Python Cookbook

Testing if a Variable Is Defined

Credit: Hamish Lawson

Problem

You want to take different courses of action based on whether a variable is defined.

Solution

In Python, all variables are expected to be defined before use. The None object is a value you often assign to signify that you have no real value for a variable, as in:

try: x
except NameError: x = None

Then it’s easy to test whether a variable is bound to None:

if x is None:
    some_fallback_operation(  )
else:
    some_operation(x)

Discussion

Python doesn’t have a specific function to test whether a variable is defined, since all variables are expected to have been defined before use, even if initially assigned the None object. Attempting to access a variable that hasn’t previously been defined raises a NameError exception (which you can handle with a try/except statement, as you can for any other Python exception).

It is considered unusual in Python not to know whether a variable has already been defined. But if you are nevertheless in this situation, you can make sure that a given variable is in fact defined (as None, if nothing else) by attempting to access it inside a try clause and assigning it the None object if the access raises a NameError exception. Note that None is really nothing magical, just a built-in object used by convention (and returned by functions that exit without returning anything specific). You can use any other value suitable for your purposes to initialize undefined variables; for a powerful and interesting example, ...