Circle-Generator | generate circles for block building games like Minecraft | Video Game library

This is an issue with how the inline backend handles closing figures and what plt.savefig does internally. The way static figures are displayed in notebooks (i.e. when not using ipympl) is that at the end of cell execution (or in this case at the end of the callback from the slider) the current figure is closed and displayed.

However plt.savefig expects there to be a currently open figure as it calls plt.gcf (get current figure) internally which either grabs the most recently active figure or creates a new empty figure if no figures are active.

So when you did this not in functions the figure wasn't closed until the cell was finished executing and so plt.savefig was able to find the figure. However when you moved to functions it was no longer able to find the current figure.

There are two basic solutions to this.

You can lift figure to the global scope and use fig.savefig - this makes sure that both the plot updating method and the saving method are refering to the same fig.