Detailed explanation of the use of Python generator decorator and recursion

1. Python generator expression

1), Python generator expression

Syntax format:

(expr for iter_var in iterable)

(expr for iter_var in iterable ifcond_expr)

2), custom generator

Using yield in the function returns 1 generator object. yieldx

Generator usage example:

In [1]:list((i**2 for i in range(1,11)))

Out[1]:[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

In [2]:def genNum(x):

 ...:  y = 0

 ...:  while y <= x:

 ...:    yield y

 ...:    y += 1

In [3]: g1= genNum(4)

In [4]:type(g1)

Out[4]:generator

In [5]:g1.next()

Out[5]:0

In [6]:g1.next()

Out[6]:1

In [7]:g1.next()

Out[7]:2

In [8]:g1.next()

Out[8]:3

In [9]:g1.next()

Out[9]:4

In [10]:g1.next()

--------------------------------------------------

StopIteration      Traceback (most recent call last)

in()

----> 1g1.next()

StopIteration:

In [11]:def genNum(n):

 ....:  i = 1

 ....:  while i <= n:

 ....:    yield i ** 2

 ....:    i += 1

In [12]:g1 = genNum(20)

In [13]:for i in g1:

 ....:  print i,

 ....:  

1 4 9 1625 36 49 64 81 100 121 144 169 196 225 256 289 324 361400

2. Python decorator

1) The decorator itself is a function for decorating other functions;

2) Function: Enhance the function of the decorated function;

Decorator 1 accepts a function object as an argument to enhance it

Example 1: Decorator usage example

In [1]:def decorater(func):

 ...:  def wrapper():

 ...:    print "Just a Decorater!"

 ...:    func()

 ...:    raw_input('Please Input your name:')

 ...:  return wrapper

 ...:

In [2]:@decorater

  ...:def show():

 ...:  print "I am from China."

 ...:  

In [3]:show()

Just aDecorater!

I am fromChina.

PleaseInput your name:Fieldyang

Example 2: Decorate a function that can pass in parameters

In [1]:def decorater(func):

 ...:  def wrapper(x):

 ...:    print "Just a Decorater!"

 ...:    func(x)

 ...:    raw_input('Please Input your name:')

 ...:  return wrapper

 ...:

In [2]:@decorater

  ...:def show(x):

 ...:  print "I am from China.%s" %x

 ...:  

In [3]:show('how are you ?')

Just aDecorater!

I am fromChina.how are you ?

PleaseInput your name:Fieldyang

3. Python recursion

Recursion requires boundary conditions, recursive forward segment and recursive return segment.

    10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 *1

    10 *  ( 10-1 ) *  (( 10-1 ) -1 ) * ...

Examples of using recursive functions:

In [1]:def recursion(n):

 ...:  if n <= 1: return 1

 ...:  else: return n * recursion(n-1)

 ...: 

A recursive function is equivalent to the following procedure:

In [2]: recursion(3) = 3 * recursion(2)= 3 * 2 *recursion(1)=3*2*1

KeyboardInterrupt

In [3]:recursion(3)

Out[3]:6

In [4]:recursion(4)

Out[4]:24

In [5]:recursion(5)

Out[5]:120

In [6]:recursion(10)

Out[6]:3628800