深入理解Python变量与常量

x = 1 #变量赋值定义一个变量x
print(id(x)) #打印变量x的标识
print(x+5) #使用变量
print("=========华丽的分割线=========")
x = 2 #量赋值定义一个变量x
print(id(x)) #此时的变量x已经是一个新的变量
print(x+5) #名称相同，但是使用的是新的变量x

x = 'hello python'
print(id(x))
print(x)

a = (1,2,3) #定义一个序列
x,y,z = a  #把序列的值分别赋x、y、z
print("a : %d, b: %d, z:%d"%(x,y,z)) #打印结果

a, b, c = 1, 2, "john"

#fileName:file1
def fun():
  local_var = 100 #定义一个局部变量
  print(local_var)
def fun2():
  zero = local_var - 100 #fun2中使用局部变量(不可以)
  print("get zero : %d"%zero)
fun()
#fun2()
print("local_var -1 = %d"%(local_var - 1)) #文件1中使用局部变量(不可以)
################################
#Traceback (most recent call last):
# File "E:/python/file1.py", line 10, in <module>
#  print("local_var -1 = %d"%(local_var - 1))
#NameError: name 'local_var' is not defined
################################
#Traceback (most recent call last):
# File "E:/python/file1.py", line 9, in <module>
#  fun2()
# File "E:/lichenli/python/file1.py", line 6, in fun2
#  zero = local_var - 100
#NameError: name 'local_var' is not defined
################################

#fileName:file2
import file1
file1.fun()
print(local_var)
########################
#运行结果
#100
#Traceback (most recent call last):
# File "E:\python\file2.py", line 4, in <module>
#  print(local_var)
#NameError: name 'local_var' is not defined
########################

#fileName:file1
g_num1 = 1 #定义全局变量
g_num2 = 2 #定义全局变量
def add_num():
  global g_num1 #引用全局变量
  g_num1 = 3 #修改全局变量的值
  result = g_num1 + 1
  print("result : %d"%result)
def sub_num():
  global g_num2
  g_num2 = 5
  result = g_num2 - 3
  print("result : %d"%result)
add_num()
sub_num()
print("g_num1:%d "%g_num1)
print("g_num2:%d "%g_num2)
#result : 4 result为局部变量
#result : 2
#g_num1:3 全局变量g_num1在执行add_num()函数时被改变
#g_num2:5 全局变量g_num2在执行sub_num()函数时被改变

#添加到sub_num()函数定义之后，add_num()函数调用之前
def other(): 
  result = g_num1 + 2 #直接适用全局变量不改变全局变量的值OK
  print("result : %d"%result)
other()

#######################
#result : 3
#result : 4
#result : 2
#g_num1:3 
#g_num2:5 
#######################

#添加到sub_num()函数定义之后，add_num()函数调用之前
def other():
  g_num1 = 10
  result = g_num1 + 2
  print("result : %d"%result)
other()
####################
#result : 12
#result : 4
#result : 2
#g_num1:3 
#g_num2:5 
####################

#fileName:file2
import file1
file1.add_num() #g_num1被改变
test = file1.g_num1 + 1
print("test :%d"%test)

#fileName:const.py
class _const:
  class ConstError(TypeError):pass
  def __setattr__(self,name,value):
    #if self.__dict__.has_key(name): 3.x之后has_key被废弃
    if name in self.__dict__:
      raise self.ConstError("Can't rebind const(%s)"%name)
    self.__dict__[name] = value
import sys
sys.modules[__name__] = _const()

#fileName:const_2.py
import const
const.name='zhangsan'
const.name='lisi'

##################################
#Traceback (most recent call last):
# File "E:/python/const_2.py", line 4, in <module>
#  const.name='lisi'
# File "E:/python\const.py", line 7, in __setattr__
#  raise self.ConstError("Can't rebind const(%s)"%name)
#const._const.ConstError: Can't rebind const(name)
##################################