Python Programming: List Comprehensions, Functions, and Classes in AI at USF - Prof. Chris, Study notes of Computer Science

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Artificial Intelligence Programming

Objects in Python

Chris Brooks

Department of Computer ScienceUniversity of San Francisco

Department of Computer Science — University of San Francisco – p.1/

3-2:^ List Comprehensions^ •^ Python has a great deal of support for

functional programming

◦^ Can do it by hand using map, filter, and lambda. ◦^ Also a built-in construct called a

list comprehension

◦^ List comprehensions are a convenient way to

map^ a

function onto a sequence. [x**2^ for^

x in^ [1,2,3,4]] phoneBook

=^ {‘‘brooks’’:’’422-5221’’,

‘‘wolber’’:’’422-1234’’,’’parr’’:’’422-3435’’

[‘‘name:

%s^ phone:

%’’^ %^

(nm,^ ph)

for^ nm,

ph^ in

phoneBook.items()][s.strip()

for^ s in

[’^ hello

’,^ ’^

in^ ’,^

’^ there^

’]]

Department of Computer Science — University of San Francisco – p.2/

3-4:^ Function Arguments^ •^ Python functions can have optional and named arguments^ def

example_fun(arg1,

arg2=’cats’,

arg3=’dogs’)

print^ arg1,

arg2,^

’ and^ ’,

arg

example_fun(’I

love’) I^ love^

cats^ and

dogs example_fun("I

love",

arg2=’fish’) I^ love^

fish^ and

dogs example_fun(’I

love’,

’fish’,

’chips’)

I^ love^

fish^ and

chips example_fun(’I

love’,

arg3=’birds’) I^ love^

cats^ and

birds

-^ This makes it easy to implement multiple versions of a function.

Department of Computer Science — University of San Francisco – p.4/

3-5:^ Functions as Objects^ •^ In Python, functions are first-class objects^ ◦

Can be assigned to variables, passed as arguments,evaluated. ◦ This allows us to create higher-order functions def^ cube(x) :

return

x^ *^ x *

x

def^ my_map(list, fn)

: list2 =

[] for^ item in

list : list2.append(fn(item))return list

Department of Computer Science — University of San Francisco – p.5/

3-7:^ Python Classes^ •^ Classes are defined with the ’class’ keyword:^ class

Mammal^

... • This defines a class with no parent class.

Department of Computer Science — University of San Francisco – p.7/

3-8:^ init^ •^ __init__is the first method called when an object is created.^ ◦

Technically, it’s not a constructor, but close enough • Takes at least one argument: self • This is a pointer to the object

Department of Computer Science — University of San Francisco – p.8/

3-10:^ Instance Variables^ •^ Instance variables

must^ be referenced with self

-^ Created when a value is assigned to them^ ◦^ No separate class definition/header file^ ◦^ This can lead to confusion^ ◦^ Convention: set up all instance variables inside init •^ Data members are public

Department of Computer Science — University of San Francisco – p.10/

3-11:^ Methods^ •^ Defined with def^ •^ Inside the scope of the class definition.^ •^ self is first argument in declaration^ ◦

Not provided when calling the method - the Pythoninterpreter fills this in. • Can use default and keyword arguments

Department of Computer Science — University of San Francisco – p.11/

3-13:^ Methods^ •^ built-in operators can be

overloaded

to provide polymorphism.

◦^ repr - controls how an object is printed. ◦^ lt, gt, le, gt, cmp : comparison operators ◦^ add, sub, mul, div : arithmetic operators

Department of Computer Science — University of San Francisco – p.13/

3-14:^ Class variables^ •^ Class variables are declared outside the scope of a method^ •^ Referenced with the class name^ •^ Useful for constant values, counters, mutex/semaphores, etc.^ •^ Places where you don’t want to create an object^ ◦

string.letters, string.ascii_lowercase, etc

Department of Computer Science — University of San Francisco – p.14/

3-16:^ Inheritance^ class^ Flyer

: "Class for^ things

that^ can

fly"

def^ init(self,

type=’’)

self.type=typedef canFly(self)

return^

True class^ Bird(Flyer)

def^ init(self,

type=’’)

Flyer.init()self.type

= type def^ sayName(self)

print^ "I

am^ a^ "

+^ self.type

Department of Computer Science — University of San Francisco – p.16/

3-17:^ Multiple Inheritance^ •^ Python also supports multiple inheritance^ •^ List multiple classes in declaration^ •^ Names are resolved from left to right

Department of Computer Science — University of San Francisco – p.17/

3-19:^ Comments^ •^ Always need to use self to refer to member variables^ •^ All variables are public^ ◦

can use __spam to indicate private variables • Classes can tell you what methods they implement with dir() • this is called

introspection

(reflection in Java)

Department of Computer Science — University of San Francisco – p.19/