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PDEP-15: Reject PDEP-10 #58623

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PDEP-10: Change status to rejected
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43 changes: 40 additions & 3 deletions web/pandas/pdeps/0010-required-pyarrow-dependency.md
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# PDEP-10: PyArrow as a required dependency for default string inference implementation

- Created: 17 April 2023
- Status: Accepted
- Created: 17 April 2023 (updated May 8, 2024)
- Status: Rejected
- Discussion: [#52711](https://github.com/pandas-dev/pandas/pull/52711)
[#52509](https://github.com/pandas-dev/pandas/issues/52509)
- Author: [Matthew Roeschke](https://github.com/mroeschke)
[Patrick Hoefler](https://github.com/phofl)
- Revision: 1
- Revision: 2

# Note

This PDEP was originally accepted on May 8, 2023. However, after reviewing feedback posted
on the feedback issue [#54466](https://github.com/pandas-dev/pandas/issues/54466), we, the members of
the core team, have not decided with moving forward with this PDEP for pandas 3.0.

The primary reasons for rejecting this PDEP are twofold:

1) Requiring pyarrow as a dependency causes installation problems.
- Pyarrow does not fit or has a hard time fitting in space-constrained environments
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I think what we could learn from this process is what caused this to change our minds? These issues were discussed leading up to the acceptance of PDEP-10.

The way this is written I think reads more as "we discovered this after the fact" instead of "we decided that X amount of negative feedback on these points was enough to revert". I think there is some value to future PDEPs to set expectations around the latter

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Within the context of recent conversation I don't think this comment about AWS is true. AWS distributes an official pandas image for lambda which already includes pyarrow, pandas, and NumPy. This is all required by their own "AWS SDK on pandas" library.

The issue more finely scoped I think is that the default wheel installation via pip into a lambda image exceeds the 256 MB limit. Either using the official AWS provided image or using miniconda should not exceed the space limits

such as AWS Lambda and WASM, due to its large size of around ~40 MB for a compiled wheel
(which is larger than pandas' own wheel sizes)
- Installation of pyarrow is not possible on some platforms. We provide support for some
less widely used platforms such as Alpine Linux (and there is third party support for pandas in
pyodide, a WASM distribution of pandas), both of which pyarrow does not provide wheels for.

While both of these reasons are mentioned in the drawbacks section of this PDEP, at the time of the writing
of the PDEP, we underestimated the impact this would have on users, and also downstream developers.

2) Many of the benefits presented in this PDEP can be materialized even with payrrow as an optional dependency.
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I personally don't find this point very convincing. Saying Many of the benefits but then following it up with one bullet point seems to miss the mark - what are the other many benefits that we don't need pyarrow for? Without pyarrow users are forgoing:

  • High performance string operations
  • Direct string creation from I/O routines (i.e. no intermediate copies)
  • Zero copy data exchange through Arrow C Data Interface
  • Performant, memory efficient, and consistent NA handling

On the larger roadmap of pandas this moves us away from tighter Arrow integration, which means we move further away from Arrow compute algorithms / joins and the larger ecosystem of tools that includes streaming, query optimizers, planners, data engines, etc...

I think this argument in its current form is saying "we don't need a car because we have a horse and buggy"

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I personally don't find this point very convincing. Saying Many of the benefits but then following it up with one bullet point seems to miss the mark - what are the other many benefits that we don't need pyarrow for? Without pyarrow users are forgoing:

  • High performance string operations
  • Direct string creation from I/O routines (i.e. no intermediate copies)
  • Zero copy data exchange through Arrow C Data Interface
  • Performant, memory efficient, and consistent NA handling

On the larger roadmap of pandas this moves us away from tighter Arrow integration, which means we move further away from Arrow compute algorithms / joins and the larger ecosystem of tools that includes streaming, query optimizers, planners, data engines, etc...

