Behavior of plt.show() When Using the Agg Backend

Goal

Determine whether invoking plt.show() after selecting the Agg backend blocks script execution or returns immediately, and what consequences this has for automated environments that rely on headless rendering.

Constraints and Uncertainty

The Agg backend is documented as providing a pure‑CPU raster buffer without a GUI event loop, which means interactive features are unavailable. However, the exact behavior of the show() function—whether it enters a blocking loop, returns instantly, or triggers any hidden side‑effects—is not explicitly defined in the current documentation, leaving room for interpretation when integrating matplotlib into CI pipelines or containerized workloads.

• Does plt.show() block execution when the Agg backend is active, or does it return immediately?
• Are there any hidden side‑effects (e.g., resource allocation, pending events) when show() is called under Agg that could affect long‑running processes?
• If a GUI‑specific method such as fig.canvas.start_event_loop() is invoked after the Agg backend has been set, what is the guaranteed outcome (exception, no‑op, etc.)?