How to ensure that a python function generates its output based only on its input?

Question

To generate an output a function usually uses only values of its arguments. However, there are also cases in which function, to generate its output, reads something from a file system or from a database or from the web. I would like to have a simple and reliable way to ensure that something like that does not happen.

One way that I see is to create a white-list of python libraries that can be used to read from file system, database or web. But if it is the way to go, where can I get this (potentially huge) list. Moreover, I do not want to disable the whole library just because it can be used to read from the file system. For example I want users to be able to use pandas library (to store and manipulate tabular data). I just do not want them to be able to use this library to read data from the file system.

Is there a solution for this problem?

Answer 1

The answer to this is no. What you are looking for is a function that tests for functional purity. But, as demonstrated in this code, there's no way to guarantee that no side effects are actually being called.

class Foo(object):
    def __init__(self, x):
        self.x = x
    def __add__(self, y):
        print("HAHAHA evil side effects here...")
        # proceed to read a file and do stuff
        return self

# this looks pure...
def f(x): return x + 1

# but really...
>>> f(Foo(1))
HAHAHA evil side effects here...

Because of the comprehensive way objects can redefine their behavior (field access, calling, operator overloading etc.), you can always pass an input that makes a pure function impure. Therefore the only pure functions are those that literally do nothing with their arguments... a class of functions that is generally less useful.

Of course, if you can specify other restrictions, this becomes easier.