Python course slides topic objects in python
- 2. Announcements for Today Assignment 1 • We have finished grading • Resubmit until correct § Read feedback in CMS § Reupload/request regrade • If you were very wrong… § You received an e-mail § More 1-on-1s this week • Finish Survey 1 Assignment 2 • Posted last Friday § Written assignment § Do while revising A1 § Relatively short (2-3 hrs) • Due Thursday § Submit as a PDF § Scan or phone picture § US Letter format! 9/20/22 Objects 2
- 3. The Basic Python Types • Type int: § Values: integers § Ops: +, –, *, //, %, ** • Type float: § Values: real numbers § Ops: +, –, *, /, ** • Type bool: § Values: True and False § Ops: not, and, or • Type str: § Values: string literals • Double quotes: "abc" • Single quotes: 'abc' § Ops: + (concatenation) 9/20/22 Objects 3 Are the the only types that exist?
- 4. Example: Points in 3D Space def distance(x0,y0,z0,x1,y1,z1): """Returns distance between points (x0,y0,y1) and (x1,y1,z1) Param x0: x-coord of 1st point Precond: x0 is a float Param y0: y-coord of 1st point Precond: y0 is a float Param z0: z-coord of 1st point Precond: z0 is a float …. """ • This is very unwieldy § Specification is too long § Calls needs many params § Typo bugs are very likely • Want to reduce params § Package points together § How can we do this? 9/20/22 Objects 4
- 5. Points as Their Own Type def distance(p0,p1): """Returns distance between points p0 and p1 Param p0: The second point Precond: p0 is a Point3 Param p1: The second point Precond: p1 is a Point3""" … This lecture will help you make sense of this spec. 9/20/22 Objects 5
- 6. Classes: Custom Types • Class: Custom type not built into Python § Just like with functions: built-in & defined § Types not built-in are provided by modules • Might seem weird: type(1) => <class 'int’> § In Python 3 type and class are synonyms § We will use the historical term for clarity introcs provides several classes 9/20/22 Objects 6
- 7. Objects: Values for a Class • Object: A specific value for a class type § Remember, a type is a set of values § Class could have infinitely many objects • Example: Class is Point3 § One object is origin; another x-axis (1,0,0) § These objects go in params distance function • Sometimes refer to objects as instances § Because a value is an instance of a class § Creating an object is called instantiation 9/20/22 Objects 7
- 8. How to Instantiate an Object? • Other types have literals § Example: 1, 'abc’, True § No such thing for objects • Classes are provided by modules § Modules typically provide new functions § In this case, gives a function to make objects • Constructor function has same name as class § Similar to types and type conversion § Example: str is a type, str(1) is a function call 9/20/22 Objects 8
- 9. Demonstrating Object Instantiation >>> from introcs import Point3 # Module with class >>> p = Point3(0,0,0) # Create point at origin >>> p # Look at this new point <class 'introcs.geom.point.Point3'>(0.0,0.0,0.0) >>> type(p) == Point3 # Check the type True >>> q = Point3(1,2,3) # Make new point >>> q # Look at this new point <class 'introcs.geom.point.Point3'>(1.0,2.0,3.0) 9/20/22 Objects 9
- 10. What Does an Object Look Like? • Objects can be a bit strange to understand § Don’t look as simple as strings or numbers § Example: <class 'introcs.Point3'>(0.0,0.0,0.0) • To understand objects, need to visualize them § Use of metaphors to help us think like Python § Call frames (assume seen) are an example • To visualize we rely on the Python Tutor § Website linked to from the course page § But use only that one! Other tutors are different. 9/20/22 Objects 10
- 11. Metaphor: Objects are Folders >>> import introcs >>> p = introcs.Point3(0,0,0) >>> id(p) id2 p id2 x 0.0 y 0.0 z 0.0 Point3 Need to import module that has Point class. Constructor is function. Prefix w/ module name. Unique tab identifier 9/20/22 Objects 11 Shows the ID of p.
