Initializer: makes new Point3""" • Start/end with 2 underscores … § This is standard in Python def __str__(self,q): § Used in all special methods """Returns: string with contents""” § Also for special attributes … • For a complete list, see def __repr__(self,q): http://docs.python.org/reference """Returns: unambiguous string""” /datamodel.html … 10/27/15 Methods and Operations 13
Returning to Fractions What We Want Operator Overloading 1 2 + 1 3 + 1 4 ∗ 5 • Python has methods that correspond to built-in ops 4 § __ add__ corresponds to + § __mul__ corresponds to * § Not implemented by default Why not use the • Implementing one allows you standard Python to use that op on your objects math operations? § Called operator overloading § Changes operator meaning 10/27/15 Methods and Operations 14
Operator Overloading: Multiplication class Fraction(object): >>> p = Fraction(1,2) """Instance attributes: >>> q = Fraction(3,4) numerator: top [int] >>> r = p*q denominator: bottom [int > 0]""” def __mul__(self,q): Python """Returns: Product of self, q converts to Makes a new Fraction; does not modify contents of self or q >>> r = p.__mul__(q) Precondition: q a Fraction""" assert type(q) == Fraction top = self.numerator*q.numerator Operator overloading uses bot = self.denominator*q.denominator method in object on left. return Fraction(top,bot) 10/27/15 Methods and Operations 15
Operator Overloading: Addition class Fraction(object): >>> p = Fraction(1,2) """Instance attributes: >>> q = Fraction(3,4) numerator: top [int] >>> r = p+q denominator: bottom [int > 0]""” def __add__(self,q): Python """Returns: Sum of self, q converts to Makes a new Fraction Precondition: q a Fraction""" >>> r = p.__add__(q) assert type(q) == Fraction bot = self.denominator*q.denominator top = (self.numerator*q.denominator+ Operator overloading uses self.denominator*q.numerator) method in object on left. return Fraction(top,bot) 10/27/15 Methods and Operations 16
Comparing Objects for Equality class Fraction(object): • Earlier in course, we saw == """Instance attributes: compare object contents numerator: top [int] § This is not the default denominator: bottom [int > 0]""" § Default : folder names • Must implement __eq__ def __eq__(self,q): """Returns: True if self, q equal, § Operator overloading! False if not, or q not a Fraction""" § Not limited to simple if type(q) != Fraction: attribute comparison return False § Ex : cross multiplying left = self.numerator*q.denominator rght = self.denominator*q.numerator 4 1 2 4 return left == rght 2 4 10/27/15 Methods and Operations 17
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