class A(object):
def show(self):
print 'base show'
class B(A):
def show(self):
print 'derived show' 
obj = B()
obj.show()

obj.__class__ = A
obj.show() 

class A(object):
def __init__(self,a,b):
self.__a = a
self.__b = b
def myprint(self):
print 'a=', self.__a, 'b=', self.__b
a1=A(10,20)
a1.myprint() 
a1(80)

class A(object):
def __init__(self, a, b):
self.__a = a
self.__b = b
def myprint(self):
print 'a=', self.__a, 'b=', self.__b
def __call__(self, num):
print 'call:', num + self.__a

class B(object):
def fn(self):
print 'B fn'
def __init__(self):
print "B INIT"
class A(object):
def fn(self):
print 'A fn'
def __new__(cls,a):
print "NEW", a
if a>10:
return super(A, cls).__new__(cls)
return B()
def __init__(self,a):
print "INIT", a 
a1 = A(5)
a1.fn()
a2=A(20)
a2.fn()

NEW 5
B INIT
B fn
NEW 20
INIT 20
A fn

ls = [1,2,3,4]
list1 = [i for i in ls if i>2]
print list1
list2 = [i*2 for i in ls if i>2]
print list2
dic1 = {x: x**2 for x in (2, 4, 6)}
print dic1
dic2 = {x: 'item' + str(x**2) for x in (2, 4, 6)}
print dic2
set1 = {x for x in 'hello world' if x not in 'low level'}
print set1

[3, 4] 
[6, 8]
{2: 4, 4: 16, 6: 36}
{2: 'item4', 4: 'item16', 6: 'item36'}
set(['h', 'r', 'd'])

num = 9
def f1():
num = 20
def f2():
print num 
f2()
f1()
f2()

9
9

num = 9
def f1():
global num
num = 20 
def f2():
print num 
f2()
f1()
f2() 
# prints:
# 9
# 20

a=8
b=9

(a,b) = (b,a)

class A(object):
def __init__(self,a,b):
self.a1 = a
self.b1 = b
print 'init'
def mydefault(self):
print 'default'
a1 = A(10,20)
a1.fn1()
a1.fn2()
a1.fn3()

default
default
default

class A(object):
def __init__(self,a,b):
self.a1 = a
self.b1 = b
print 'init'
def mydefault(self):
print 'default'
def __getattr__(self,name):
return self.mydefault
a1 = A(10,20)
a1.fn1()
a1.fn2()
a1.fn3()

class A(object):
def __init__(self,a,b):
self.a1 = a
self.b1 = b
print 'init'
def mydefault(self,*args):
print 'default:' + str(args[0])
def __getattr__(self,name):
print "other fn:",name
return self.mydefault
a1 = A(10,20)
a1.fn1(33)
a1.fn2('hello')
a1.fn3(10)

__all__ = ['mod1','mod3']

def mulby(num):
def gn(val):
return num * val
return gn
zw = mulby(7)
print(zw(9));

def strtest1(num):
str='first'
for i in range(num):
str+="X"
return str