Fix NameError: “Python List Is Undefined” – With Examples

How do you fix the NameError: Python list is undefined type errors?

When you get this error in your Python code there are a few ways to fix the problem and in essence, the easy fix is to check you’re working on a list variable that has been initiated as a list .

The three common causes I have found in my own Python code for why this error pops up is due to: 1. not instantiating a variable as a list; 2. incorrectly typing the name of the list variable; or 3. using a variable in a list that hasn’t been declared.

I’ll demonstrate these in a little more detail with examples below.

Understanding 'Python List is Undefined' Error

Common Causes of this Error

In Python, the List is undefined error usually occurs when you attempt to access or manipulate a list that has not been properly defined or declared.

This can happen for a variety of reasons, such as improper list initialisation, typographical errors, or incorrect use of list elements.

It is essential to understand these common causes in order to troubleshoot and resolve the error effectively.

Examples of Situations Leading to this Error

Let’s look at some scenarios demonstrating the ‘Python list is undefined’ error:

1. You forget to initialise a list before using it in your code.

For example, if you try to append an item to a list that hasn’t been declared yet, you will encounter this error.

>>> my_list.append(1)
NameError: name 'my_list' is not defined

2. A typo or spelling error in the list’s name.

You have correctly defined the variable, but you mistakenly typed the wrong name for the list variable later in your code. Python will raise the same type of error and interpret it as an undefined list and raise the error.

>>> my_list = []
>>> my_lst
NameError: name 'my_lst' is not defined. Did you mean: 'my_list'?

3. Incorrectly referencing list elements. If you’re trying to set list elements using variables that do not yet exist:

>>> a = 1
>>> b = 2
>>> my_list = [a, b, c]
NameError: name 'c' is not defined

By familiarising yourself with these situations, you will be better equipped to identify and resolve the ‘Python list is undefined’ error as you encounter it in your programming journey.

Syntax and Python List Basics

Defining and Creating Lists

In Python, a list is a versatile and widely used data structure that can store multiple elements of any data type, including numbers, strings or even other lists. They are ordered, mutable, and allow duplicate elements.

To create a list, you simply need to wrap comma-separated values within square brackets. Here’s an example:

>>> my_list = [1, 'apple', 3.14, [4, 5]]

In this example, my_list is a Python list containing four elements: an integer, a string, a float, and another list as its elements.

Accessing and Modifying List Elements

Each element in a Python list has a corresponding index, starting at zero for the first element . You can access individual elements using their index.

Here’s how:

>>> my_list = [1, 'apple', 3.14, [4, 5]]
>>> print(my_list[1])
'apple'

In this example, 'apple' is printed because it has an index of 1 (the second element of the list).

To modify an element in the list, you just need to assign a new value to that index. Here’s an example:

An example of modifying an element:

>>> my_list = [1, 'apple', 3.14, [4, 5]]
>>> my_list[1] = 'banana'
>>> print(my_list)
'banana'

After running this code, my_list has become [1, 'banana', 3.14, [4, 5]] since the element at index 1 (previously defined as 'apple' ) has now been replaced with 'banana' .

Troubleshooting the 'Python List is Undefined' Error

Properly Declaring and Initialising Lists

To avoid the 'Python List is Undefined' error, always ensure that you’ve correctly declared and initialised your list before using it .

Start by creating an empty list using either square brackets:

>>> my_list = []
>>> type(my_list)
<class 'list'>

Or you can use the constructor method list() :

>>> my_list = list()
>>> type(my_list)
<class 'list'>

Alternatively, you can initialise your list with some predetermined values:

>>> my_list = [1, 2, 3]
>>> type(my_list)
<class 'list'>

Or you can convert other types into a list using the list() constructor:

>>> my_list = list({'a': 1, 'b': 2})
>>> type(my_list)
<class 'list'>
>>> print(my_list)
['a', 'b']

By taking these steps, you’ll prevent any issues related to undefined lists.

Checking for Spelling Errors

Always double-check your code for spelling errors, as even the smallest mistakes can cause an undefined list error. It’s essential to verify that you’re using the correct list names when referencing or modifying your list elements.

It helps if you can use an IDE (like PyCharm ) or a text editor (like Visual Studio Code ) that can help to highlight your code as your type and display prompts when you enter a variable that hasn’t been defined.

List Is Undefined Error In Python: Summary

If you get the NameError: List is Undefined in Python check to make sure your code has properly declared the variable you are using previously as a list variable.

Check the variable has been declared with either square brackets [] or with the list() constructor. Check also the contents of your list elements have also been declared so that you are not inserting undeclared items as this will also cause NameErrors around your list too.