COMP519 Web Programming Lecture 21: Python (Part 5) Handouts

Transcription

1 COMP519 Web Programming Lecture 21: Python (Part 5) Handouts Ullrich Hustadt Department of Computer Science School of Electrical Engineering, Electronics, and Computer Science University of Liverpool

2 Functions Classes Revision Contents 1 Functions Variable Scope and Functions Static Variables 2 Classes Defining and Instantiating a Class Class and Instance Attributes Deleting Instances Class Methods Inheritance Magic Methods 3 Further Reading Ullrich Hustadt COMP519 Web Programming L21 1

3 Functions Classes Revision Variable Scope and Functions Static vars Scope of Variables Sope of a Variable (Occurrence) The region of a program or the sequence of execution steps of a program in which we can validly refer to that variable (occurrence) public class Test { int x; void method1 () {... A... int x; int y;... B... } void method2 () {... C... } int z; } Where in this Java class can we validly refer to the variables x, y and z, for example, print them out? Can we refer to x, y, and z in method2()? Which occurrence of x do we refer to? Which occurrence of x would we refer to in method1() either in section...a... or...b...? Such questions become even more complicated when function definitions can be nested as in JavaScript and Python Ullrich Hustadt COMP519 Web Programming L21 2

4 Functions Classes Revision Variable Scope and Functions Static vars A Look Back to JavaScript function state (s,a,k,b,l,c,m) { document. writeln (s+ : +a+ = +k+, + b+ = +l+, +c+ =[ +m+ ]<br /> ) } function outer (x,y,z) { function inner () { state ( Inner (1), x,x, y,y, z,z) x = 3 z [1] = 3 state ( Inner (2), x,x, y,y, z,z) } state ( Outer (1), x,x, y,y, z,z) inner () y = 4 z [2] = 4 state ( Outer (2), x,x, y,y, z,z) } u = 1; v = 2; w = [0,1,2] state ( Main (1), u,u, v,v, w,w) outer (u,v,w) state ( Main (2), u,u, v,v, w,w) Main(1): u=1, v=2, w=[0,1,2] Outer(1): x=1, y=2, z=[0,1,2] Inner(1): x=1, y=2, z=[0,1,2] Inner(2): x=3, y=2, z=[0,3,2] Outer(2): x=3, y=4, z=[0,3,4] Main(2): u=1, v=2, w=[0,3,4] x, y, z are local variables of outer x, y, z in inner are those of outer x, y, z are accessible in inner Ullrich Hustadt COMP519 Web Programming L21 3

5 Functions Classes Revision Variable Scope and Functions Static vars Variable Scope in Python: Example def state (s,a,k,b,l,c,m): print (s+ : +a+ = + str (k)+, + b+ = + str (l)+, +c+ = + str (m)) def outer (x,y,z): def inner (): state ( Inner (1), x,x, y,y, z,z) x = 3 z [1] = 3 state ( Inner (2), x,x, y,y, z,z) state ( Outer (1), x,x, y,y, z,z) inner () y = 4 z [2] = 4 state ( Outer (2), x,x, y,y, z,z) u, v, w = 1, 2, [0,1,2] state ( Main (1), u,u, v,v, w,w) outer (u,v,w) state ( Main (2), u,u, v,v, w,w) Main(1): u=1, v=2, w=[0,1,2] Outer(1): x=1, y=2, z=[0,1,2] Traceback (most recent call last): line 20, in <module> outer(u,v,w) line 13, in outer inner() line 8, in inner state( Inner(1), x,x, y,...) UnboundLocalError: local variable x referenced before assignment x in inner is a local variabe of inner x in inner is not the same as x in outer y in inner is the same as y in outer z in inner is the same as z in outer Ullrich Hustadt COMP519 Web Programming L21 4

6 Functions Classes Revision Variable Scope and Functions Static vars Variable Scope in Python: Analysis By default, any assignment to a variable within a function creates a new local variable def outer (x,y,z): def inner (): x = 3 # x is a new local variable Not all statements that look like assignments are assignments def bubble_sort ( arr ): def swap (i, j): arr [ i] = tmp # why is arr not a new local variable? is syntactic sugar for def bubble_sort ( arr ): def swap (i, j): arr. setitem (i,tmp ) that is, the assignment is a method call for the object arr arr[i] = tmp does not result in the creation of a new local variable arr Ullrich Hustadt COMP519 Web Programming L21 5

