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?

Welcome! I think you would benefit from taking a look at this article: https://thepythonguru.com/python-builtin-functions/eval/ — bracco23, Apr 08 '19 at 12:49
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 — Chris_Rands, Apr 08 '19 at 13:00

score 2 · Answer 1 · answered Apr 08 '19 at 12:55

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!'

armitus · Answer 2 · 2019-04-08T14:06:35.660

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.

score 0 · Answer 3 · answered Apr 08 '19 at 13:36

0

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?

answered Apr 08 '19 at 13:36

Rahul chandra

13
2

Python code for allowing a user to input a function

3 Answers3