Re: [Matplotlib-users] animation.FuncAnimation example --- how does it work?

[Benjamin R. <ben...@ou...>]

The documentation should say "the number", not "a number". This particular
argument expects either a generator, an iterable, or an integer. If none is
given, it will try and figure out something for itself. The integer is used
to create a number generator, and so you effectively have an iterable that
supplies the first argument to the animation function.

Often times, the first argument to this function is just a frame index. It
is quite possible that you don't need it at all, but it is a part of the
defined API that the FuncAnimation assumes to be able to call the
user-supplied function.

Does that help?

Ben Root

<shameless_plug>
P.S. - My new book "Interactive Applications using Matplotlib" has an
entire chapter devoted to animations
</shameless_plug>


On Thu, Apr 23, 2015 at 12:05 PM, Virgil Stokes <vs...@it...> wrote:

>  Thanks for your reply to my post, Jerzy.
>
> On 23-Apr-2015 13:18, Jerzy Karczmarczuk wrote:
>
>
>
> Le 23/04/2015 12:22, Virgil Stokes a écrit :
>
> The following interesting example (random_data.py) is posted at:
>
> http://matplotlib.org/1.4.2/examples/animation/random_data.html
>
>
> import matplotlib.pyplot as plt
> import matplotlib.animation as animation
>
> import numpy as np
>
> Yes, I forgot to include this
>
>
> fig, ax = plt.subplots()
> line, = ax.plot(np.random.rand(10))
> ax.set_ylim(0, 1)
>
> def update(data):
>     line.set_ydata(data)
>     return line,
>
> def data_gen():
>     while True: yield np.random.rand(10)
>
> ani = animation.FuncAnimation(fig, update, data_gen, interval=100)
> plt.show()
>
> This codes works; but, I am very confused by it. For example:
>
> 1. There are 3 positional arguments given for animation.FuncAnimation;
> but, in the
> API documentation for this class (
> http://matplotlib.org/api/animation_api.html), only
> two positional arguments are shown.
>
> The third one is the third one,
> "*frames* can be a generator, an iterable, or a number of frames."
>
> This makes very little sense to me --- what does "or a number of frames"
> mean?
>
> The name "data_gen" could suggest its meaning (after having read the doc).
>
> I am not sure what you are referencing as "the doc"; but I did read the
> documentation several times and English is my native language.
>
> Note please that the keyword parameters are specified extra.
>
> I am aware of this.  Perhaps, I am a Python dummy --- when I see something
> like value = None in a Python API argument, I interpret this as a keyword
> argument and not a generator.
>
>
>  2. data, the argument to the update function seems to be undefined.
>
> FuncAnimation usually passes the frame number: 0, 1, 2, ... as the first
> parameter of the update function, when "frames" is None, or the number of
> frames. If - as here - the third parameter is a generator, it passes the
> yielded data to update.
>  It may be used or not.
>
> Ok, I understand that better now. But, you say "or *the number* of
> frames" but the documentation reads "or *a number* of frames" --- what
> does this mean?
>
> And I still do not understand how to use the first argument of the
> function to be called for the animation. In another animation example (
> histogram.py), the animation function is defined by:
>
> def animate(i):
>     # simulate new data coming in
>     data = np.random.randn(1000
>     n, bins = np.histogram(data, 100)
>     top = bottom + n
>     verts[1::5,1] = top
>     verts[2::5,1] = top
>
> This works of course; but, why is the "i" required? There is no other
> reference to it in the entire script. If I remove it; i.e. use def
> animate(): I get the following error:
>
> TypeError: animate() takes no arguments (1 given)
>
> I do not understand how this explains the fact that the function no longer
> has any arguments. Please explain the meaning of this error message?
>
>
> Please, in such cases test your programs by adding some simple tracing
> contraptions, say, print(data) inside update.
>
> I did this and more before posting my email. I would not have posted this
> unless I thought it was necessary. And I thought one purpose of this user
> group was to help people --- even if they ask a question which may be
> annoying or "stupid" in some subscribers opinion. I try to remember what a
> very wise teacher once said --- "there is no such thing as a stupid
> question".
>
>
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