feat: Use `_repr_html_` when native supports it by dangotbanned · Pull Request #2776 · narwhals-dev/narwhals

dangotbanned · 2025-07-03T13:07:24Z

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Code follows style guide (ruff)
Tests added
Documented the changes

If you have comments or can explain your changes, please do so below

I discovered this method in (#2572), when I was trying to work out why polars.Expr looked so much better that what I had 😅

Thinking we can get more immediate benefits now by allowing this option when a backend supports it for:

DataFrame
- (pandas, polars)

LazyFrame
- (pandas, polars)

Series
- (~~pandas~~, polars)

- Related #1702 - https://ipython.readthedocs.io/en/stable/config/integrating.html#rich-display - https://github.com/pandas-dev/pandas/blob/22f12fc5d3f7fda3f198760204e7c13150c78581/pandas/core/frame.py#L1189-L1232 - https://github.com/pola-rs/polars/blob/8011fa34e0c5f1270ef52e2d3b0b2946bb2faa72/py-polars/polars/dataframe/frame.py#L1580-L1605

dangotbanned · 2025-07-03T19:16:07Z

narwhals/_utils.py

+    style_css = (
+        ".dataframe caption { "
+        "caption-side: bottom; "
+        "text-align: center; "
+        "font-weight: bold; "
+        "padding-top: 8px;"
+        "}"
+    )


If anyone has any suggestions for styling - feel free to experiment/comment 🙂

The only decision I'd made so far was putting the <caption> below the table

With the default polars formatting, it appeared between the table and the shape tuple when above - which I thought looked odd

That's very reasonable!

Realized I never followed this up with an example

Now that I'm looking at it again, maybe above isn't so bad?

- `pandas` reuses the eager version - `pyarrow` doesn't support - `ibis` requires changing global config, so skipping that - `dask` does have a `_repr_html_`, but doesn't parse well

dangotbanned · 2025-07-09T16:21:14Z

narwhals/_utils.py

+    if header == "Narwhals LazyFrame" and "LazyFrame" in native_html:
+        html = native_html.replace("LazyFrame", "LazyFrame.to_native()")
+        return f"{html}<p><b>{header}</b></p>"


Had to add this branch for pl.LazyFrame as it wasn't parsing with my naive wrapper:

import io import xml.etree.ElementTree as ET import polars as pl data = {"a": [1, 2, 3], "b": ["fdaf", "fda", "cf"]} ldf = pl.LazyFrame(data) >>> ET.parse(io.StringIO(ldf._repr_html_())) ParseError: junk after document element: line 1, column 25

Seems to fail on the first <p> in https://github.com/pola-rs/polars/blob/dfa5efe71156c654a1ba3a54b865eae723a818e9/py-polars/polars/lazyframe/frame.py#L783

- `pandas` only supports it for `pd.DataFrame`

dangotbanned · 2025-07-09T16:53:14Z

Possible follow-ups

Just some loose ideas, nothing I'm planning to work on any time soon 😅

Support dask.dataframe.DataFrame._repr_html_
It has one, but it is a bit strange and I wasn't able to parse it (3769326)
Normalize the styling
- The code in polars is fairly short - vendoring it so we can render all the eager backends the same isn't that unrealistic and would be a nice DX improvement
- https://github.com/pola-rs/polars/blob/dfa5efe71156c654a1ba3a54b865eae723a818e9/py-polars/polars/dataframe/_html.py

dangotbanned · 2025-08-02T20:43:55Z

@MarcoGorelli, @FBruzzesi

Thought I'd do one last check before closing this one, here's a few options to choose from:

Don't support this
Do it, but less (defer entirely to polars, pandas)
Do it, but style differently (feat: Use _repr_html_ when native supports it #2776 (comment))
Do it, but increase the scope and make pyarrow + pd.Series look pretty too (feat: Use _repr_html_ when native supports it #2776 (comment))

No worries if we don't want it 🙂

FBruzzesi

@dangotbanned thank to your ping - I got reminded that I once started to look at this, and never finished. I am not against having a good support, yet I am not very useful nor opinionated about this.

I would try to aim for a pareto optimum that balances usefulness and maintainability 😂

FBruzzesi · 2025-07-18T20:02:25Z

narwhals/_utils.py

+    header: Literal["Narwhals DataFrame", "Narwhals LazyFrame", "Narwhals Series"],
+    native_html: str,
+) -> str | None:  # pragma: no cover
+    if header == "Narwhals LazyFrame" and "LazyFrame" in native_html:
+        html = native_html.replace("LazyFrame", "LazyFrame.to_native()")
+        return f"{html}<p><b>{header}</b></p>"


I am mostly nitpicking here but... isn't the header actually a footer? 😂

You're quite right 😂

It started as a header until I ran into (#2776 (comment))

I should've updated that to footer or caption

Oh I forgot, the name header actually came from generate_repr

narwhals/narwhals/_utils.py

Line 1515 in e32e139

def generate_repr(header: str, native_repr: str) -> str:

Anyway - updated it in (57e333d)

FBruzzesi · 2025-07-18T20:04:08Z

narwhals/_utils.py

+        tree.getroot().insert(0, style)
+    buf = io.BytesIO()
+    tree.write(buf, "utf-8", method="html")
+    return buf.getvalue().decode()


Everything else in this function is a new language to me - I am not very helpful

Ah yeah xml.etree.elementtree is a bit of a strange one

I had to learn a bit of lxml once to fix a particularly broken file.
The API of that is based on this stdlib module, but was more ergonoic than this mess 😄

To simplify this:

