POC of PDEP-9 (I/O plugins)
Proof of concept of what's being discussed in #51799.

An example of a plugin is implemented in https://github.com/datapythonista/lance-reader

Both together allow this code:

```python
import pandas

pandas.load_io_plugins()

df = pandas.read_lance('sample.lance')
df.to_lance('sample_copy.lance')
```
A couple other thoughts:

1) Do users have a way to tell which things are 3rd party?  Many people don't use virtualenvs etc or keep track of what they have installed or why.

2) When users upgrade a pandas version, can/should we warn or something on existing connectors and tell them to upgrade those too?  Or maybe have the connector define a `max_(tested|supported)_pandas_version`?   > A couple other thoughts:
> 
>     1. Do users have a way to tell which things are 3rd party?  Many people don't use virtualenvs etc or keep track of what they have installed or why.
> 
>     2. When users upgrade a pandas version, can/should we warn or something on existing connectors and tell them to upgrade those too?  Or maybe have the connector define a `max_(tested|supported)_pandas_version`?

I guess we don't really do either of these for the matplotlib backends, nor do I think pandas necessarily needs to enforce these. I think users knowing where things come from is a good point. Ideally we'd like to make things be as little as magic as possible even for users without a deep understanding of Python. Users can iterate entrypoints in the same way as done in this PR to see what connectors are available in their environtment. Afaik entrypoints can provide the name of the module, but not the name of the pip/conda package that added them. I'd personally start simple, but depending on the feedback could be reasonable to have something like `pandas.list_connectors()` or whatever to make users life easier.

For the versioning, I find it out of scope. Compatibility of packages should be at the package level (pip/conda dependencies). And if we want to get into the business of recommending users to have newer versions it'd probably make more sense to start by recommending updates to pandas, or to numpy, if users aren't using the latest versions, and not here, don't you think?

@mroeschke I'll remove the PDEP label, iirc our website will believe this is a PDEP, and publish it in the list of proposals under discussion (or fail because the title doesn't start by PDEP-x. @pandas-dev/pandas-core I updated this POC to show in practice what PDEP-9 proposes regarding Arrow, which based on the feedback received I don't think it was clear.

The current approach is that pandas will expect connectors to implement `pandas_reader` and `pandas_writer` which communicate with pandas with a pandas DataFrame.

In case a connector doesn't implement functions that exchange a pandas dataframe, then pandas will fallback to see if the connector can exchange data with a PyArrow object. If PyArrow is installed (it will be, since it'll be a dependency of the connector), then pandas will read the PyArrow table from the connector, cast it to a dataframe, and return it for the `pandas.read_whatever(...)` call. This design opens the door to support nanoarrow, a Rust Arrow implementation and others in the future, if that is useful, with minimal changes required in the pandas side. 

This PyArrow fallback options adds value in at least two different ways:
- It makes the API between connectors and pandas simpler, as an Arrow structure (PyArrow Table) is more concise than a pandas dataframe, which could have custom extension arrays, different backend types...
- It makes this I/O interface available to projects that don't want or can't depend on pandas. Possibly increasing its adoption, and having more projects/devs testing and helping maintain connectors

@jbrockmendel @simonjayhawkins @jreback does this seem reasonable to you? I think you were the main people who expressed concerns about using Arrow as an interface.

Finally, in this version I added a function `pandas.load_io_plugins()` that need to be called to load the I/O connectors. The alternative was to load them at `import pandas` which doesn't make it possible to disable it via options, which would be executed when connectors are already loaded. I think making this opt-in initially is a good idea, and if in a future version we want to make plugins load by default, the deprecation cycle should be trivial, warning when users call the function so they remove the call. > I'm fine with the current implementation, but I do feel strongly against making this the default with the current behavior of raising when there is a conflict. To me, that is an entire new level of dependency hell where say pip installs just fine but I can't use package X because I have package Y and Z installed.
> 
> More generally, I think requiring users to modify their environment to use this feature should be avoided at all costs. Would it be possible to have an option specifying which ones to skip?

I agree, but I don't see an easy solution to this. The current version doesn't load plugins by default, so if I understand you correctly, you're talking in a future version (e.g. 2.2 or later) if we remove `pandas.load_io_plugins()` and we call it automatically at `import pandas`.

