How to Check if Variable is Defined in Python

Checking if a variable is defined in Python is easy. To check if a variable is defined, the easiest way is with exception handling.

try:
    print(variable)
except NameError:
    print("variable isn't defined")

#Output:
variable isn't defined.

You can use the Python globals() functions to check if a variable is defined globally.

variable = "this is a variable"

if 'variable' in globals():
   print("variable is defined in global variables!")

#Output:
variable is defined in global variables!

Finally, if you want to check if a variable is defined locally, you can use the Python locals() function.

def someFunction():
    variable = "this is a variable"
    if 'variable' in locals():
        print("variable is defined in local variables!")

someFunction()

#Output:
variable is defined in local variables!

When working in Python, one of the worst experiences is when you run a piece of a code and you receive an error because a variable isn’t defined.

Luckily, we can check if a variable is defined in Python easily so we don’t have errors in our code.

Checking to see if a variable is defined can be done in Python in a few different ways, but the easiest way is with exception handling.

In a try block, we can try and use the variable. If the variable isn’t defined, then a NameError will be raised and we can handle the error in the except block.

Below is how to check if a variable is defined in Python with exception handling.

try:
    print(variable)
except NameError:
    print("variable isn't defined")

#Output:
variable isn't defined.

If instead, you are trying to check if an attribute is defined in an object, you can check if an attribute is defined in an object with the hasAttr() function.

Using Python to Check if Variable is Defined as Global Variable

In many programming languages, the concept of scope is very important and affects how the flow of data occurs in a program. Depending on the language, there will be both global and local variables available in a program.

In Python, we can check if a variable is defined as a global variable with the help of the globals() function. The globals() function returns a dictionary of all global variables.

print(globals())

#Output:
{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>}

To check for if a variable is defined as a global variable, we can check if the variable is in globals.

Below is an example of how to check if a variable is defined as a global variable in Python.

variable = "this is a variable"

if 'variable' in globals():
   print("variable is defined in global variables!")
else:
   print("variable isn't defined in global variables!")

if 'other_variable' in globals():
   print("variable is defined in global variables!")
else:
   print("other variable isn't defined in global variables!")

#Output:
variable is defined in global variables!
other variable isn't defined in global variables!

Using Python to Check if Variable is Defined as Local Variabley

We can also check if a variable is defined as a local variable with the help of the locals() function. The locals() function returns a dictionary of all local variables.

The locals() function can be useful if we are in a function or object and want to check if a variable is defined only in that object or function.

To check if a variable is defined as a local variable, we can check if the variable is in locals().

Below is an example of how to check if a variable is defined as a local variable in Python.

def someFunction():
    variable = "this is a variable"
    if 'variable' in locals():
        print("variable is defined in local variables!")
    else:
        print("variable isn't defined in local variables!")
    if 'other_variable' in locals():
        print("other variable is defined in local variables!")
    else:
        print("other variable isn't defined in local variables!")

someFunction()

#Output:
variable is defined in local variables!
other variable isn't defined in local variables!

Hopefully this article has been useful for you to learn how to check if a variable is defined in Python.