COMP 364: Classes, Objects, and Names

Transcription

1 COMP 364: Classes, Objects, and Names Carlos G. Oliver, Christopher Cameron September 13, /26

2 Outline vs Development Environment Recap 3. Basic Data Types 4. Variables 2/26

3 Your Development Environment I Your OS manages file storage and program execution. I Python is a language for writing instructions. I The instructions are read and executed by the Python interpreter (executable program) I 3 ways to execute you Python commands I Save.py file! command line $ python my file.py I Execute commands interactively one at a time with Python or ipython. $ python (good for quick tests) I Jupyter Notebook (example) I In the course we will use all three methods. 3/26

4 Data Types The Python interpeter can handle information and perform operations on 4 broad categories. I Numbers! integers, floating points I Text! characters and strings I Booleans! True or False Data can be stored in memory where it is forgotten after Python closes. Or it can be kept in storage (i.e. hard drive) where it lives permanently. 4/26

5 How does this actually work? I What happens when I do this? I >>> I Every line it sees, Python evaluates it, stores the result as an Object and gives it a unique label ID. I An Object is any piece of data that you can tell Python to remember in memory (numbers, strings, booleans, etc.) Object Memory ID: Value: /26

6 Abriefinterludeonfunctions I Programming is super boring without functions. We ll go into much more detail later but for now a rough outline so things can be a bit more interesting. I A function is also an Object which can execute some commands given an (optional) input to produce an output. I function name(input) I e.g. >>> print("hello world") Input Function Output 6/26

7 id() I Like the print(object) function, Python has a built-in function called id(object). I id(object) outputs the unique ID of any input object. 1 >>> id(pi) >>> id(id) /26

8 AbriefinterludeonComments I The special symbol # tells Python to ignore the rest of the line (to the right). I This is called a comment and it lets us annotate code. I Again, more on this later. 1 #this is a comment and will be ignored 2 x = True #this will also get ignored 8/26

9 Remembering values: variables I If we wanted to re-use the value we have to either re-type it or use its super long ID. I We can instead assign a name to the object ID to be called at alatertime. 1 >>> >>> id(3.1415) >>> pi = >>> id(pi) I single equals says assign the name on the left side of = to the object on the right hand side. 9/26

10 Under the Covers: Namespaces I This is accomplished by what we call a namespace I A namespace is simply a mapping from names to objects. I A name is NOT an object. I Namespaces remember which Object IDs belong to which names (e.g. pi) I When we say >>> pi we are saying Python, get me the object whose name is pi 10/26

11 Namespaces Memory Namespace pi /26

12 Namespaces I We can assign multiples names to the same object. 1 >>> bob = pi 2 >>> id(bob) == id(pi) Namespace Memory pi bob /26

13 1 1 Source: 13/26

14 Built-in Data Types I We saw that Objects have the property of a unique ID I Python needs to know what kind of data it is dealing with to be able to do operations on it properly. I Clearly strings can t be handled the same way as decimal numbers or booleans (try it). I Objects have another property called the type which tells Python how to deal with the objects. I Python has a set of pre-defined (built-in) data types, or classes which we have already started playing with. 14/26

15 type(object) The type() function tells us the type or class of an Object. 1 >>> type(5) 2 <class int > 3 >>> type("bob") 4 <class str > 5 >>> x = True 6 >>> type(x) 7 <class bool > 8 >>>y=x 9 >>> type(y) 10 <class bool > Python uses the type to know how to perform operations on the Objects. 15/26

16 AnoteonBooleans The boolean values True and False are treated as 0 and 1. Try it! 1 >>> 6 - True 16/26

17 String slicing I We can select parts of a string using slicing. I s[a:b] evaluates to the sub-string of s from the a-th character and not including the b-th character. 1 >>> bob = "This is a string" 2 >>> bob[0:2] # Note: the first index is 0, NOT 1 3 "Th" 4 >>> bob[0] 5 "T" 6 >>> bob[4:] # from the fourth till the end 7 " is a string" You can get the last character using bob[-1] Life Hack 1 17/26

18 Objects have attributes I I I What is Kanye West? He is a Person (Object type), with a specific set of attributes (which are themselves objects). Objects of the same type behave similarly. (integers can be added/subtracted, strings can be concatenated,people can jump and rap) 18/26

19 Recap of definitions I Objects hold information, a.k.a. attributes. They are unique data containers. (e.g. >>> ) I Classes define the common properties of a set of objects. (i.e. the class of bool objects can do and and or operations I Names are labels to a specific object. Multiple names can be bound to the same object. I Namespace is the mapping from names to objects. A program can have multiple namespaces (more later) 19/26

20 Summary Figure: Names, Namespaces, Objects class / type o1 o2 o3 Objects bob alice eve Names Namespace 20/26

21 Viewing attributes You can see all the attributes of an object using the function dir(object). 1 >>> dir("bob") 2 [ add,..., len, lt, mod,,!,!,!,!,!,!,!,!,! mul, ne, new, reduce,..., str, capitalize, casefold, center, count, encode, endswith, expandtabs, find, format, format_map, index, isalnum, isalpha, isdecimal, isdigit,..., isupper, join, ljust, lower, lstrip, maketrans, partition, replace,..., split, splitlines, startswith, strip,..., upper, zfill ] 21/26

22 Accessing object attributes I You can access an attribute of an Object using the. (dot) operator. I Syntax: object name.object attribute I Since strings are a type of object. Let s see what kinds of skills and properties they posess. 1 >>> s = "Hi my name is Kanye" #let s access the,! isupper() function of this string 2 >>> s.isupper() 3 False 22/26

23 More useful string attributes 1 >>> s.upper() # try s.lower() 2 "HI MY NAME IS KANYE" 3 >>> s.startswith("hi") 4 True 5 >>> " hello ".strip() 6 "hello" 7 >>> s.split() #more on the output of this function,! later 8 ["Hi", "my", "name", "is", "Kanye"] I Notice anything weird with these function calls? Where is the input? I The object s before the dot is also passed as in as input to the function! 23/26

24 Collections I At this point all we have is basically a fancy calculator. Not very special. I However, the world is full of collections of things. We would like to be able to work with such things e ciently. 2 2 Rick and Morty 24/26

25 How can we keep track of all the Jerrys? I We know how to store data as objects. So let s make an object for each Jerry. 1 jerry1 = "Original" 2 jerry2 = "First clone" 3 jerry3 = "Second clone" This doesn t seem like a very e cient way of doing things. Thankfully Python lets us store collections of things very easily. 25/26

26 Lists I Just like everything in Python, a list is an Object of type list. I We store a list using square brackets []. 1 >>> jerrys = ["Original jerry", "Clone one", "Clone,! two"] 2 >>> type(jerrys) 3 <class list > 4 >>> id(jerrys) /26