Inline::Python - Testing call method with __builtin__ getattr - fails if called repeatedly on same object / attribute combination

Matplotlib_reduced.pm6

use Inline::Python;

my $py = Inline::Python.new();
$py.run('import matplotlib.pyplot');

class Matplotlib::Plot {
    method cm {
        class {
            # this getattr method, called like so : 'say $plt.cm.getattr($cmap, 'N');'
            method getattr($obj, $name) {
                $py.call('__builtin__', 'getattr', $obj, $name);
            }
            method FALLBACK($name, $idx) {
                $py.run("matplotlib.pyplot.cm.{$name}($idx)", :eval);
            }
        }.new();
    }
    method FALLBACK($name, |c) {
        $py.call('matplotlib.pyplot', $name, |c)
    }
}

class Matplotlib {
    method FALLBACK($name, |c) {
        $py.call('matplotlib', $name, |c)
    }
}

perl6_matplotlib_getattr_test.p6

use v6;
use lib '.';
use Matplotlib_reduced;

my $plt = Matplotlib::Plot.new;

my $cmap = $plt.get_cmap('seismic', 5);

# -- this works

#say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 
#say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 

# -- this fails on the 3rd call

#say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 
#say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 
#say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 

# -- this works

#say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 
#say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 

# -- this fails on the 3rd call

#say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 
#say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 
#say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 

# -- this works

#say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 
#say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 

# -- this fails on the 3rd call

say 'Colormap Name    : ' ~ $plt.cm.getattr($cmap, 'name'); 
say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 
say 'Number of Colors : ' ~ $plt.cm.getattr($cmap, 'N'); 

# -- etc....

Upon further experimentation, it seems like problems happen as soon as you start doing calls to methods on __builtin__. If I avoid those methods, I don't seem to hit the same problem.

For example, in my FALLBACK in the earlier comment, if I don't check __builtin__.hasattr then it works, but of course, Python will throw an exception if the attribute doesn't exists. You can put a try in front of the call to __getattribute__ and it will return an undefined value (instead of throwing) if the attribute doesn't exists.

I'm not yet sure how to put all these ideas together in a cohesive whole, but play around with them and see what you come up with.

btw python's getattr supports a third argument which is a default value in case the attribute doesn't exist.

If you write a wrapper, maybe the simplest thing to do is to create a sentinel value Any.new as the default, and if that is returned, interpret it as not found.

More thanks 0racle, and Moritz, I definitely got way more out of this than I put in :-) I hope this thread has somehow kickstarted a future new article on your Perl6/Matplotlib blog 0racle. I agree about not bugging the Inline::Python maintainer over this when there is a work around.

It all seems so straightforward when it is explained, like you have both done in your comments, I'm not sure after a month of researching I'd have come to the same understanding :-) It's given me a whole ton of concepts and reasoning to investigate. I'll carry on with my efforts on this, and try to add some results to this gist when I have them. Thanks again.