7 Functions Classes Revision Variable Scope and Functions Static vars Variable Scope in Python A variable that is a function parameter or defined inside a function is local to that function the variable (only) exists as long as a function execution does the variable can only be accessed inside the function A variable that is outside any function (or class) is global the variable can be accessed anywhere if is not hidden by a local variable with the same name The declaration global var1, var2,... causes the listed variables to be interpreted as global variables The declaration nonlocal var1, var2,... causes the listed variables to refer to previously bound variables in the nearest enclosing scope excluding globals Ullrich Hustadt COMP519 Web Programming L21 6

8 Functions Classes Revision Variable Scope and Functions Static vars Global vs Local Variables in Python def state (s,a,k,b,l,c,m): print (s+ : +a+ = + str (k)+, + b+ = + str (l)+, +c+ = + str (m)) def outer (x,y,z): def inner (): nonlocal x state ( Inner (1), x,x, y,y, z,z) x = 3 z [1] = 3 state ( Inner (2), x,x, y,y, z,z) state ( Outer (1), x,x, y,y, z,z) inner () y = 4 z [2] = 4 state ( Outer (2), x,x, y,y, z,z) u, v, w = 1, 2, [0,1,2] state ( Main (1), u,u, v,v, w,w) outer (u,v,w) state ( Main (2), u,u, v,v, w,w) Main(1): u=1, v=2, w=[0,1,2] Outer(1): x=1, y=2, z=[0,1,2] Inner(1): x=1, y=2, z=[0,1,2] Inner(2): x=3, y=2, z=[0, 3, 2] Outer(2): x=3, y=4, z=[0, 3, 4] Main(2): u=1, v=2, w=[0, 3, 4] x in inner is now the same as x in outer y in inner is the same as y in outer z in inner is the same as z in outer Ullrich Hustadt COMP519 Web Programming L21 7

9 Functions Classes Revision Variable Scope and Functions Static vars Python Functions and Static Variables Python does not have a ^Astatic^A keyword to declare a variable to be static and preserve its value between different calls of a function The solution is to use a function attribute instead def counter (): try : counter. count += 1 except AttributeError : counter. count = 1 return counter. count print ("1: static count =", counter ()) print ("2: static count =", counter ()) print ("3: global counter. count = "+ counter. count ) 1: static count = 1 2: static count = 2 3: global counter. count = 2 As the example shows the function attribute is global/public Ullrich Hustadt COMP519 Web Programming L21 8

10 Functions Classes Revision Variable Scope and Functions Static vars Anonymous Functions Python supports a weak form of anonymous functions: lambda param1, param2,...: expression Parameters work as for named functions The return value of an anonymous function is the value of expression lambda x, y: x**y if y > 0 else x **(1/ y) Both named and anonymous functions can be assigned to variables: def exp (base, exp ): return base ** exp func1 = exp func2 = lambda x: 2* x print ( func1 (2,2) + func2 (3)) # prints 10 Both named and anonymous functions can be passed to other functions as arguments or used as values in complex data structures Ullrich Hustadt COMP519 Web Programming L21 9

11 Defining and Instantiating a Class Python is an object-oriented language with classes A class can be defined as follows: class identifier : docstring suite The class name identifier is case-sensitive docstring is a string describing the class and will be returned by help(identifier) or identifier. doc suite is a non-empty sequence of statements, typically defining attributes and methods An object of a class is created using identifier (arg1,arg2,...) where arg1,arg2,... is a possibly empty list of arguments passed to the constructor of the class identifier Ullrich Hustadt COMP519 Web Programming L21 10

12 A Closer Look at Class Definitions In more detail, the definition of a class typically looks as follows class id: docstring # Class Attributes cattrib = value1 # Constructor def init (self,p1,p2,...): # Instance Attribute self. iattrib = value2 suite1 # Methods def method2 (self,p1,p2,...): suite2 def methodn (self,p1,p2,...): suiten Every instance obj of this class will have an attribute iattrib accessible as obj.iattrib cattrib is a class attribute accessible as id.cattrib or obj.cattrib init is the constructor of the class and will be called whenever id(a1,a2,...) is executed Via self the constructor receives a reference to the newly created object p1,p2,... will be instantiated by a1,a2,... Ullrich Hustadt COMP519 Web Programming L21 11

