Python code for allowing a user to input a function

Question

In my introduction to python course we were covering the input function last time and I saw that whatever you type as an input is stored as a string. You can use the int() and float() before the input function to store as a number but I was wondering how can it be used to store a function the user enters. Is there a way to use input to allow me to define a function?

• bracco23: Welcome! I think you would benefit from taking a look at this article: thepythonguru.com/python-builtin-functions/eval
• Chris_Rands: This is nearly always a bad idea- if you're writing the input then you can easily edit the source. If another 'user' is writing the input then they can easily write malicious code that you will then execute. I suggest take the string input and parse it carefully to only validate the kinds of input you want

Answer 1

You can do this using the exec() built in function.

https://docs.python.org/3/library/functions.html#exec

my_function_def = '''
def my_function():
 print('Greetings from my function!')
'''
exec(my_function_def)
my_function() # -> 'Greetings from my function!'

Answer 2

One potential solution is to allow the user to call a pre-defined method by name. This is more secure than using the exec() command outright, as it strictly limits the user's ability to trigger code blocks.

def SomeAction():
 print("SomeAction has been executed.")
userInput = input("Enter method name: ")
if(userInput == SomeAction.__name__):
 method = getattr(sys.modules[__name__], SomeAction.__name__) #Get method by name
 method()

However, if you're talking about having the user define a method completely from input calls, then you'll need to create an input loop to get the method body.

print("Enter your method declaration: ")
methodDeclaration = ""
methodLine = "X"
i = 1
while(len(methodLine) != 0):
 methodLine = input(str(i) + ": ")
 i = i+1
 methodDeclaration = methodDeclaration +"\n"+ methodLine
print("Method to be executed:")
print(methodDeclaration)
exec(methodDeclaration)
myMethod()

The output of the above code is as below:

Enter method name: SomeAction
SomeAction has been executed.
Enter your method declaration: 
1: def myMethod():
2: print("MyMethod is running!")
3: 
Method to be executed:
def myMethod():
 print("MyMethod is running!")
MyMethod is running!

Be advised that this is not a secure practice. Using exec() allows the user to input any desired command into your python script. That being said, it is indeed possible to have user-defined methods from input.

Answer 3

def A(arg):
 """function definations """
 return arg 
def B(func):
 """ function defination B with func as argument """
 return func(input())
B(A)

more ref: Python function as a function argument?