How to Fix – NameError name ‘plt’ is not defined

If you are working with Python and trying to use the matplotlib library, you may encounter the “NameError: name ‘plt’ is not defined” error. In this tutorial, we will explore why this error occurs and the steps required to fix it such that your Python code can successfully run without errors.

We will cover common causes of the error and provide solutions to help you get your code up and running quickly. So, let’s get started!

Why does the NameError: name 'plt' is not defined error occur?

This error occurs when you try to use the pyplot module from Matplotlib which is generally imported as plt, but Python does not recognize it. This error can be frustrating, but it is usually easy to fix. In this tutorial, we will go through the steps to fix this error. The following can be the possible scenarios that may lead to this error.

1. You have not imported the pyplot module from Matplotlib.
2. You have imported the pyplot module using a different name.

How to fix the NameError: name 'plt' is not defined?

Before we proceed with the above scenario and their solutions, make sure that you have the matplotlib module installed. If matplolib is not installed and you try to import it, you’ll get a ModuleNotFoundError. Make sure that you have installed Matplotlib correctly and that it is up to date. You can check your installation by running the following command in your terminal or command prompt:

pip show matplotlib

This command will show you information about your Matplotlib installation, including the version number.

If you are using a virtual environment or a different Python environment, make sure matplotlib is installed in that environment.

Let’s now look at the above scenarios in detail that could result in the above error.

The plt module is not imported

It can happen that you are trying to use the pyplot module from Matploltib without even importing it. This is because Python does not recognize the a given library and its functions until it is imported into the code.

For example, let’s try to use pyplot module without importing it and see what we get.

import numpy as np
# note that plt is not imported

# Generating synthetic data
x = np.random.rand(50)
y = x + np.random.rand(50)*0.2

# Creating scatter plot
plt.scatter(x, y)
plt.xlabel('X')
plt.ylabel('Y')
plt.title('Scatter plot of X and Y')
plt.show()

Output:

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

Cell In[1], line 9
      6 y = x + np.random.rand(50)*0.2
      8 # Creating scatter plot
----> 9 plt.scatter(x, y)
     10 plt.xlabel('X')
     11 plt.ylabel('Y')

NameError: name 'plt' is not defined

We get a NameError stating that the name plt is not defined. This is because we are trying to use plt which is not imported. Use import matplotlib.pyplot as plt to import the pyplot module with the name plt.

import numpy as np
import matplotlib.pyplot as plt

# Generating synthetic data
x = np.random.rand(50)
y = x + np.random.rand(50)*0.2

# Creating scatter plot
plt.scatter(x, y)
plt.xlabel('X')
plt.ylabel('Y')
plt.title('Scatter plot of X and Y')
plt.show()

Output:

Here, we are importing the pyplot module with the name plt first and then using it. You can see that we did not get any errors here.

The plt module is imported using a different name

In the import statement import matplotlib.pyplot as plt we are importing the pyplot module from matplotlib with the name plt. Now, if you’re using a different name or just importing the pyplot module without the name plt. You’ll also run into the NameError: name 'plt' is not defined if you try to use plt in your code.

Let’s look at an example.

import numpy as np
import matplotlib.pyplot as plot

# Generating synthetic data
x = np.random.rand(50)
y = x + np.random.rand(50)*0.2

# Creating scatter plot
plt.scatter(x, y)
plt.xlabel('X')
plt.ylabel('Y')
plt.title('Scatter plot of X and Y')
plt.show()

Output:

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

Cell In[1], line 9
      6 y = x + np.random.rand(50)*0.2
      8 # Creating scatter plot
----> 9 plt.scatter(x, y)
     10 plt.xlabel('X')
     11 plt.ylabel('Y')

NameError: name 'plt' is not defined

We get a NameError: name 'plt' is not defined. This is because we have imported the pyplot module with the name plot but we’re trying to use it using the name plt.

To fix this error, you can either use the name that you have used in the import statement or import pyplot with the name plt. It is recommended that you use import matplotlib.pyplot as plt as this is the syntax that is generally used and following this convention makes the code easier to read and debug.

Conclusion

In conclusion, the “NameError: name ‘plt’ is not defined” error is a common issue that can occur when using the Matplotlib library in Python. This error typically occurs when the user forgets to import the library or incorrectly imports it. To fix this error, you can simply import the Matplotlib library using the correct syntax and ensure that it is installed in your Python environment. By following the steps outlined in this tutorial, you should now be able to resolve the “NameError: name ‘plt’ is not defined” error and continue working with Matplotlib in your Python projects.