条件循环控制语句。一般需要和break一起使用，不然会进入死循环。

格式：【 while <条件>：

<内容>

break 】

x=int(input('请输入一个数字：'))
while x>0:
 print('正数')
 break

2.if语句

流程分支的条件控制，一般和elif和else使用。

x=int(input('请输入一个数字：'))
if x<0:
 print('负数')
elif x==0:
 print('零')
else :
 print('正数')

3.for语句

循环控制语句，可用来遍历某一对象，和in一起使用。

格式： 【 for <> in <对象集合>：】

x=['a','b','c','d']for i in x : 
 # i 位置的字符，只要不是关键字，可以随意用字符代表
 print(i)

4.range()函数

数字序列迭代器,当你迭代它时，它是一个能够像期望的序列返回连续项的对象,但为了节省空间，它并不真正构造列表。

格式： range(stop) 给出结束数值，开始数值默认为0，间隔为1。

range(start,stop) 给出开始数值和结束数值，间隔为1。

range(start,stop,step) 给出开始数值和结束数值，间隔为step数值。

class range(object):
 """
 range(stop) -> range object
 range(start, stop[, step]) -> range object
 
 Return an object that produces a sequence of integers from start (inclusive)
 to stop (exclusive) by step. range(i, j) produces i, i+1, i+2, ..., j-1.
 start defaults to 0, and stop is omitted! range(4) produces 0, 1, 2, 3.
 These are exactly the valid indices for a list of 4 elements.
 When step is given, it specifies the increment (or decrement).
 """
 def count(self, value): # real signature unknown; restored from doc
 """ rangeobject.count(value) -> integer -- return number of occurrences of value """
 return 0

 def index(self, value, start=None, stop=None): # real signature unknown; restored from doc
 """
 rangeobject.index(value, [start, [stop]]) -> integer -- return index of value.
 Raise ValueError if the value is not present.
 """
 return 0

 def contains(self, *args, **kwargs): # real signature unknown
 """ Return key in self. """
 pass

 def eq(self, *args, **kwargs): # real signature unknown
 """ Return self==value. """
 pass

 def getattribute(self, *args, **kwargs): # real signature unknown
 """ Return getattr(self, name). """
 pass

 def getitem(self, *args, **kwargs): # real signature unknown
 """ Return self[key]. """
 pass

 def ge(self, *args, **kwargs): # real signature unknown
 """ Return self>=value. """
 pass

 def gt(self, *args, **kwargs): # real signature unknown
 """ Return self>value. """
 pass

 def hash(self, *args, **kwargs): # real signature unknown
 """ Return hash(self). """
 pass

 def init(self, stop): # real signature unknown; restored from doc
 pass

 def iter(self, *args, **kwargs): # real signature unknown
 """ Implement iter(self). """
 pass

 def len(self, *args, **kwargs): # real signature unknown
 """ Return len(self). """
 pass

 def le(self, *args, **kwargs): # real signature unknown
 """ Return self<=value. """
 pass

 def lt(self, *args, **kwargs): # real signature unknown
 """ Return self<value. """
 pass

 @staticmethod # known case of new
 def new(*args, **kwargs): # real signature unknown
 """ Create and return a new object. See help(type) for accurate signature. """
 pass

 def ne(self, *args, **kwargs): # real signature unknown
 """ Return self!=value. """
 pass

 def reduce(self, *args, **kwargs): # real signature unknown
 pass

 def repr(self, *args, **kwargs): # real signature unknown
 """ Return repr(self). """
 pass

 def reversed(self, *args, **kwargs): # real signature unknown
 """ Return a reverse iterator. """
 pass

 start = property(lambda self: object(), lambda self, v: None, lambda self: None) # default

 step = property(lambda self: object(), lambda self, v: None, lambda self: None) # default

 stop = property(lambda self: object(), lambda self, v: None, lambda self: None) # default

for i in range(3): #运行

5.break和continue语句，以及循环中的else语句

1）break语句和 C 中的类似，用于跳出最近的一级 for 或 while 循环。

while True:
 print('hello')
 break

2）continue语句表示循环继续执行下一次迭代:

for x in range(1, 4):
 print(x, 'for语句')
 continue
 print(x, 'continue语句后')
else:
 print(x, 'else语句')
 
#运行结果
for语句
for语句
for语句
else语句

3）循环中的else

如continue的例子里，有for-else语句，else语句会在循环跳出后执行，但是break跳出循环则不会执行else,所以else可以用来处理循环中的一些异常跳出。

for x in range(1, 4):
 print(x)
else:
 print(x)

#运行结果
2
3

6.pass语句

pass语句什么也不做。它用于那些语法上必须要有什么语句，但程序什么也不做的场合，通常用于创建最小结构的类。

另一方面，pass可以在创建新代码时用来做函数或控制体的占位符。可以让你在更抽象的级别上思考。

class EmptyClass：
 pass