Eval function, global, local variables

eval() function ：

a=10;
print(eval("a+1"))

eval Function will “ String text ” Change to code variable

  stay Python in evel() The syntax format of the function is eval(expression, globals=None, locals=None)

global：g   local：t

a=10
b=20
c=30
g={'a':6,'b':8}
t={'b':100,'c':10}
print(eval('a+b+c',g,t))

a and c Go to the dictionary respectively g And the dictionary t The value in , When globals and locals There are parameters in b You take locals The value in . therefore a=6,b=100,c=10. The running result is 116

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application ： You can use one line of code to implement the functions of the calculator print(eval(input(' Please enter '))).

shortcoming ： But because it has the ability to convert strings into expressions for execution , So it can execute system commands . This can easily be used by people with ulterior motives to execute system commands , Delete key system files .