- 12. Metaphor: Objects are Folders id2 p id2 x 0.0 y 0.0 z 0.0 Point3 • Idea: Data too “big” for p § Split into many variables § Put the variables in folder § They are called attributes • Folder has an identifier § Unique (like a netid) § Cannot ever change § Has no real meaning; only identifies folder Unique tab identifier Attribute 9/20/22 Objects 12
- 13. Object Variables • Variable stores object name § Reference to the object § Reason for folder analogy • Assignment uses object name § Example: q = p § Takes name from p § Puts the name in q § Does not make new folder! • This is the cause of many mistakes for beginners id2 p id2 x 0.0 y 0.0 z 0.0 Point3 id2 q 9/20/22 Objects 13
- 14. Objects and Attributes • Attributes live inside objects § Can access these attributes § Can use them in expressions • Access: <variable>.<attr> § Look like module variables § Recall: math.pi • Example >>> p = introcs.Point3(1,2,3) >>> a = p.x + p.y id3 x 1.0 y 2.0 z 3.0 id3 p Point3 9/20/22 Objects 14
- 15. Objects and Attributes • Attributes live inside objects § Can access these attributes § Can use them in expressions • Access: <variable>.<attr> § Look like module variables § Recall: math.pi • Example >>> p = introcs.Point3(1,2,3) >>> a = p.x + p.y id3 x 1.0 y 2.0 z 3.0 id3 p Point3 3.0 a 9/20/22 Objects 15
- 16. Objects and Attributes • Can also assign attributes § Reach into folder & change § Do without changing p • <var>.<attr> = <exp> § Example: p.x = p.y+p.z § See this in visualizer • This is very powerful § Another reason for objects § Why need visualization id3 x 1.0 y 2.0 z 3.0 id3 p Point3 5.0 x 9/20/22 Objects 16
- 17. Exercise: Attribute Assignment • Recall, q gets name in p >>> p = introcs.Point3(0,0,0) >>> q = p • Execute the assignments: >>> p.x = 5.6 >>> q.x = 7.4 • What is value of p.x? 9/20/22 Objects 17 id4 p id4 q A: 5.6 B: 7.4 C: id4 D: I don’t know id4 x 0.0 y 0.0 z 0.0 Point3
- 18. Exercise: Attribute Assignment • Recall, q gets name in p >>> p = introcs.Point3(0,0,0) >>> q = p • Execute the assignments: >>> p.x = 5.6 >>> q.x = 7.4 • What is value of p.x? 9/20/22 Objects 18 id4 p id4 q A: 5.6 B: 7.4 C: id4 D: I don’t know id4 x 0.0 y 0.0 z 0.0 Point3 5.6 CORRECT x
- 19. Exercise: Attribute Assignment • Recall, q gets name in p >>> p = introcs.Point3(0,0,0) >>> q = p • Execute the assignments: >>> p.x = 5.6 >>> q.x = 7.4 • What is value of p.x? 9/20/22 Objects 19 id4 p id4 q A: 5.6 B: 7.4 C: id4 D: I don’t know id4 x 0.0 y 0.0 z 0.0 Point3 5.6 7.4 CORRECT x x
- 20. Objects Allow for Mutable Functions • Mutable function: alters the parameters § Often a procedure; no return value • Until now, this was impossible § Function calls COPY values into new variables § New variables erased with call frame § Original (global?) variable was unaffected • But object variables are folder names § Call frame refers to same folder as original § Function may modify the contents of this folder 9/20/22 Objects 20
- 21. Example: Mutable Function Call • Example: def incr_x(q): q.x = q.x + 1 >>> p = Point3(0,0,0) >>> p.x 0.0 >>> incr_x(p) >>> p.x 1.0 1 incr_x 2 id1 q Global STUFF Call Frame id1 p id1 0.0 … Point3 x 2 9/20/22 Objects 21
- 22. Example: Mutable Function Call • Example: def incr_x(q): q.x = q.x + 1 >>> p = Point3(0,0,0) >>> p.x 0.0 >>> incr_x(p) >>> p.x 1.0 1 incr_x id1 q Global STUFF Call Frame id1 p id1 0.0 … Point3 x 2 1.0 x 9/20/22 Objects 22
