feat: add knit_print method for DataFrame (experimental)

[eitsupi]

Add the ability to output pretty HTML tables on knitr documents.

[eitsupi]

I am confused about the errors in the test.
Locally the test does not fail.

[sorhawell]

I am confused about the errors in the test. Locally the test does not fail.

I think I have had that error before also, I can try fiddle with it this evening.

[sorhawell]

@eitsupi I also updated the snapshot, which had slightly different spacings on my machine. At worst polars could have some machine-specific inference of line widths which is good for users and bad for testing. In such case, maybe we can use a DataFrame with only 1-3 columns as test case.

[eitsupi]

Thanks!

Definitely a small data frame seems to be the right target for snapshot testing here. I will update.

[eitsupi]

Sorry, but I found that the current approach will fail if there is a column of list or long column name.
We can check with dplyr::starwars for example.

> Sys.setenv(POLARS_FMT_TABLE_FORMATTING = "ASCII_MARKDOWN")

> Sys.setenv(POLARS_FMT_TABLE_INLINE_COLUMN_DATA_TYPE = "1")

> pl$DataFrame(dplyr::starwars)
shape: (87, 14)
| name (str)     | height (i32) | mass (f64) | hair_color | ... | species (str) | films          | vehicles       | starships     |
|                |              |            | (str)      |     |               | (list[str])    | (list[str])    | (list[str])   |
|----------------|--------------|------------|------------|-----|---------------|----------------|----------------|---------------|
| Luke Skywalker | 172          | 77.0       | blond      | ... | Human         | ["The Empire   | ["Snowspeeder" | ["X-wing",    |
|                |              |            |            |     |               | Strikes Back", | , "Imperial    | "Imperial     |
|                |              |            |            |     |               | "Rev...        | Speede...      | shuttle"]     |
| C-3PO          | 167          | 75.0       | null       | ... | Droid         | ["The Empire   | []             | []            |
|                |              |            |            |     |               | Strikes Back", |                |               |
|                |              |            |            |     |               | "Att...        |                |               |
| R2-D2          | 96           | 32.0       | null       | ... | Droid         | ["The Empire   | []             | []            |
|                |              |            |            |     |               | Strikes Back", |                |               |
|                |              |            |            |     |               | "Att...        |                |               |
| Darth Vader    | 202          | 136.0      | none       | ... | Human         | ["The Empire   | []             | ["TIE         |
|                |              |            |            |     |               | Strikes Back", |                | Advanced x1"] |
|                |              |            |            |     |               | "Rev...        |                |               |
| ...            | ...          | ...        | ...        | ... | ...           | ...            | ...            | ...           |
| Poe Dameron    | null         | null       | brown      | ... | Human         | ["The Force    | []             | ["T-70 X-wing |
|                |              |            |            |     |               | Awakens"]      |                | fighter"]     |
| BB8            | null         | null       | none       | ... | Droid         | ["The Force    | []             | []            |
|                |              |            |            |     |               | Awakens"]      |                |               |
| Captain Phasma | null         | null       | unknown    | ... | null          | ["The Force    | []             | []            |
|                |              |            |            |     |               | Awakens"]      |                |               |
| Padmé Amidala  | 165          | 45.0       | brown      | ... | Human         | ["Attack of    | []             | ["H-type      |
|                |              |            |            |     |               | the Clones",   |                | Nubian        |
|                |              |            |            |     |               | "The Ph...     |                | yacht",       |
|                |              |            |            |     |               |                |                | "Naboo s...   |

Perhaps we may need to write out the HTML in the same way as python-polars...
https://github.com/pola-rs/polars/blob/81b4102ac03f79361645e709f05061d9bb876a51/py-polars/polars/dataframe/_html.py#L48-L115

[eitsupi]

Some additional functions need to be implemented.

• [.DataFrame (Like https://github.com/apache/arrow/blob/0bf777a5952be012e41f5b1ad443d4fec38e6f5a/r/R/arrow-tabular.R#L105) ([.DataFrame #140)
• dtype_strings (Like https://github.com/pola-rs/polars/blob/f09fd6f66f8fc31f51b69fe93a09e955c9be3490/py-polars/src/dataframe.rs#L126, maybe -> typeof()) (feat: add DataFrame$dtype_strings() #141)
• get_fmr (Like https://github.com/pola-rs/polars/blob/f09fd6f66f8fc31f51b69fe93a09e955c9be3490/py-polars/src/series.rs#L384, maybe -> as.charactor()) (feat: add an internal method Series$get_fmt #143, feat: add as.charactor.Series #162)

[sorhawell]

@eitsupi
colname and value(s) are right-aligned, whereas colname and type are left-aligned. Would it look good to right-align all three (colname, type, value(s))?

... or left align

[eitsupi]

@eitsupi colname and value(s) are right-aligned, whereas colname and type are left-aligned. Would it look good to right-align all three (colname, type, value(s))?

... or left align

Good point, that bothered me too.

But I think this looks exactly the same as the current py-polars.
For example, this is what happens when we render the following Rmd file.

---
title: test
output:
  github_document:
    df_print: paged
---

```{r}
#| include: false
nycflights13::flights[1, ] |> arrow::write_parquet("flights.parquet")
devtools::load_all()
```

```{python}
import polars as pl

pl.read_parquet("flights.parquet")
```

```{r}
polars::scan_parquet("flights.parquet")$collect()
```

(Only the R's have [ and ] escaped with a backslash.......needs to be fixed.)

On the other hand, the following qmd file is right-aligned when rendered by Quarto CLI (1.3, prerelease version).

---
title: test
format:
  html:
    self-contained: true
    df-print: paged
---

```{r}
#| include: false
nycflights13::flights[1, ] |> arrow::write_parquet("flights.parquet")
devtools::load_all()
```

```{python}
import polars as pl

pl.read_parquet("flights.parquet")
```

```{r}
polars::scan_parquet("flights.parquet")$collect()
```

So this is more like a CSS (?) problem of rmarkdown package.

[eitsupi]

Fixed to look exactly the same as python-polars!

[sorhawell]

looks fine to me
Worked as expected on my machine

[vincentarelbundock]

Looks good to me. This is a very nice improvement!