I think this argument in its current form is saying "we don't need a car because we have a horse and buggy"

In PDEP-10, there are 3 benefits listed

  1. pyarrow strings (possible to provide users this benefit without making pyarrow required)
  2. Nested datatypes (can't have this without arrow, but this is a bit niche)
  3. Interopability (the alternative is the dataframe interchange protocol, which is more widely adopted at the moment. Not sure about the zero-copy stuff for that, though. I think it also might be possible to implement Arrow C Data interface support without taking on a hard dep on pyarrow)
    • Also, the primary beneficiary of this is other dataframe libraries (as opposed to us).

So, IMO, this argument is accurate, in that most of the benefits in PDEP-10 can be made possible (for those user that have pyarrow installed) without making pyarrow required.

The future benefits of Arrow are very compelling, but decisions on making a dependency required should be based on immediate and not future benefits. Like I said before, it is easy to reconsider this decision in a years time if those future benefits are materialize.

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If you think points 1 and 3 are possible without pyarrow then the alternatives for that should be laid out in this PDEP, at least at a super high level. I'm assuming point 1 refers to the nanoarrow POC I was sharing; point 3 requires reimplementing the conversions that pyarrow already has. (I personally don't think building either of those from scratch is a good long term solution but it can at least be discussed)

For point 2 how do you know those are niche applications? Its easy to dismiss things that don't exist today as not worthwhile, but I get the feeling that there could be plenty of use cases for the aggregate types, since they have a natural fit with many of the Python containers.

On interoperability the long term prospects for the dataframe interchange protocol seem dubious, and we have even discussed moving that out of pandas (see #56732).

  • Also, the primary beneficiary of this is other dataframe libraries (as opposed to us).

The Arrow interchange protocol can be used by any library that needs to work with Arrow data - there is no limit to it being used by other dataframe libraries. It provides a standardized API so that third parties don't need to hack into our internals, which is a direct benefit for us. It also works in two directions - we can be a consumer just as much as a producer.

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Nested datatypes (can't have this without arrow, but this is a bit niche)

Also wanted to point out that arrow has a decimal128 and decimal256 type which is especially useful for financial calculations where floating point inaccuracies cannot be tolerated, and the arrow decimal types are an extremely significant improvement over using object.

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Sure, will update and add a note in the PDEP when I get time again.


For example, as detailed in PDEP-14, it is possible to create a new string data type with the same semantics
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PDEP 14 does not change performance or memory savings if you do not have pyarrow installed

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added a note in parentheses at the end of that sentence.

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Did you push this up? I don't see anything in parentheses.

The way I am interpreting this now is "we don't need/care for pyarrow strings because we have always had a string data type using Python strings" - is that correct?

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I updated the PDEP-15 text, and forgot to remove the PDEP-10 changes.

I've removed the PDEP-10 changes now.

as our current default object string data type, but that allows users to experience faster performance and memory savings
compared to the object strings.

While we've decided to not move forward with requiring pyarrow in pandas 3.0, the rejection of this PDEP
does not mean that we are abandoning pyarrow support and integration in pandas. We, as the core team, still believe
that adopting support for pyarrow arrays and data types in more of pandas will lead to greater interoperability with the
ecosystem and better performance for users. Furthermore, a lot of the drawbacks, such as the large installation size of pyarrow
and the lack of support for certain platforms, can be solved, and potential solutions have been proposed for them, allowing us
to potentially revisit this decision in the future.

However, at this point in time, it is clear that we are not ready to require pyarrow
as a dependency in pandas.


## Abstract

Expand Down Expand Up @@ -210,6 +246,7 @@ before releasing a new pandas version.

- 17 April 2023: Initial version
- 8 May 2023: Changed proposal to make pyarrow required in pandas 3.0 instead of 2.1
- 8 May 2024: Changed status to rejected

[^1] <https://pandas.pydata.org/docs/development/roadmap.html#apache-arrow-interoperability>
[^2] <https://arrow.apache.org/powered_by/>