# Parallelize pandas apply() and map() with Dask DataFrame

• November 22, 2021

TLDR; Dask DataFrame can parallelize pandas *apply()* and *map()* operations, but it can do much more. With Dask’s *map_partitions()*, you can work on each partition of your Dask DataFrame, which is a pandas DataFrame, while leveraging parallelism for various custom workflows.

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Dask DataFrame helps you quickly scale your single-core pandas code, while keeping the API familiar. The *map() *and *apply()* functions are at the core of data manipulation with pandas. In this post, we’ll take a closer look at how you can perform these operations efficiently with Dask DataFrame. We’ll also look at some additional features that Dask provides to level-up your computations.

In particular,

- Dask’s
*map_partitions()*function, and - Dask’s implementation of pandas
*apply()*,*map()*, and*applymap()*.

Follow along with the code in this notebook!

## Quick overview of pandas *apply()* and *map()*

You can use **pandas’ ***apply()*** function **to apply any in-built or custom Python function across a pandas one-dimensional array, i.e., a Series or a single dimension of a DataFrame. This can be applied across columns (axis=0), or rows (axis=1).

Consider a pandas DataFrame ‘df’ with 10 rows and 2 columns: ‘a’ and ‘b’, where each element in the DataFrame is a random integer:

Also, consider a function *minmax()* that sleeps for 1 second and returns the difference between the largest and smallest value:

def minmax(x): sleep(1) return x.max() - x.min()

We’ll use *apply()* to do the *minmax()* operation on all the rows in df:

df.apply(minmax, axis=1)

This computation takes ~10s because we have 10 rows. (We’re noting the time to compare it against Dask later!)

You can use the** ***map()*** **and** ***applymap()*** **functions** **for element-wise operations across a pandas Series and DataFrame respectively.

Consider a function ‘inc’ that sleeps for 1s and returns the input incremented by 1:

def inc(i): sleep(1) return i+1

We’ll use *map()* to do this *inc()* operation across a column in the pandas DataFrame (which is a pandas Series):

df.a.map(inc)

And then, across the entire dataframe using *applymap()*:

df.applymap(inc)

We noted the time for these operations as well, the Series map took ~10 seconds and DataFrame applymap took ~20 seconds.

## Dask Map Partitions

A Dask DataFrame consists of multiple pandas Dataframes, and each pandas dataframe is called a partition. This mechanism allows you to work with larger-than-memory data because your computations are distributed across these pandas dataframes and can be executed in parallel. This includes our *apply()* and *map()* computations!

First, we need to convert our pandas dataframe into a dask dataframe ‘ddf’:

ddf = dd.from_pandas(df, npartitions=5) # Dask DataFrame has 5 partitions

We can manually access and work on these individual partitions using Dask’s *map_partitions()* function. *map_partitions() *maps any function across each partition, so you can operate on each partition as you would on a pandas DataFrame.

We can define a new function called *minmax2 *that operates on a pandas DataFrame and applies the previous *minmax *function on the DataFrame:

def minmax2(df): return df.apply(minmax, axis=1)

Now, we’ll use *map_partitions()* to map this function *minmax2* across ddf:

ddf.map_partitions(minmax2, meta=(None, 'int64'))

The visualization below shows how *minmax2 *was mapped across all 5 partitions of ddf, and hence, was almost 5x as fast as the original pandas *apply()*.

In the visualizations, rectangular nodes represent Python objects, circular nodes represent functions, and the arrows between them represent task dependencies.

## Meta keyword argument

*map_partitions()* and all the functions mentioned below have an important keyword argument called ‘meta’. It describes the structure of the expected output of your computation. These functions are very flexible and can present many different types of outputs, which is why the ‘meta’ argument is essential.

It is an optional argument. If it’s not provided, Dask DataFrame tries to infer the output type with a sample input (which can also take some time), but it’s good practice to describe it yourself. You can describe ‘meta’ as a pandas DataFrame, pandas Series, Python dictionary, Python iterable, or a Python tuple.

## Dask’s apply() and map()

Dask also implements the pandas *apply(),* *map()*, and *applymap() *functions which (similar to *map_partitions()*) operate on each partition of the dataframe in a parallel fashion. Let’s look at some examples below:

Dask DataFrame *apply()*:

q = ddf.apply(minmax, axis=1, meta=(None, 'int64'))

Dask DataFrame Series* map:*

r = ddf.a.map(inc)

Dask DataFrame *applymap:*

s = ddf.applymap(inc)

Notice how these computations are ~5x as fast as the corresponding pandas operations!

## map_partitions() vs apply()

We have used the pandas and Dask *apply() *functions in this blog post only for understanding purposes. In actual use cases, it’s not a good idea to use *apply()* with custom Python code because of its slow performance. We recommend using *map_partitons()* instead.

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