Dynamic function creation at run time with Python's exec built-in

By Vasudev Ram

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Hi, readers,
I was browsing Paul Graham's essay about his book "On Lisp". It's about advanced uses of Lisp and some of the features that make it unique.

In it, he talks about many Lisp features, like the ability to treat code as data, the fact that functions are first-class objects, so, just like other objects such as scalar variables, lists, maps, etc., they too can be passed as arguments to functions, returned from functions, stored in variables, etc., and the fact that Lisp programs can write Lisp programs. (There's a lot more to the book, of course.) This made me think of experimenting a bit with Python's dynamic features, whether similar to Lisp or not. One man's Lisp is another man's Python ...

I did know from a while ago that Python has many dynamic features, and have blogged about some of them in the past. (Search my blog for topics like Python tricks, the trace module and chained decorators, Python introspection, the inspect module, finding the caller of a Python function, Python generators are pluggable, function-based callbacks, class-based callbacks, and other fun stuff. Try using 'site:jugad2.blogspot.com' and suchlike search engine techniques.) This is one more such exploration.

I got the idea of writing a program that can dynamically create a function at run time, based on some user input, and then run that function. The program uses the exec built-in of Python.

Here is the program, dyn_func_with_exec.py:

# dyn_func_with_exec.py 
# Purpose: To dynamically create (and run) a function at run time. 
# Author: Vasudev Ram
# Copyright 2018 Vasudev Ram
# Web site: https://vasudevram.github.io
# Product store: https://gumroad.com/vasudevram

from __future__ import print_function
import sys

major = sys.version_info.major
if major == 2:
    input_func = raw_input
elif major == 3:
    input_func = input
else:
    print("Unsupported version, go back to the future.")
    sys.exit(0)

func_name = input_func("Enter name of function to create dynamically: ")
func_def = """
def {}(args):
    print("In function", func_name, ": args:", args)
""".format(func_name)

exec(func_def)
print(func_name, ":", eval(func_name))

args1 = (1, "a"); args2 = (2, "b")

# Two ways of calling the dynamically created function:
print("\nCalling {} via eval():".format(func_name))
eval(func_name + "(args1)")
print("\nCalling {} via globals() dict:".format(func_name))
globals()[func_name](args2)

Here is the output of 3 runs:
(py is the Python launcher for Windows.)

$ py -2 dyn_func_with_exec.py
Enter name of function to create dynamically: mu
mu : <function mu at 0x0000000002340518>

Calling mu via eval():
In function mu : args: (1, 'a')

Calling mu via globals() dict:
In function mu : args: (2, 'b')

$ py -3 dyn_func_with_exec.py
Enter name of function to create dynamically: _
_ : <function _ at 0x000000000039C1E0>

Calling _ via eval():
In function _ : args: (1, 'a')

Calling _ via globals() dict:
In function _ : args: (2, 'b')

$ py dyn_func_with_exec.py
Enter name of function to create dynamically: |
Traceback (most recent call last):
  File "dyn_func_with_exec.py", line 28, in <module>
    exec(func_def)
  File "<string> line 2
    def |(args):
        ^
SyntaxError: invalid syntax

So it works in both Python 2 and 3. The last run (with an invalid function name) shows how the user input gets interpolated into the function definition string func_def.

The if statement used shows another dynamic aspect of Python: the ability to call different functions (via the same name, input_func), based on some condition which is only known at run time (the Python version, in this case).

Why mu for the input? Well, if foo, why not mu? :)
See my HN comment here

Another way to dynamically decide which function to run:

Driving Python function execution from a text file

Enjoy.

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- Vasudev Ram - Online Python training and consulting

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