13 A Closer Look at Class Definitions In more detail, the definition of a class typically looks as follows class id: docstring # Class Attributes cattrib = value1 # Constructor def init (self,p1,p2,...): # Instance Attribute self. iattrib = value2 suite1 # Methods def method2 (self,p1,p2,...): suite2 def methodn (self,p1,p2,...): suiten Every instance obj of this class will have methods method2 to methodn For an instance obj of this class, method calls will take the form obj.method2(a1,a2,...) to obj.methodn(a1,a2,...) p1,p2,... will be instantiated by a1,a2,... in the normal way for function calls Via self each method receives a reference to obj as additional argument Ullrich Hustadt COMP519 Web Programming L21 12

14 Defining and Instantiating a Class: Example class Rectangle : """ A simple rectangle class """ type = Rectangle def init (self, width =0, height =0): self. width = width self. height = height def area ( self ): return self. height * self. width # Using the class r1 = Rectangle () print ( Area of r1:,r1. area ()) r2 = Rectangle (5,2) print ( Area of r2:,r2. area ()) r3 = Rectangle ( width =5, height =2) print ( Area of r3:,r3. area ()) Area of r1: 0 Area of r2: 10 Area of r3: 10 Ullrich Hustadt COMP519 Web Programming L21 13

15 Class and Instance Attributes Class attributes are owned by the class itself and shared by all instances of the class Instance attributes are owned by the specific instances of a class Instances can be given additional attributes (and methods) at any time Assigning a value to a class attribute via an instance creates a new instance variable For an instance obj, obj. dict returns a dictionary of all instance variable and their values of obj print (" Dictionary : ",r2. dict," Type :", r2. type ) Dictionary : { width : 5, height : 2} Type : Rectangle r1. colour = " blue " r1. type = " Point " print (r1. dict ) { width : 0, height : 0, colour : blue, type : Point } Ullrich Hustadt COMP519 Web Programming L21 14

16 Private Attributes Python has no private class or instance attributes By convention, attributes whose name starts with a double underscore should be considered private Python supports this convention by name mangling: Within the definition of a class id an attribute attrib is textually replaced by _id attrib class Hide : chide = 1 def init ( self ): self. ihide = 2 h1 = Hide () print (" Class : ", Hide. dict ) print (" Instance :", h1. dict ) print (" Value of ihide for h1 :", h1. _Hide ihide ) print (h1. ihide ) Class : { _Hide chide : 1, init :... } Instance : { _Hide ihide : 2} Value of ihide for h1: 2 AttributeError : Hide object has no attribute ihide Ullrich Hustadt COMP519 Web Programming L21 15

17 Private Attributes: Example class Mapping : def init (self, iterable ): self. items_ list = [] self. update ( iterable ) def update ( self, iterable ): for item in iterable : self. items_list. append ( item ) update = update # private copy of original update () class MappingSubclass ( Mapping ): def update ( self, keys, values ): # provides a new signature and definition for update () # but does not break init () in Mapping for item in zip ( keys, values ): self. items_list. append ( item ) Section 9.6. Private Variables ( of Python Software Foundation: The Python Tutorial. Python Software Foundation, 21 December [accessed 21 December 2017] Ullrich Hustadt COMP519 Web Programming L21 16

18 Deleting Instances An instance obj can be deleted using del obj Deleting an instance will implicitly call the method del class Employee : totalnumber = 0; def init (self, name ): self. name = name type ( self ). totalnumber += 1 def del ( self ): type ( self ). totalnumber -= 1 A: # employees : 2 B: # employees : 1 C: # employees : 0 e1 = Employee (" Ada ") e2 = Employee (" Ben ") print ("A: # employees : ", Employee. totalnumber ) del e1 print ("B: # employees : ", Employee. totalnumber ) e2 = None print ("C: # employees : ", Employee. totalnumber ) Ullrich Hustadt COMP519 Web Programming L21 17

