is alt.renderers.enable('altair_saver', ['vega-lite', 'png']) on README outdated?

[zyxue]

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-53-092c93936f9f> in <module>
----> 1 altair.renderers.enable('altair_saver', ['vega-lite', 'png'])

TypeError: enable() takes from 1 to 2 positional arguments but 3 were given

https://github.com/altair-viz/altair/blob/582b590a29dfbfc06aad27be79371f98a763a0aa/altair/utils/plugin_registry.py#L156

[jakevdp]

Maybe it needs to be passed as a keyword argument?

[zyxue]

what would the keyword be?
tried

>>> altair.renderers.enable('altair_saver', fmts=['vega-lite', 'png'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/lib/python3.8/site-packages/altair/utils/plugin_registry.py", line 177, in enable
    return PluginEnabler(self, name, **options)
  File "/path/to/lib/python3.8/site-packages/altair/utils/plugin_registry.py", line 27, in __init__
    self.registry._enable(name, **options)
  File "/path/to/lib/python3.8/site-packages/altair/utils/plugin_registry.py", line 142, in _enable
    ep = entrypoints.get_single(self.entry_point_group, name)
  File "/path/to/lib/python3.8/site-packages/entrypoints.py", line 217, in get_single
    raise NoSuchEntryPoint(group, name)
entrypoints.NoSuchEntryPoint: No 'altair_saver' entry point found in group 'altair.vegalite.v4.renderer'

[jakevdp]

The NoSuchEntryPoint error suggests that the altair_saver package is not installed in the environment where you're trying to use it.

[zyxue]

I see, thanks. it's unclear what the keyword should be, and how it works. All these seem to run

>>> import altair
>>> altair.renderers.enable('altair_saver', abc=['vega-lite', 'png'])
RendererRegistry.enable('altair_saver')
>>> altair.renderers.enable('altair_saver', xyz=['vega-lite', 'png'])
RendererRegistry.enable('altair_saver')
>>> altair.renderers.enable('altair_saver', fmts=['vega-lite', 'png'])
RendererRegistry.enable('altair_saver')

[jakevdp]

I think fmts is the correct argument. I suspect the others will fail when you try displaying a chart.

[zyxue]

I see. you're right.

Is

altair.renderers.enable('altair_saver', fmts=['svg'])

the same as

altair.renderers.enable('svg`)

?

Both seem to work

[jakevdp]

Yes, they do the same thing. altair_saver provides a superset of the functionality of altair's built-in renderers.

[jakevdp]

(and under the hood, altair.renderers.enable('svg') is using altair_saver)

[zyxue]

I find it a bit confusing. How could I better understand it given that altair_saver is the name of a saver while svg is the name of a format? How is it structured under the hood?

[jakevdp]

In order to display a chart as SVG, it has to first be saved to SVG in memory.