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

[Virgil S. <vs...@it...>]

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".