r/vignettes/python.Rmd

---
title: "Apache Arrow in Python and R with reticulate"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Apache Arrow in Python and R with reticulate}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

The `arrow` package provides `reticulate` methods for passing data between
R and Python in the same process. This document provides a brief overview.

## Installing

To use `arrow` in Python, at a minimum you'll need the `pyarrow` library.
To install it in a virtualenv,

```r
library(reticulate)
virtualenv_create("arrow-env")
install_pyarrow("arrow-env")
```

If you want to install a development version of `pyarrow`,
add `nightly = TRUE`:

```r
install_pyarrow("arrow-env", nightly = TRUE)
```

`install_pyarrow()` also works with `conda` environments
(`conda_create()` instead of `virtualenv_create()`).

For more on installing and configuring Python,
see the [reticulate docs](https://rstudio.github.io/reticulate/articles/python_packages.html).

## Using

To start, load `arrow` and `reticulate`, and then import `pyarrow`.

```r
library(arrow)
library(reticulate)
use_virtualenv("arrow-env")
pa <- import("pyarrow")
```

The package includes support for sharing Arrow `Array` and `RecordBatch`
objects in-process between R and Python. For example, let's create an `Array`
in `pyarrow`.

```r
a <- pa$array(c(1, 2, 3))
a

## Array
## <double>
## [
##   1,
##   2,
##   3
## ]
```

`a` is now an `Array` object in our R session, even though we created it in Python.
We can apply R methods on it:

```r
a[a > 1]

## Array
## <double>
## [
##   2,
##   3
## ]
```

We can send data both ways. One reason we might want to use `pyarrow` in R is
to take advantage of functionality that is better supported in Python than in R.
For example, `pyarrow` has a `concat_arrays` function, but as of 0.17, this
function is not implemented in the `arrow` R package. We can use `reticulate`
to use it efficiently.

```r
b <- Array$create(c(5, 6, 7, 8, 9))
a_and_b <- pa$concat_arrays(list(a, b))
a_and_b

## Array
## <double>
## [
##   1,
##   2,
##   3,
##   5,
##   6,
##   7,
##   8,
##   9
## ]
```

Now we have a single `Array` in R.

"Send", however, isn't the correct word. Internally, we're passing pointers to
the data between the R and Python interpreters running together in the same
process, without copying anything. Nothing is being sent: we're sharing and
accessing the same internal Arrow memory buffers.

## Troubleshooting

If you get an error like

```
Error in py_get_attr_impl(x, name, silent) :
  AttributeError: 'pyarrow.lib.DoubleArray' object has no attribute '_export_to_c'
```

it means that the version of `pyarrow` you're using is too old.
Support for passing data to and from R is included in versions 0.17 and greater.
Check your pyarrow version like this:

```r
pa$`__version__`

## [1] "0.16.0"
```

Note that your `pyarrow` and `arrow` versions don't need themselves to match:
they just need to be 0.17 or greater.