Undefined Variable Nameerror In Python

Encountering the “NameError: name ‘var’ is not defined” is a common experience for Python users. In Python, variable declaration is a straightforward process of assigning a value. This error arises when attempting to access a non-existent variable or one not defined in the program. Surprisingly, it can occur even when the variable is present, owing to various reasons. In this discussion, we explore the nuances of the NameError and strategies to resolve this issue in Python programming.

Why Does the Python NameError: name ‘var’ is not defined Arise?

Various reasons might lead to the Python NameError: name ‘var’ is not defined. Let us have a look at these common reasons:

• When an undefined variable is used
• When there is an spelling mistake in the variable or function name
• When the variable is accessed first but defined later
• When the variable is used out of its defined scope

When Undefined Variables are Used

As already mentioned, the NameError: name ‘var’ is not defined signifies that the variable we are trying to access is not defined. In other words, this error occurs when the Python interpreter fails to find a variable that we are trying to access. Here is a simple example of the same that leads to the NameError: name ‘var’ is not defined. In the below example, we get the NameError: name ‘var’ is not defined simply because we are trying to access the variable ‘b’ when it is not even defined in the program:

Python3

a = 'GeeksforGeeks!'
print(a)

#note that we have not declared the variable b
print(b)

Output:

Traceback (most recent call last):
  File "Solution.py", line 5, in <module>
    print(b)
NameError: name 'b' is not defined

When there is a Spelling Mistake

Now look at the code given below. Here, we first define the variable, ‘name_1’ and then access it using the print statement. But still, we run into the NameError: name ‘var’ is not defined. Python, being a strictly typed language, treats ‘name_1’ and ‘name_I’ as two unique variables. Thus, due to this slight mistype, the Python interpreter tries to access the variable ‘name_I’, which naturally it is unable to find. And hence, we get the NameError: name ‘name_I’ is not defined.

Python3

#define the variable
name_1 = 'GeeksforGeeks!'

#access the variable
print(name_I)

Output:

Traceback (most recent call last):
  File "Solution.py", line 2, in <module>
    print(name_I)
NameError: name 'name_I' is not defined

A similar error will arise even if we misspell any in-built function like print() or input().

Python3

x = int(inpet("What is your age?"))

Output:

Traceback (most recent call last):
  File "Solution.py", line 1, in <module>
    x = int(inpet("What is your age?"))
NameError: name 'inpet' is not defined

When the Variable is Accessed First and Defined Later

In this code, the variable is declared and accessed without any error in the spelling, yet, we get the NameError: name ‘var’ is not defined. This happens because Python interpreter executes the code line by line. Meaning, when the print statement in the above code runs, the presence of the variable ‘a’ is not known by the interpreter and thus, it throws the NameError:

Python3

print(a)
a = 'GeeksforGeeks'

Output:

Traceback (most recent call last):
  File "Solution.py", line 1, in <module>
    print(a)
NameError: name 'a' is not defined

When a Variable is used out of its defined Scope

In the code given below, we have defined a variable inside the function demo(). But, we use the print statement outside this function to access the variable ‘a’. On running this code, you will find that the code throws the NameError: name ‘var’ is not defined. This happens because the variable ‘a’ is defined inside the demo() function and hence, its scope is limited to that function only:

Python3

def demo():
  a = 'GeeksforGeeks'

demo()
print(a)

Output:

Traceback (most recent call last):
  File "Solution.py", line 5, in <module>
    print(a)
NameError: name 'a' is not defined

How to Solve an Undefined Variable NameError in Python?

To solve the NameError: name ‘var’ is not defined, you should take care of the pointers mentioned above. The previous examples can be updated to get rid of the error as follows:

Define the Variable before using it

Make sure to define a variable before you use it:

Python3

a = 'GeeksforGeeks!'
print(a)

b = 'GFG'
print(b)

Output

GeeksforGeeks!
GFG

Avoid Spelling Mistakes in Variable Names and Function Names

Make sure you do not misspell any variable name:

Python3

#define the variable
name_1 = 'GeeksforGeeks!'

#access the variable
print(name_1)

Output

GeeksforGeeks!

Make sure you do not misspell any in-built function or user-defined function:

Python3

x = int(input("What is your age?"))
print(x)

Output:

What is your age?20
20

Access a Variable only when its first defined

Make sure you first create the variable and then use it and not the other way around:

Python3

a = 'GeeksforGeeks'
print(a)

Output

GeeksforGeeks

Access the Variable in its defined scope

Make sure you do not access a variable out of its defined scope.

Python3

a = 'GFG'
def demo():
  b = 'GeeksforGeeks'
  print(b)

demo()
print(a)

Output

GeeksforGeeks
GFG

Apart from taking care of all these pointers, you can also setup the try-else block to resolve this error in a more efficient manner. Read all about it here – Handling NameError Exception in Python.

Conclusion

In this post, we looked at the different reasons that might lead to an undefined variable nameerror in Python. Although in most cases this error is thrown because of a wrong spelling, the use of scope is also often overlooked, which naturally leads to the nameerror. Although avoiding this error is the best that you can do, especially now that you are aware of the scenarios that lead to it, exploring the use of exception handling is also a great option to save you from this error for once and for all.