19 Static Methods (Class Methods) The Pythonic way is to define getter and setter method only for private class and instance attributes For private instance variables this is done in the standard way For private class variables we need to declare the getter / setter method to be static e3 = Employee (" Ada ") e4 = Employee (" Ben ") print (" Number of employees : ", Employee. getnumber ()) print (" Number of employees : ", e3. getnumber ()) Number of employees : 2 Number of employees : 2 class Employee : tnum = 0; def init (self, name ): self. name = name type ( self ). tnum += 1 def del ( self ): type ( self ). tnum -= 1 def getname ( self ): return self. name def setname (self, name ): self. name = staticmethod def getnumber (): return Employee. tnum Ullrich Hustadt COMP519 Web Programming L21 18

20 Static Methods (Class Methods) A static declaration should also be used for methods that are independant of specific instances class Circle : PI = c1 = Circle (1.2) c2 = Circle (1.0) def init (self, radius ): self. r = radius def area ( self ): return round ( Circle. PI * \ self.r * staticmethod def max (cx,cy ): if cx.r > cy.r: return cx else : return cy print (" c1 radius :", c1.r) print (" c1 area : ",c1. area ()) print (" c2 radius :", c2.r) print (" c2 area : ",c2. area ()) bg = Circle. max (c1,c2) print (" bg radius :", bg.r) c1 radius : 1.2 c1 area : c2 radius : 1.0 c2 area : bg radius : 1.2 Ullrich Hustadt COMP519 Web Programming L21 19

21 Inheritance In a class definition it is possible to specify one or more superclasses from which a class should inherit attributes and methods: class identifier ( Super1, Super2,...): docstring suite class Rectangle : """ A simple rectangle class """ type = Rectangle def init (self, width =0, height =0): self. width = width self. height = height def area ( self ): return self. height * self. width class Square ( Rectangle ): type = Square def init (self, size ): super (). init (size, size ) sq1 = Square (5) print (" The type of sq1 is", sq1. type ) print (" The area of sq1 is", sq1. area () ) The type of sq1 is Square The area of sq1 is 25 Ullrich Hustadt COMP519 Web Programming L21 20

22 Magic Methods init and del are examples of magic methods: Methods that are invoked when a certain language constructs is used or a particular condition becomes true Other examples include: getitem setitem str allows instances to use the [] (indexer) operators to get or set a value called by str(), print(), format() to obtain a string representation of an instance eq called by comparison operator == lt called by comparison operator < add called by binary operator + bool called for truth value testing and by bool(), should return True or False iter called by for-loop to iterate over the elements next of an iterable object Ullrich Hustadt COMP519 Web Programming L21 21

23 Magic Methods: Index Operators A class implementing getitem (self,key) setitem (self,key,value) allows instances to use the [] operator to access values class Building : def init (self, floors ): self. floors = [ None ]* floors def setitem (self, floor_number, data ): self. floors [ floor_number ] = data def getitem (self, floor_number ): return self. floors [ floor_number ] building1 = Building (3) building1 [0] = Reception building1 [1] = ABC Corp building1 [2] = DEF Ltd print ( building1 [2] ) # prints DEF Ltd Ullrich Hustadt COMP519 Web Programming L21 22

24 Magic Methods: Iterators A for-loop starts by calling the iter () on an object to obtain an iterator object At each iteration (including the first), a for-loop calls the next () method on the iterator object to get the next item If next () raises the exception StopIteration, the for-loop stops class PowTwo : def init (self, max = 0): self. max = max def iter ( self ): self.n = 0 return self def next ( self ): if self.n <= self. max : result = 2 ** self. n self.n += 1 return result else : raise StopIteration for x in PowTwo (2): print (x) 1 # 2 ** 0 2 # 2 ** 1 4 # 2 ** 2 Ullrich Hustadt COMP519 Web Programming L21 23

25 Functions Classes Revision Revision and Further Reading Read Section 9: Classes ( of Python Software Foundation: The Python Tutorial. Python Software Foundation, 21 December [accessed 21 December 2017] Read Chapter: Object-Oriented Programming ( eu/python3_object_oriented_programming.php) and all remaining chapters of Bernd Klein: Python 3 Tutorial. bodenseo Python Training Couses, 21 December [accessed 21 December 2017] Ullrich Hustadt COMP519 Web Programming L21 24