# How to Pass Multiple Arguments to a map Function in Python

## Introduction

The `map()` function is everywhere in Python. It's a built in, it's part of the `concurrent.futures.Executor`, and also `multiprocessing.Pool`; but... it's limited!

It's limited because you cannot pass multiple arguments to it. However, what if I told you that there's some easy ways you can do that?

In this post, I’m going to show what you can do to map a function that expects multiple arguments. By the end of this article, you'll know:

- [what is a map function and the problem with it](#what-does-map-do-in-python-and-the-problem-with-it)
- [how to map two or more arguments with `itertools.starmap()`](#solution-1-mapping-multiple-arguments-with-itertoolsstarmap)
- [how to use `functools.partial` to "freeze" and pass multiple arguments to map](#solution-2-using-functoolspartial-to-freeze-the-arguments)
- [the way to map multiple arguments by "repeating" them](#solution-3-mapping-multiple-arguments-by-repeating-them)
- [how to pass multiple args to multiprocessing `pool.map`](#problem-2-passing-multiple-parameters-to-multiprocessing-poolmap)
- [how to pass multiple arguments to a concurrent futures ProcessPoolExecutor (or ThreadPoolExecutor)?](#problem-3-how-to-pass-multiple-arguments-to-a-concurrent-futures-processpoolexecutor-or-threadpoolexecutor)

Let's go!

## What Is a Map Function and the Problem With It

A `map()` is a function that expects one or more iterables and a function as arguments. 

For each item in these iterables, `map` applies the function passed as argument. The result is an iterator where each element is produced by the function you provided as argument. If you pass multiple iterables, you must pass a function that accepts that many arguments.

### The Problem

Let’s imagine that you have a function called `sum_four` that takes 4 arguments and returns their sum.

```python
>>> def sum_four(a, b, c, d):
        return a + b + c + d
```
Let’s also suppose that you are solving a very specific problem that requires the first 3 arguments to be fixed. In this problem, you want to compare how the function behaves when you vary only the last parameter.

```python
>>> a, b, c = 1, 2, 3

>>> sum_four(a=a, b=b, c=c, d=1)
 7

>>> sum_four(a=a, b=b, c=c, d=2)
 8

>>> sum_four(a=a, b=b, c=c, d=3)
 9

>>> sum_four(a=a, b=b, c=c, d=4)
 10
```

Now, say that you want to use `map`, because you like functional programming, or maybe because you come from a language that encourages this paradigm. 

Since only `d` varies, we could store all potential values for `d` we want to test in a list like this `all_d_values = [1, 2, 3, 4]`. 

The issue is, given a function and a list of single elements, if you want to pass that list to a `map` function and it takes only one element, what can you do?

### Solution 1 - Mapping Multiple Arguments with `itertools.starmap()`

The first solution is to *not* adopt the `map` function but use `itertools.starmap` instead. This function will take a function as arguments and an iterable of tuples.  Then, `starmap` will iterate over each tuple `t` and call the function by unpacking the arguments, like this `for t in tuples: function(*t)`.

To make things more clear, consider the following example.

```python
>>> import itertools

>>> all_d_values = [1, 2, 3, 4]

>>> items = ((a, b, c, d) for d in all_d_values)

>>> list(items)
 [(1, 2, 3, 1), (1, 2, 3, 2), (1, 2, 3, 3), (1, 2, 3, 4)]

>>> list(itertools.starmap(sum_four, items))
 [7, 8, 9, 10]
```

As you can see, there’s a lot of repetition, which may inevitably consume a lot of memory if the list is big. To improve that I made `items` as a generator, this way we only hold in memory the element we’ll be processing.

### Solution 2 - Using `functools.partial` to “Freeze” the Arguments

The second solution is to use currying and create a new partial function. According to the docs, [`partial()`](https://docs.python.org/3/library/functools.html#functools.partial) will "freeze" some portion of a function’s arguments and/or keywords resulting in a new function with a simplified signature.

```python
>>> import functools

>>> partial_sum_four = functools.partial(sum_four, a, b, c)

>>> partial_sum_four(3)
9

>>> list(map(partial_sum_four, all_d_values))
[7, 8, 9, 10]
```

### Solution 3 - Mapping Multiple Arguments by "Repeating" Them

The third alternative is to use the [`itertools.repeat()`](https://docs.python.org/3/library/itertools.html#itertools.repeat). 

This function produces an iterator that returns object over and over again. It will run indefinitely if you don’t specify the times argument. 

If we take a closer look at `map()`'s  [signature](https://docs.python.org/3/library/functions.html#map), it accepts a function and multiple iterables, `map(function, iterable, ...)`. 

