How to Fix the 'Generating a PNG with matplotlib when DISPLAY is undefined' Error?

How to Resolve the "Generating a PNG with matplotlib when DISPLAY is undefined" Error

This error arises when attempting to generate a PNG image using matplotlib when the DISPLAY environment variable is not set or is undefined. This typically occurs when running matplotlib in a headless environment such as a server or a batch script.

The primary cause of this error is that matplotlib chooses a backend that requires an X Windows display by default. To resolve this, you must explicitly force matplotlib to use a backend that does not require X Windows.

Solution:

To fix this issue, include the following code before importing any other matplotlib modules:

import matplotlib

# Force matplotlib to use a backend that does not require X Windows.
matplotlib.use('Agg')

This will set the backend to the Agg (Anti-Grain Geometry) backend, which is designed for generating images without the need for an X Windows display.

Alternative Solution:

An alternative solution is to set the backend directly in the .matplotlibrc configuration file. This file is typically located in the ~/.config/matplotlib directory. Add the following line to the file:

backend: Agg

This will permanently set the backend to Agg for all matplotlib instances.

Important

Remember that these solutions must be implemented before you import any other matplotlib modules, such as pyplot. Otherwise, matplotlib will have already chosen its backend, and the use('Agg') directive will have no effect.

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