How to check if a variable exists

When calling a variable, if the variable is not defined, python will report an error.

#The solution is also very simple, which is to assign a null value to the variable in advance.

But you can also determine whether a variable name has been defined by calling the system's built-in function. There are 3 built-in functions that can be implemented.

res1 = 'test' in locals().keys()
res2 = 'test' in dir()
res3 = 'test' in vars().keys()
print(res1,res2,res3)  # 变量test暂时还没有定义，返回False
test = ""  # 定义变量test
res4 = 'test' in locals().keys()
res5 = 'test' in dir()
res6 = 'test' in vars().keys()
print(res4,res5,res6)  # 变量test已经被定义了，返回True

Above, if you need to call a variable that is not sure whether it has been defined, you can first use the above function to add a judgment.

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