According to its description, 
> If additional iterable arguments are passed, function must take that many arguments and is applied to the items from all iterables in parallel. With multiple iterables, the iterator stops when the shortest iterable is exhausted.

Bingo! We can make `a`, `b` and `c` infitnite iterables by using `itertools.repeat()`. As soon as `all_d_values` is exhausted, which is the shortest iterable, `map()` will stop.

```python
>>> import itertools
>>> list(map(sum_four, itertools.repeat(a), itertools.repeat(b), itertools.repeat(c), all_d_values))
 [7, 8, 9, 10]
```
To put it another way, using `repeat()` is roughly equivalent to:

```python
>>> list(map(sum_four, [1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3], all_d_values))
 [7, 8, 9, 10]
```

You don't need to worry too much about memory as `repeat` produces the elements on the go.  In fact, it returns a `repeatobject`, not `list`  [[ref]](https://github.com/python/cpython/blob/3.9/Modules/itertoolsmodule.c#L4226) .

## Problem 2: Passing Multiple Parameters to multiprocessing `Pool.map`

This problem is very similar to using the regular `map()`. The only difference is that we need to pass multiple arguments to the multiprocessing's pool map.

Suppose that we want to speed up our code and run `sum_four` in parallel using processes. 

The good news is, you can use the solutions above, with one exception: `Pool.map` only accepts one iterable. This means we cannot use `repeat()` here. Let's see the alternatives.

### Using `pool.starmap`

The `Pool` class from `multiprocessing` module implements a `starmap` function that works the same way as its counterpart from the `itertools` module. 

```python
>>> from multiprocessing import Pool

>>> import itertools

>>> def sum_four(a, b, c, d):
                return a + b + c + d
    
>>> a, b, c = 1, 2, 3

>>> all_d_values = [1, 2, 3, 4]

>>> items = [(a, b, c, d) for d in all_d_values]

>>> items
 [(1, 2, 3, 1), (1, 2, 3, 2), (1, 2, 3, 3), (1, 2, 3, 4)]

>>> with Pool(processes=4) as pool:
         res = pool.starmap(sum_four, items)

>>> res
 [7, 8, 9, 10]
```

### Using `partial()`

As alternative, we can also rely on the good `partial` function.

```python

>>> import functools

>>> partial_sum_four = functools.partial(sum_four, a, b, c)

>>> with Pool(processes=4) as pool:
         res = pool.map(partial_sum_four, all_d_values)

>>> res
 [7, 8, 9, 10]
```

## Problem 3: How to Pass Multiple Arguments to concurrent futures `Executor.map`?

The [`concurrent.futures`](https://docs.python.org/3/library/concurrent.futures.html) module provides a high-level interface called `Executor` to run callables  asynchronously. 

There are two different implementations available, a `ThreadPoolExecutor` and a `ProcessPoolExecutor`. 

Contrary to `multiprocessing.Pool`, a `Executor` does not have a `startmap()` function. However, its `map()` implementation supports multiple iterables, which allow us to use `repeat()`. Another difference is that `Executor.map` returns a generator, not a list.

### Using `partial()` With a ProcessPoolExecutor (or ThreadPoolExecutor) 

By "freezing" the arguments using `partial` we use the `map` method from `ProcessPoolExecutor` like a regular map function. Since they both share the same interface, you can do the same interchangeably with a `ThreadPoolExecutor`

```python
>>> from concurrent.futures import ProcessPoolExecutor

>>> import functools

>>> def sum_four(a, b, c, d):
                return a + b + c + d
    
>>> a, b, c = 1, 2, 3

>>> all_d_values = [1, 2, 3, 4]

>>> partial_sum_four = functools.partial(sum_four, a, b, c)

>>> with ProcessPoolExecutor(max_workers=4) as pool:
              res = list(pool.map(partial_sum_four, all_d_values))

>>> res
 [7, 8, 9, 10]
```

### Using `repeat()`

Again, we can just use `itertools.repeat` to get the job done like the previous solutions.

```python
>>> from concurrent.futures import ProcessPoolExecutor

>>> from itertools import repeat

>>> def sum_four(a, b, c, d):
                return a + b + c + d

>>> a, b, c = 1, 2, 3

>>> all_d_values = [1, 2, 3, 4]

>>> with ProcessPoolExecutor(max_workers=4) as pool:
              res = list(pool.map(sum_four, repeat(a), repeat(b), repeat(c), all_d_values))

>>> res
 [7, 8, 9, 10]
```

## Conclusion

That’s it for today, folks! I hope you’ve learned something different and useful. The `map()` function makes Python feel like a functional programming language. `map()` is available not only as a built-in function but also as methods in the `multiprocessing` and `concurrent.futures` module. In this article, I showed what I do to map functions that take several arguments. 

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