What is the problems with my code and how to fix it?-CodePudding

def ask_a():
    a = input("""Please enter a:
a = """)
    a = eval(a)
    if a == 0:  
        print("Please input the correct number! \n")
        ask_a()
    elif isinstance(a, str):
        print("Please input the correct number! \n")
        ask_a()
    else:
        print(a)
        return a

ask_a()

I'm making a quadratic equation solver by python 3 and that is what i write for asking for a (a is not 0 and a is not a string)

This is what the error message:

Please enter a:
a = sgafdf
Traceback (most recent call last):
  File "c:\Users\jbtua\OneDrive\Desktop\wut is this\Personal Folder\Programming Projects\Python\blank-1.py", line 15, in <module>
    ask_a()
  File "c:\Users\jbtua\OneDrive\Desktop\wut is this\Personal Folder\Programming Projects\Python\blank-1.py", line 4, in ask_a
    a = eval(a)
  File "<string>", line 1, in <module>
NameError: name 'sgafdf' is not defined
PS C:\Users\jbtua\OneDrive\Desktop\wut is this\Personal Folder\Programming Projects> & C:/Users/jbtua/AppData/Local/Programs/Python/Python310/python.exe "c:/Users/jbtua/OneDrive/Desktop/wut is this/Personal Folder/Programming Projects/Python/blank-1.py"
Please enter a:
a = a
Please input the correct number! 

Please enter a:
a = w
Traceback (most recent call last):
  File "c:\Users\jbtua\OneDrive\Desktop\wut is this\Personal Folder\Programming Projects\Python\blank-1.py", line 15, in <module>
    ask_a()
  File "c:\Users\jbtua\OneDrive\Desktop\wut is this\Personal Folder\Programming Projects\Python\blank-1.py", line 10, in ask_a
    ask_a()
  File "c:\Users\jbtua\OneDrive\Desktop\wut is this\Personal Folder\Programming Projects\Python\blank-1.py", line 4, in ask_a
    a = eval(a)
  File "<string>", line 1, in <module>
NameError: name 'w' is not defined

What should I do to avoid this? I will appreciate all the answer.

CodePudding user response：

Problem (1.)

You're calling eval(). Don't do that. It opens too many doors to remote exploits. Specify the business problem you're trying to solve, and propose a solution that doesn't involve eval.

Problem (2.)

You never assigned a value to sgafdf, nor to w, yet you tried to evaluate them.

That won't work.

Stick to defined variables in the expressions you try to evaluate.

CodePudding user response：

You are using the eval() function which will evaluate the string that is passed to it as code. For example, if you enter the input print(9) your code will print 9:

Please enter a:
a = print(9)
9
None

When you pass it invalid code as input it will throw that exception.

CodePudding user response：

Here you are using eval which evaluates the input as integer or float while the data that you entered is string so it won't work.