Custom column formatters for HTML in IPython, e.g. can show np 2D-array as image
I've created a [gist](https://gist.github.com/d1manson/f2589d4e4b44ff3ba85b) to explain in more detail, and with a screenshot.

this would need to be implemented like:

`DataFrame.to_html(......, formatters=a_callable_or_dict_of_callables)`

so if a formatter exists for a particular column, or genreally then it would get called to generate the html.

this would be then fairly generic.
 Yes, that makes sense, but you also need some way to attach the `callable_or_dict_of_callables` to the `DataFrame` "permanently" so that `DataFrame._repr_html` can pass it to `DataFrame.to_html` when it is called without any arguments.  Note that these custom formatters may/probably will be specific to the output type (i.e. html in this case).
Is there a recommended way to do this such that it will automatically be copied across to views/copies of the `DataFrame`?
 you cannot copy meta-data like this. It is in theory possible, but this introduces all kinds of issues. Much better to simply pass it to `.to_html()` no?
 this is related to #6488 and #3195
 If it's a lot more complicated to do it as meta-data then I'll settle for what you are suggesting. 

In fact that would probably be ok for my usage case, as I'm using a DataFrame as the backend of a custom DataSet class, i.e. I can explicitly call  `.to_html` from inside my `DataSet._repr_html`, and pass it whatever meta data I need to.

Since you seem to have a pretty good idea of what you'd like to see here, and it doesn't seem that far off what is already suggested, can I ask you to finish it off?   

p.s. This is my first ever pull request to an open-source project, so I don't know what the etiquette is.
 @d1manson well, would love to have you contribute. why don't you change around a bit to use it in calling `.to_html()` and see how it goes...
 is that any better?
 looks closer. What would help this is a few tests for this kind of behavior. E.g. you assert that the generated html is correct. see `test_formats.py` (you don't have to exactly put the generated html, just that certain elements that you are testing are in there).
 Any progress on this? I think this is a VERY useful feature, that will make IPython notebook a lot more powerful as a data presentation tool - I see a LOT of use cases:
- row identification
  - e.g. if you have a data frame with rows corresponding to countries, could repr a flag column
- data summarization
  - add a e.g. spark chart to a row easily
- repr objects in dataframe
  - e.g. if have e.g. a column of chemical objects, repr the structure

In my opinion, this should be quite a priority for pandas.
 @richlewis42 would love to have another contributor to this. There are several linked issues. This requires a bit of a comprehensive, though not too difficult feature set.
 I would love to contribute, although I am very new to open source, and to the internals of pandas.  If there is anything specific that I could do to push this forward, please let me know. 
 here what would help

a code example which shows an api of what a user would do

eg construct a sample frame programmatically 
then df.to_html(.....) but with all of the details

this is not a difficult issue to fix but needs a reasonably rich api spec
 I will give this a try when I get back from work, hopefully over the weekend.  Thanks for the ideas.
 if you guys want to update for 0.17.0 that would be gr8!
 Hi, sorry I've been off the case for ages, I've been really busy at work.  I thought about what could be useful, and it seems to me that the simplest thing would be that if an object in a DataFrame offers an HTML representation (i.e. implements `_repr_html_`), then that should be used by default (instead of the output from prettyprint, which I think is used at the moment) - this may be a slightly different use case to the original pull request however.  I put an example in a [gist](http://nbviewer.ipython.org/gist/richlewis42/b806185b847096498318).  I couldn't make the change in `pandas.core.format` easily, as the representation is generated, then gets escaped by default - there would need to be some handling to make sure html provided by the object isn't escaped, but strings generated from the object is.  Any opinions on this?
 @richlewis42 that is a really nice idea, actually. For example, it would work well with geopandas, which holds shapely objects inside a column. These objects already have an `_repr_html_` that shows a picture of the shape, so the dataframe HTML view could show those shapes as well.
 I would vote for both: if a custom formatter is provided, then use that, otherwise if a `_repr_html_` exists, use that, otherwise use `repr` or `str` or whatever.  

The advantage of the custom formatter is that it gives you a lot of control: 
- if the elements are images, then you could resize them, or add a caption giving their dimensions etc.  
- if the elements can be treated as keys into a map, e.g. the string `"br"` could be treated as a key into a map of flag images, you could then render the flag with the caption "br"...I think this is what @richlewis42  meant in the first comment.
- if the elements are large data structures (e.g. numpy arrays, trees, dictionaries etc.) there may not be a single obvious `_repr_html_` for the given class, but there may be a sensible way of showing the specific data in your column.

Of course one approach would be to wrap each of the objects in the column in a custom object which handles the `_repr_html_` call, but that would be a pretty ugly solution... the custom `formatters` suggestion is not really much more complicated than finding and using the `_repr_html_`.
 I agree, these are not necessarily mutually exclusive. Recursive support for `_repr_html_` should go in another PR, in any case.
 Agreed, I still like the custom formatters, and think they would be very helpful.  

Could the recursive `_repr_html_` be implemented by using a formatter by default for all columns, that would call each object's `_repr_html_`, and fall back to an escaped pretty printed str (thus killing two birds with one stone)?

Or shall I open another pull request for recursive `_repr_html_`?  I had a look at the code, but couldn't see an easy way to modify it without rewriting quite a bit of it, due to the escaping being applied after the object is converted to string representation.  
 This PR needs a bit of extra work to go in -- notably it needs some tests and slightly better docs. I'll add comments inline in the code.

@richlewis42 I haven't taken a look at the code carefully yet, so up to you on the best order to add this. This PR is also probably going to make it in shortly and might help a title bit (see the `notebook` argument): https://github.com/pydata/pandas/pull/10232

Edit: to clarify, I do think it's probably a good idea to add `_repr_html_` later, unless @d1manson wants to add it to this PR. The main work here will probably be documentation, not the actual code changes.
 Mostly this just needs tests to verify the stated functionality. 
 anyone want to update (or present a new PR)?

@d1manson @richlewis42 
@jd 
 I think this could be done as part of #10250 with just a formatter method(s) (as you have several functionailites added), so closing, but welcome extensions as part of that PR (or after)
