Python NameError

Introduction to Python NameError

The following article provides an outline for Python NameError. NameError is a kind of error in python that occurs when executing a function, variable, a library or a string without quotes that have been typed in the code without any previous Declaration. In other words when the global or a local name cannot be identified by the interpreter upon execution throws a NameError. It can be viewed in the last line of the error message to understand the NameError where the function, variable, package or string that has not been declared will be shown in the message saying the respective function, package, variable “is not defined”.

Syntax of Python NameError

When writing a function with a name we often miss to call the exact function name in the future which will lead to a NameError.

Example:

Code:

## Functions which return values
def calc_sum(x,y):
op = x + y
return(op)
ss = calc_sum(5,10)
print(ss)

Output:

For the same function lets see the NameError.

Code:

## Functions which return values
def calc_sum(x,y):
op = x + y
return(op)
ss = calc_su(5,10)
print(ss)

Output:

It was originally written to perform some operation between two numbers and named as calc_sum but upon calling it we used the name calc_su so it is not defined previously and it will throw us a NameError.

How NameError Works?

When working with user-defined variables in coding the NameError often occurs due to the difficulty in identifying the local/global values for the interpreter.

Example:

Code:

l1 = [1,2,5,8,9] l2 = [1,5.6,"hello"] # mix of data types
l1[1] = 100 # muting the 1st position of l1 to 100
print(l1)
del l1[2] # delete from a position 2
print(l1)

Output:

Code:

l1 = [1,2,5,8,9] l2 = [1,5.6,"hello"] # mix of data types
l1[1] = 100 # muting the 1st position of l1 to 100
print(l1)
del l[2] # delete from a position 2

Here we are performing a delete operation on a variable l that is not defined.

Only l1 & l2 have been defined and the interpreter can only identify l1 & l2 variables. So if we operate on l which is not defined it will throw us a NameError.

Output:

There are cases of NameError that also occurs during operations done with python library or a package. Where if we miss to import the package or library or if we haven’t defined the name of that package or library and upon executing an operation using that library, we will get a NameError

Example:

Code:

l10 = [1,2,6,7,4,5,7,8,6,3] l11 = [1,2,6,7,4,5,7,8,6,3,1000] ## numpy operations
np.mean(l11)
np.median(l11)
np.std(l11)

Output:

We haven’t imported the numpy library as np and upon executing the operation we will the NameError.

Code:

import numpy as np
l10 = [1,2,6,7,4,5,7,8,6,3] l11 = [1,2,6,7,4,5,7,8,6,3,1000] ## numpy operations
print(np.mean(l11))
print(np.median(l11))
print(np.std(l11))

Output:

There is another kind of NameError that occurs when we fail to insert a string value inside the quotes and the interpreter identifies it as a variable and throws a NameError.

Example:

Code:

print('Its a Beautiful day')
name=('Bala')
print(name,'Its a Beautiful day')

Output:

Code:

Error,
print('Its a Beautiful day')
name=(Bala)
print(name,'Its a Beautiful day')

Here we fail to put the string inside the quotes so the console will throw us the NameError.

Output:

Avoiding NameErrors in Python

The NameError can be avoided easily by using the Python Error handling technique or exception handling which denotes the user about the NameError that is occurring in the block of code without actually throwing an error.

The most common errors that occur in python is the SyntaxError & NameError. When we write our code and if it is not accepted by the programming language, then arises the SyntaxError. SyntaxError can be corrected by following the guidelines of the programming language in a way the interpreter could understand. The NameError can be avoided by using a technique called Exception Handling.

Code:

defmy_func():
x="Name Error Exception"
print(y)
my_func()

Output:

Even if we write code without any SyntaxError, the program can result in runtime errors. These are called Exceptions. There are numerous built-in exceptions available in Python and One such exception is NameError Exception.

In Python, the NameError exception comes into picture when we try to use or refer a name that is not defined locally or globally.

Example:

For NameError Exception handling.

Code:

try:
x="Name Error Exception"
print(y)
except NameError:
print("Name Error Exception is Caught")

Output:

The try block allows us to check and see that our code will throw an error or not. We can use this block to trail our code to see if there may be potential for errors to occur.

The Except Block will allow us to write the part where we want to handle the error. We can use this block to denote the user on want went wrong without the console throwing errors.

We can also use a Finally block along with try and except to continuously run the code without termination by indicating only the exception message.

Example:

Finally block.

Code:

name='Smith'
try:
print("Hello" " "+ name)
except NameError:
print("Name is not denoted")
finally:
print("Have a nice day")

Output:

Since the name is denoted the code has successfully run without throwing the exception.

Below we have deleted the name denoted we can see the exception message thrown.

Code:

name='Smith'
try:
print("Hello" " "+ name)
except NameError:
print("Name is not denoted")
finally:
print("Have a nice day")
del name
try:
print("Hello " + name)
except NameError:
print("Name is not denoted")

Output:

The finally block allows us to run the code without termination since the name denoted is deleted. The finally blocks will get executed even if the try block raises an exception message. We can use this technique to overcome the NameError.

Conclusion

The main take away to remember in python NameError is the failure of the interpreter to identify the name or text we have used in our code. We have seen in detail about the NameError that occurs in Python programming language and the techniques to overcome the NameError.

This is a guide to Python NameError. Here we discuss how NameError works and avoiding NameErrors in python respectively. You may also have a look at the following articles to learn more –

1. Python Input String
2. Python String Operations
3. Python Sort List
4. Python Constants