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

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

On 23-Apr-2015 20:09, Jerzy Karczmarczuk wrote:
> Animation problem of Virgil Stokes.
>
> Since I began the answer sequence, here's a little more. If this overlaps 
> other answers, I apologize.
>
> About the third parameter
>>> "/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.
> I am not trying to accuse you of anything, I simply underline that if the 
> documentation - as cited - mentions the word *generator*, and the program 
> contains the argument *data_gen*, which is OBVIOUSLY a generator which 
> *yields* some data, then this might have give you some clues concerning its 
> meaning.
Ok Jerzy --- I apologize for my reaction and you are correct, I should have 
taken "generator" more seriously.
>
>>> 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.
>
> 1. Generators might be or not function arguments, keyword or not; this is not 
> an issue here. Generators are entities which produce iterators.  Here you 
> have: *def data_gen(): while True: yield np.random.rand(10)* , and this is all.
Yes, again Jerzy.
>
> 2. When you see "frames=None", etc., this doesn't absolutely mean that it is a 
> kw parameter, just a positional parameter with a default value.
> Brendan Barnwell commented this already.
> If you look at the definition of the FuncAnimation class, which is readable, 
> in the file ... matplotlib/animation.py, you will see
> *def __init__(self, fig, func, frames=None, init_func=None, fargs=None, 
> save_count=None, **kwargs):**
> **   ...*
> which explicitly confirms this.
>
> I think that you got the answers you demanded. The FuncAnimation callback 
> *requires* one argument, even if you don't use it. It may be the frame number 
> (or index if you wish), or explicit, specific data, such as in your first 
> example.
> If, as in your second example, the parameter passed is the frame index, and 
> you want to see how you can use it, test this:
>
> *x=np.linspace(0,20,300)***
>
> *   def update(i):*
>
> **
>
> **
>
> *    data=(np.sin(x+i/3)+1)/2*
>
> **
>
> *    line.set_ydata(data)*
>
> **
>
> *
> *
>
> *ani = animation.FuncAnimation(fig, update, interval=40)*
>
>
> You should see a moving sinusoid.
Yes, I have written something like this myself in an attempt to better 
understand Matplotlib animation. IMHO, the documentation for FuncAnimation 
should explicitly state that one argument is required for the animation 
function. The documentation does state:

   "Makes an animation by repeatedly calling a function /func/, passing in 
(optional) arguments in /fargs/."

But of course this is my opinion.
>
>>>
>>> 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".
>
> I don't understand your reaction.  I teach computer science for many years, 
> and several times per week I suggest to my students to do this, to trace their 
> programs. I trace mine. 
I have also taught programming and I often make the same suggestion and of 
course this is a suggestion that I follow myself. And I did this before posting. 
One thing that led me astray was the error message that was generated when I 
removed the argument to the animate function. I even went into the source of 
Matplotlib in an attempt to answer this; but, I still could not resolve this issue.
> Nobody feels offended. You said: data is undefined. But it WAS DEFINED, you 
> agree? So, printing its value was the most natural suggestion. You would 
> immediately see that it had been generated by data_gen.
Yes, I did see this and this was one thing that confused me.
>
> Also, I recommend that you look upon the sources of Matplotlib from time to 
> time. I do it regularly, it is nicely commented, and complements very well the 
> documentation.
I have done this several times; but, I find this package very complex and 
difficult to unravel. I use Matplotlib a lot and it is a fantastic package --- 
great work by the developer team. The fact that I do not always need to know 
what is "under the cover" of Matplotlib, is IMHO a positive thing, and a good 
indicator that the documentation (including the examples and gallery) is rather 
well done.
>
> Jerzy Karczmarczuk
>
> PS. Some blizzard around positional/keyword parameters in Python is quite 
> persistent.  "Dive into Python" of Mark Pilgrim covers this, and offers some 
> useful references.
I recently obtained this book; but, have not "dived" into it yet  It now seems 
clear that I should take the plunge soon :-)

Best regards, and thanks for your follow-up Jerzy,
--V