Defining __getattr__ breaks _repr_html_

[mcleonard]

I created a class where I am storing rows of data with column headers. I want to display the contents in an HTML table in notebooks, so I'm using _repr_html_. I'd also like to access columns using the headers as attributes, like foo.column1. However, defining __getattr__ stops _repr_html_ from being called and the notebook falls back to __repr__. This shouldn't happen because _repr_html_ is defined in the class instance, so __getattr__ shouldn't be called. I checked if __getattr__ is being called and it appears that _ipython_canary_method_should_not_exist_ is being called and then it falls back to the normal __repr__.

Here's an example:

[mcleonard]

Ah, nevermind. I needed to raise AttributeError in __getattr__.

[takluyver]

Yep. To clarify, your __getattr__ implementation effectively pretends that the object has any attribute we check for (so hasattr(a, x) works for any x). If we detect that an object is doing that, we don't trust it when it claims to have methods like _repr_html_.

[connerxyz]

Okay but what's the solution?

[connerxyz]

If it checks for permissive attribute handling to mitigate potentially not having a __repr_html__ implementation, why doesn't it check for the __repr_html__ implementation itself?

[zlatko-minev]

So what is the resolution? I have the same issue

[kechan]

So what is the resolution? I have the same issue

In my def getattr(self, attr), I just did this:

if attr == '_ipython_canary_method_should_not_exist_':
  return

to suppress that message. This is a hack I don't know any unintended consequence. For my case, I will post back if i see any weird behavior downstream.

[takluyver]

In general, your __getattr__ should either accept only names in some category, e.g.

if name in self.columns:
    return self.columns[name]
raise AttributeError(name)

Or at least have a broad refusal for e.g. anything that starts with an underscore:

if name.startswith('_'):
    raise AttributeError(name)

Special-casing the 'canary' method defeats the point of it. IPython deliberately doesn't trust objects which claim to have an attribute with any name, because it can cause all sorts of problems if we call special methods which aren't really defined and do something unexpected.