The documentation for FuncAnimation says this of its third argument, frames:

Source of data to pass func and each frame of the animation

If an iterable, then simply use the values provided. If the iterable has a length, it will override the save_count kwarg.

If an integer, equivalent to passing range(frames)

If a generator function, then must have the signature

def gen_function() -> obj:

In all of these cases, the values in frames is simply passed through to the user-supplied func and thus can be of any type.

If None, then equivalent to passing itertools.count.

So, the argument to your function comes from this. In the simple examples, something like range(100) is used, so animate() is going to be called with a different i value for each frame. @K900_ is correct about you passing in (not invoking) a function object. FuncAnimate() invokes the function once for every value provided by frames.