Parameter validation, Best practices in Python

Question

Lets take an example of a API

def get_abs_directory(self, path):
    if os.path.isdir(path):
       return path
    else:
       return os.path.split(os.path.abspath(path))[0]

My question is what is the pythonic way of validating parameters, should I ignore any type of validations (I observed that all the python code does no validation at all)

1. Should I check for "path" to be empty and not null
2. Should I check the "type" of path to be string always
3. In general should I check for type of parameters ? (I guess not as python in dynamically typed)

This question is not specific to File IO, instead FileIO is used only as an example

Answer 1

As mentioned by the documentation here, Python follows an EAFP approach. This means that we usually use more try and catch blocks instead of trying to validate parameters. Let me demonstrate:

import os


def get_abs_directory(path):
    try:
        if os.path.isdir(path):
            return path
        else:
            return os.path.split(os.path.abspath(path))[0]
    except TypeError:
        print "You inserted the wrong type!"


if __name__ == '__main__':
    get_abs_directory(1)  # Using an int instead of a string, which is caught by TypeError

You could however, wish to code in a LBYL (Look Before You Leap) style and this would look something like this:

import os


def get_abs_directory(path):

    if not isinstance(path, str):
        print "You gave us the wrong type, you big meany!"
        return None

    if os.path.isdir(path):
        return path
    else:
        return os.path.split(os.path.abspath(path))[0]

if __name__ == '__main__':
    get_abs_directory(1)