Python, Part 2

Python Object Equality

x == yx is y

In Java:(x.equals(y))(x == y)

Python Exceptions

try: do_stuff() if something_bad_has_happened(): raise MyException, "something happened" except MyException, e: print "My exception occurred, value: ", e.value

Python Exceptions

while True: try: x = int(raw_input("Please enter a number: ")) except ValueError: print "That was no valid number. Try again."

Functional programming features: map

lst = ["1", "2", "3", "4", "5"] nums = map(string.atoi, lst) # nums is now [1, 2, 3, 4, 5]

map

def add(x, y): return x + y

lst1 = [1, 2, 3, 4, 5] lst2 = [6, 7, 8, 9, 10]

lst_sum = map(add, lst1, lst2) # lst_sum == [7, 9, 11, 13, 15]

reduce

def add(x, y): return x + y

lst1 = [1, 2, 3, 4, 5] sum = reduce(add, lst1) # sum == 15

filter

nums = range(0,101) # [0, 1, ... 100] def is_odd(x): return x % 2 == 1

odd_nums = filter(is_odd, nums) # odd_nums == [1, 3, 5, ... 99]

Lambda: Anonymous functions

t = lambda x,y: x*y

def t: return x*y

answer = t(2,3)

LambdaUseful for map, reduce, filter:

nums = range(0,101) # [0, 1, ... 100]

odd_nums = filter(lambda x: x%2==1, nums) # odd_nums == [1, 3, 5, ... 99]

Pass Comparators to Sort:

#return data which has groups of a name followed by 2 scores.def get_data(): return [('fred',10,1),('bill',3,0),('betty',5,8)]

a=get_data()

a.sort(lambda x, y: cmp(x[2],y[2])) print a

>>>[('bill', 3, 0), ('fred', 10, 1), ('betty', 5, 8)]

Each “file” is a module.

Consider module called “hello.py”:

def printer(message): print message

In module main.py write:import hellohello.printer(“hello”)

-OR- from hello import printerprinter(“hello”)

from hello import *

Main Module

What about interactive session? Is it a module??

Yes: __main__

Modules executed on import. In many modules you will see:

if __name__ == "__main__": main()

Python Classes:

Attributes can be added/deleted dynamically (no declaring)

Keyword self (like this) passed into all class methods

Special methods __init__ (Constructor)__del__ (Destructor)

class Student: def __init__(self, name, hours, qpts): self.name = name self.hours = hours self.qpts = qpts

def getName(self): return self.name

def gpa(self): return self.qpts/self.hours

from student import *

aStudent = Student(“Mary Adams”, 127, 228)gpa = aStudent.gpa()print gpa

s = Student(…) calls the constructor. To call the destructor:s = 0

Creating a list the old way…

>>> squares = [] >>> for x in range(10): squares.append(x**2) >>> squares [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

List Comprehensions

>>> squares = [] >>> for x in range(10):

squares.append(x**2) >>> squares [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>>squares = [x**2 for x in range(10)]

List Comprehensions

>>> vec = [-4, -2, 0, 2, 4] >>> [x*2 for x in vec]

List Comprehensions

>>> vec = [-4, -2, 0, 2, 4] >>> [x*2 for x in vec] [-8, -4, 0, 4, 8]

>>> vec = [-4, -2, 0, 2, 4] >>> [x for x in vec if x >= 0]

>>> vec = [-4, -2, 0, 2, 4] >>> [x for x in vec if x >= 0] [0, 2, 4]

Can do embedded loops too

>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]

Can do embedded loops too

>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]