Can you dynamically create a python function, based on user input?

Question

Is it possible to create functions dynamically at runtime, based on user input?

For example:

I have a function 'add_i(x)', which adds i to x. Now, if a user calls 'add_2(x)' I want to create the function, that adds 2 to x. Is it possible to create this function at runtime, without the user noticing it?

The add_i(x) function is used as an argument for another function:

    def K(*f:Callable):
      g,*h = f
      def k(*x):
        h_res = [i(*x) for i in h]
        return g(*h_res)
      return k

So K takes some functions as parameters and executes them on some input x. This input should be the same for every possible function, so using the user input as parameter would crash every other function.

look for the Eval function

Roland Kwee
– Roland Kwee

2022-11-25 12:58:04 +00:00
Commented Nov 25, 2022 at 12:58 — Roland Kwee
– Roland Kwee, Commented Nov 25, 2022 at 12:58

BSimjoo · Accepted Answer · 2022-11-25 13:39:05Z

1

Yes, you can define functions at runtime, you can use eval but it is insecure and if you just want to add some function with specific job like adding i to x you can do something better! look at this code:

def define(i):
   def add_i(x):
      return i+x
   return add_i

When you call define(2) it will define a function that returns 2+x. But what can we do for variables? I don't know want do you mean of using x but if x is a variable that user could use from input I could recommend you to use a dictionary to save variable names and values, Then we use regex to get i and variable name:

import re

def define(i):
   def add_i(x):
      return i+x
   return add_i

variables = {'x':4}    # for example now we add x to variables with value 4
# all commands that user could call are saved in `commands` dictionary
# you can add commands for user for example we add print function that prints variable value
commands = {
  # <command_name>:<function>
  'print':lambda x:x  # script will print returned value
}

while (True):
  cmd = input('> ')
  cmd_name, var_name = re.match(r'(\w[\w\d_]*)\((\w[\w\d]*)\)',cmd).groups()
  # optional: you can create variable if it is not in dictionary
  if not var_name in variables:
    variables[var_name]=0
  if not cmd_name in commands:
    _,i = cmd_name.split('_')
    commands[cmd_name] = define(int(i))
  print(commands[cmd_name](variables[var_name]))  # call function and print output

> add_5(x)
9
> print(x)
9

answered Nov 25, 2022 at 13:39

BSimjoo

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3 Comments

hannah Over a year ago

Hi @BSimjoo :) That's almost what I needed! There is only one problem left: When someone imports my module for another python-project and tries to invoke the add_i - Funktion. How can I deal with that?

hannah Over a year ago

Is there something like a placeholder for function names?

BSimjoo Over a year ago

if you are using a class e.g my_class then if you use self.__setattr__(cmd_name,define(...)) you will add attribute to a instance on runtime, then calling my_class.add_5(5) will call defined function and returns 10. You can use if getattr(my_call,'add_5'): to check attribute (or function) exists or not.

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