- 23. Example: Mutable Function Call • Example: def incr_x(q): q.x = q.x + 1 >>> p = Point3(0,0,0) >>> p.x 0.0 >>> incr_x(p) >>> p.x 1.0 1 Global STUFF Call Frame id1 p id1 0.0 … Point3 x 2 1.0 x ERASE WHOLE FRAME Change remains 9/20/22 Objects 23
- 24. Methods: Functions Tied to Objects • Have seen object folders contain variables § Syntax: ⟨obj⟩.⟨attribute⟩ (e.g. p.x) § These are called attributes • They can also contain functions § Syntax: ⟨obj⟩.⟨method⟩(⟨arguments⟩) § Example: p.clamp(-1,1) § These are called methods • Visualizer will not show these inside folders § Will see why in November (when cover Classes) 9/20/22 Objects 24
- 25. Understanding Method Calls • Object before the name is an implicit argument • Example: distance >>> p = Point3(0,0,0) # First point >>> q = Point3(1,0,0) # Second point >>> r = Point3(0,0,1) # Third point >>> p.distance(r) # Distance between p, r 1.0 >>> q.distance(r) # Distance between q, r 1.4142135623730951 9/20/22 Objects 25
- 26. Recall: String Method Calls • Method calls have the form string.name(x,y,…) • The string in front is an additional argument § Just one that is not inside of the parentheses § Why? Will answer this later in course. method name arguments argument Are strings objects? 9/20/22 Objects 26
- 27. Surprise: All Values are Objects! • Including basic values § int, float, bool, str • Example: >>> x = 1000 >>> id(x) 2.5 x 2.5 id5 id5 x float 9/20/22 Objects 27
- 28. This Explains A Lot of Things • Basic types act like classes § Conversion function is really a constructor § Remember constructor, type have same name • Example: >>> type(1) <class 'int'> >>> int('1') 1 • Design goals of Python 3 § Wanted everything an object § Makes processing cleaner • But makes learning harder § Objects are complex topic § Want to delay if possible 9/20/22 Objects 28
- 29. But Not Helpful to Think This Way • Number folders are immutable § “Variables” have no names § No way to reach in folder § No way to change contents >>> x = 1000 >>> y = 1000 >>> id(x) 4497040368 >>> id(y) 4497040400 >>> y = y+1 >>> id(y) 4497040432 1000 4497040368 4497040368 x int Makes a brand new int folder 9/20/22 Objects 29
- 30. But Not Helpful to Think This Way • Number folders are immutable § “Variables” have no names § No way to reach in folder § No way to change contents • Remember purpose of folder § Show how objects can be altered § Show how variables “share” data § This cannot happen in basic types • So just ignore the folders § (The are just metaphors anyway) >>> x = 1000 >>> y = 1000 >>> id(x) 4497040368 >>> id(y) 4497040400 >>> y = y+1 >>> id(y) 4497040432 9/20/22 Objects 30
- 31. Basic Types vs. Classes Basic Types • Built-into Python • Refer to instances as values • Instantiate with literals • Are all immutable • Can ignore the folders Classes • Provided by modules • Refer to instances as objects • Instantiate w/ constructors • Can alter attributes • Must represent with folders 9/20/22 Objects 31 In doubt? Use the Python Tutor
- 32. Where To From Here? • Right now, just try to understand objects § All Python programs use objects § The object classes are provided by Python • OO Programming is about creating classes § But we will not get to this until after Prelim 1 • Similar to the separation of functions § First learned to call functions (create objects) § Then how to define functions (define classes) 9/20/22 Objects 32