Conditional code execution can generate unexpected errors

Classification

Affected Components : builtin
Operating System : Linux
Python Versions : 2.6.x, 2.7.x
Reproducible : Yes

Source code

import sys

def test(first_arg, second_arg):
    return (first_arg + second_arg)

def main():
    if len(sys.argv) > 3:
        return test(1, test)
    else:
        return test(1, 2)

sys.exit(main())

Steps to Produce/Reproduce

To reproduce the problem copy the source code in a file and execute the script using the following command syntax:

$ python -OOBRtt test.py

Alternatively you can open python in interactive mode:

$ python -OOBRtt <press enter>

Then copy the lines of code into the interpreter.

And to generate the error issue copy the following command in a terminal and press `Enter```:

python -W error -OOBRtt test.py 0 1 2

Description

No error if the script is executed with no arguments:

python -W error -OOBRtt test.py

No error if the script is executed with one argument:

python -W error -OOBRtt test.py 0

No error if the script is executed with two arguments:

python -W error -OOBRtt test.py 0 1

But if the code is executed by passing three arguments we have an error:

python -W error -OOBRtt test.py 0 1 2
Traceback (most recent call last):
  File "test.py", line 12, in <module>
    sys.exit(main())
  File "test.py", line 8, in main
    return test(1, test)
  File "test.py", line 4, in test
    return first_arg + second_arg
TypeError: unsupported operand type(s) for +: 'int' and 'function'

In python variables are not statically typed therefore is possible to have a valid part of code pointing to another section of the code only under certain conditions, thus escaping normal testing procedures.

A possible solution would be to implement a module to check object type, length and reference, and to raise an exception (TypeError or Value Error) avoiding intermediate operations.