function_ClosureAndAnonymous.py

#function as return 
def lazy_sum(*args):
    def sum():
        ax = 0
        for n in args:
            ax = ax + n
        return ax
    return sum
f = lazy_sum(1,3,5,7,9)
f
#<function lazy_sum........>
f()
#25
#sum keeps parameters and variables of lazy_sum.
#every time lazy_sum will return different function.


def createCounter():
    n = [0]
    def counter():
        n[0] = n[0] + 1
        return n[0]
    return counter
cA = createCounter()
print(cA(), cA(), cA(), cA())   # 1 2 3 4
cB = createCounter()
print(cB(), cB(), cB(), cB())   # 1 2 3 4

s = 0 
def createCounter():
    def counter():
        global s
        s = s + 1
        return s
    return counter
cA = createCounter()
print(cA(), cA(), cA(), cA())   # 1 2 3 4
cB = createCounter()
print(cB(), cB(), cB(), cB())   # 5 6 7 8


#   lambda x: x * x
#   same as
def f(x):
    return x * x
#   lambda means anonymous function.
#   lambda has only one expression

def _not_divisible(n):
    return lambda x: x % n > 0
n = 2
it = [1,2,3,4,5]
it = filter(_not_divisible(n), it)
#   _not_divisible(n) return lambda x: x % n > 0, still be a function.

#   function has one attribute named __name__.
def log(func):
    def wrapper(*args, **kw):
        print('call %s():' % func.__name__)
        return func(*args, **kw)
    return wrapper
@log
def now(t):
    print(t)
    print('2018-7-6')    
now(t) # t is not defined, but still work
''' result:
    call now():
    <function build.<locals>.<lambda> at 0x000002525D658AE8>
    2018-7-6
'''