Element: Is a HTML Element

Tree: Refers to a document/webpage, but in this case it is just a table

So I'm essentially doing a fancy find/replace, but trying to preserve the structure of the document

FBruzzesi · 2025-08-02T21:23:23Z

narwhals/_utils.py

+    style_css = (
+        ".dataframe caption { "
+        "caption-side: bottom; "
+        "text-align: center; "
+        "font-weight: bold; "
+        "padding-top: 8px;"
+        "}"
+    )


That's very reasonable!

dangotbanned · 2025-08-03T12:51:28Z

#2776 (comment)

Do it, but increase the scope and make pyarrow + pd.Series look pretty too

I've started #2925 and came up against the pyarrow.ChunkedArray repr again, while writing an example 🤦‍♂️

import pyarrow as pa

import narwhals as nw

>>> nw.Series.from_iterable("a", [4, 1, 3, 2], dtype=nw.UInt32, backend=pa)
┌───────────────────────────────────────────────────────┐
|                    Narwhals Series                    |
|-------------------------------------------------------|
|<pyarrow.lib.ChunkedArray object at 0x0000017129497880>|
|[                                                      |
|  [                                                    |
|    4,                                                 |
|    1,                                                 |
|    3,                                                 |
|    2                                                  |
|  ]                                                    |
|]                                                      |
└───────────────────────────────────────────────────────┘

Even if we don't go ahead with _repr_html_ - I'd really like to be displaying Series.name and Series.dtype in __repr__

The polars one manages to fit in both of those + shape, while taking up waaaay less horizontal space and 1 fewer lines:

>>> nw.Series.from_iterable("a", [4, 1, 3, 2], dtype=nw.UInt32, backend="polars")
┌─────────────────┐
| Narwhals Series |
|-----------------|
|shape: (4,)      |
|Series: 'a' [u32]|
|[                |
|        4        |
|        1        |
|        3        |
|        2        |
|]                |
└─────────────────┘

#2776 (comment)

Related #2776 (comment)

dangotbanned · 2025-08-15T14:05:30Z

#2776 (comment)

If we just wanted pa.ChunkedArray to look nicer, I've got a very naive new repr (not html) for nw.Series:

shape: (365,)
dtype: Datetime(time_unit='us', time_zone=None)
name: 'time series'
nw.Series[pyarrow]
[
	2009-01-02 00:00:00
	2009-01-03 00:00:00
	2009-01-04 00:00:00
	2009-01-05 00:00:00
	2009-01-06 00:00:00
	…
	2009-12-28 00:00:00
	2009-12-29 00:00:00
	2009-12-30 00:00:00
	2009-12-31 00:00:00
	2010-01-01 00:00:00
]

shape: (30,)
dtype: UInt32
name: 'lower max rows'
nw.Series[pyarrow]
[
	0
	1
	2
	…
	27
	28
	29
]

shape: (30,)
dtype: Int16
name: 'oh pandas too???'
nw.Series[pandas]
[
	29
	28
	27
	26
	25
	24
	…
	5
	4
	3
	2
	1
	0
]

Would be nicer-er if we used the short type codes from polars

dangotbanned added enhancement New feature or request pandas-like Issue is related to pandas-like backends polars Issue is related to polars backend labels Jul 3, 2025

dangotbanned added 2 commits July 3, 2025 14:29

Merge branch 'main' into df-repr-html

04e1f51

Merge branch 'main' into df-repr-html

bf7e87a

dangotbanned commented Jul 3, 2025

View reviewed changes

dangotbanned added 4 commits July 9, 2025 14:42

Merge remote-tracking branch 'upstream/main' into df-repr-html

edb7ae4

feat: Support pl.LazyFrame._repr_html_

3769326

- `pandas` reuses the eager version - `pyarrow` doesn't support - `ibis` requires changing global config, so skipping that - `dask` does have a `_repr_html_`, but doesn't parse well

Merge remote-tracking branch 'upstream/main' into df-repr-html

3559cf8

chore: add note on testing

ea42e83

dangotbanned commented Jul 9, 2025

View reviewed changes

feat: Support pl.Series._repr_html_

3748f23

- `pandas` only supports it for `pd.DataFrame`

dangotbanned marked this pull request as ready for review July 9, 2025 16:53

dangotbanned added 9 commits July 11, 2025 20:58

Merge branch 'main' into df-repr-html

5bb73d8

Merge branch 'main' into df-repr-html

e77c8de

Merge branch 'main' into df-repr-html

d8f09a4

Merge branch 'main' into df-repr-html

4227f10

Merge branch 'main' into df-repr-html

690e6e4

Merge branch 'main' into df-repr-html

6f2ab6b

Merge branch 'main' into df-repr-html

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Merge remote-tracking branch 'upstream/main' into df-repr-html

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Merge branch 'main' into df-repr-html

f73d564

FBruzzesi reviewed Aug 2, 2025

View reviewed changes

dangotbanned marked this pull request as draft August 3, 2025 15:22

dangotbanned added 2 commits August 3, 2025 16:05

Merge remote-tracking branch 'upstream/main' into df-repr-html

e32e139

rename header -> caption_text

57e333d

#2776 (comment)

dangotbanned added 2 commits August 3, 2025 16:50

change caption-side to top

7758265

Related #2776 (comment)

Merge remote-tracking branch 'upstream/main' into df-repr-html

dfb5f5e

dangotbanned mentioned this pull request Aug 11, 2025

feat: Add {Expr,Series}.is_close #2962

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dangotbanned mentioned this pull request Oct 16, 2025

nw.DType, pl.DataType feature parity #3214

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