Python中的上下文变量

Python中的上下文变量对象是一种有趣的变量类型，它根据上下文返回变量的值。根据单线程或执行中的上下文，它可能有多个值。 contextvars 模块中的 ContextVar 类，用于在Python中声明和使用上下文变量。

注意：这在Python版本 >= 3.7 中受支持。

以下是声明复杂变量的简单语法：

Syntax: contextvars.ContextVar(name, default)

Parameter:

name: Name of the variable which can be used for reference or debug process.
default: Returns the value of this variable in particular context. If there is no value in the context then it will return the default value.

Return: contextvars class object.

编程需要懂一点英语

注意：上下文变量应该总是在程序的顶部创建，而不是在程序的函数或中间块中。

以下是 ContextVar 类中定义的方法：

get(): Returns the value of context variable in the particular context, if it does not contain any then It will return the default value provided if provided, else raise a lookup error.
set(value): This method is called to set a new value provided as parameter to the context variable in the particular context it is called in.
reset(token): This method resets the value of context variable to that of before ContextVar.set() was called in reference to token returned by set() method.

编程需要懂一点英语

例子：

Python3

# import module
import contextvars
  
# declaring the variable 
# to it's default value
cvar = contextvars.ContextVar("cvar",
                              default = "variable")
  
print("value of context variable cvar: \n",
      cvar.get())
  
# calling set method
token = cvar.set("changed")
  
print("\nvalue after calling set method: \n",
      cvar.get())
  
# checking the type of token instance
print("\nType of object instance returned by set method: \n",
      type(token))
  
# calling the reset method.
cvar.reset(token)
  
print("\nvalue after calling reset method: \n",
      cvar.get())

Python3

import contextvars
  
# declaring the variable
cvar = contextvars.ContextVar("cvar", default = "variable")
  
# calling set function to get a token object
token = cvar.set("changed")
  
# token.var returns the ContextVar
# object through which token is generated.
print("ContextVar object though which token was generated: ")
print(token.var)
  
# As only one time set is called
# it should return token.MISSING.
print("\nAfter calling token.old_value : ")
print(token.old_value)
  
# Recalling set method again to
# test other side of token.old_value
token = cvar.set("changed_2")
print("\nPrinting the value set before set method called last : ")
print(token.old_value)
  
print("\nThe current value of context variable is : ")
print(cvar.get())

输出：

value of context variable cvar: 
variable

value after calling set method: 
changed

Type of object instance returned by set method: 


value after calling reset method: 
variable

contextvars 中的令牌类：

令牌对象由 ContextVar.set() 方法返回，并且可以在 ContextVar.reset(token) 方法中作为参数重用，以将其值重置为之前的令牌上下文变量，如上所述。

以下是可与令牌对象一起使用的属性：

Token.var：它返回用于创建此令牌的 ContextVar 对象，由 CotextVar.set() 方法返回。这具有只读属性，这是不可调用的。
Token.old_value：设置为变量在调用 ContextVar.set() 方法之前所具有的值，因此生成了令牌。它是不可调用的，它具有只读属性。如果在调用之前未设置变量，则它指向 Token.MISSING。
Token.MISSING：被 Token.old_value 用作标记。

例子：

Python3

import contextvars
  
# declaring the variable
cvar = contextvars.ContextVar("cvar", default = "variable")
  
# calling set function to get a token object
token = cvar.set("changed")
  
# token.var returns the ContextVar
# object through which token is generated.
print("ContextVar object though which token was generated: ")
print(token.var)
  
# As only one time set is called
# it should return token.MISSING.
print("\nAfter calling token.old_value : ")
print(token.old_value)
  
# Recalling set method again to
# test other side of token.old_value
token = cvar.set("changed_2")
print("\nPrinting the value set before set method called last : ")
print(token.old_value)
  
print("\nThe current value of context variable is : ")
print(cvar.get())

输出：

ContextVar object though which token was generated:


After calling token.old_value :

Python中的上下文变量

Python3

Python3

contextvars 中的令牌类：

Python3

contextvars 中的上下文类：