I don't think pandas should know anything about third-party connectors (let me know if you disagree with that). If that's the case, we don't have any way in the pandas side to prevent two connectors using the same name. So, as I see things the optionsif two installed packages request to use the name `pandas.read_X` are:

1. We raise an exception. This hopefully will cause frustration of the users using the connector that duplicated the name, and the authors will probably change it.
2. We load one of them with an arbitrary criteria (the first or last to be returned by the Python entrypoints API)
3. We rename them to something like `pandas.read_X_1` and `pandas.read_X_2`

While none of the options are perfect, 1 is the only reasonable option to me (regardless of whether we load plugins at import or explicitly, that it's probably a separate discussion). What do you think @rhshadrach? > so if I understand you correctly, you're talking in a future version (e.g. 2.2 or later) if we remove `pandas.load_io_plugins()` and we call it automatically at `import pandas`.

Correct. I'm +1 as long we either require calling `pandas.load_io_plugins()` or finding a good way to handle conflicting packages. With requiring `pandas.load_io_pluging()`, can we also have an option to determine what one loads in case of a conflict?

> 1. We raise an exception. This hopefully will cause frustration of the users using the connector that duplicated the name, and the authors will probably change it.

As expressed in https://github.com/pandas-dev/pandas/pull/53005#pullrequestreview-1437372863, I do not find this approach viable. Relying on inducing a bad user experience for our users to encourage change in other packages does not seem like a good strategy to me.

> 2. We load one of them with an arbitrary criteria (the first or last to be returned by the Python entrypoints API)

While not great, I think this would be okay if users had a way (e.g. a pandas option?) to change which one is loaded. Is this possible?

> We rename them to something like `pandas.read_X_1` and `pandas.read_X_2`

This could mean installing a package breaks functioning code; -1 here. Even if you leave one named `pandas.read_X`, I think this is still not viable. We do have somewhat similar behavior here: e.g. calling DataFrame.to_excel without specifying the engine argument also depends on what packages are installed (e.g. if you only have openpyxl vs only have xlxswriter installed). But at least I can specify `engine="openpyxl"` which will then guarantee the desired engine (or raise if not installed). I don't think that is possible here. > While not great, I think this would be okay if users had a way (e.g. a pandas option?) to change which one is loaded. Is this possible?

Sounds good in theory, but I fail to see how users could specify which option to use, either when loading the plugins, or as an option later. If we only have the name (e.g. two implementations for `pandas.read_deltalake`) how can I reference one of them individually? We could add an `engine` value to this API, but I think that creates more problems than solutions:
- If we can't expect packages to do the right thing and not duplicate names, why we should expect engines to be unique? We are in the same problem (with a more complex solution)
- The current approach assumes all engines of a single format to have the same signature, which I think it's far from optimal. For example, I'd like to have a CSV reader with a `schema` parameter, but this is not possible in pandas since `read_csv(engine='*', ...)` has the same signature for all CSV readers.

The only thing that comes to my mind would be to use PyPI names to enforce uniqueness. Not sure if the entrypoints API allows me to see the package name, or what to do with conda-forge, so probably a bad idea. But that's the only way I can think of. A fourth option would be to warn when loading conflicting plugins, and only raise when calling the conflicting name (the name will will then not be a real one but a shim that raises an exception). 

There will no doubt be situations when want to access some plugin, but not the conflicting one, so I can see @rhshadrach point that it will be annoying to get an error in those situations, but as warning first and the raise if you actually access it could be a way forward?  Good point. A warning sounds good too. I think it's simpler instead of an exception if the function/method is accessed, to simply not create it. What do you think? I think that's also possible. The warning text should be clear about what the problem is.

One thing though I like about raising errors is that it gives a nice entry point into the debugger and let's you go back to where the  error occurred. I'm sure that is also possible with warnings, though I could google it, my point is just that the work flow in debugging errors is more natural than debugging warnings. Or that's just me who knows.

I  could accept both just a warning or a warning plus exception if the name is called.

 @rhshadrach @topper-123 thanks a lot for the feedback, it was very useful. I updated the PR, and now it only warns if connectors conflict and are being ignored (not registered). I think this is surely better, and I could get the name of the pip/conda package, so the error message should be quite informative on what needs to be done.

I didn't try it yet, as it's not trivial, but it'd be great to know your feedback on this approach before moving forward. Closing PDEP-9 as rejected, so not moving forward with this. See #51799.