Parallel Python

Part 1: Mapping Functions

In many situations you would like to apply the same function to lots of different pieces of data. For example, let's create two arrays of numbers, and use our add function to add pairs of numbers together. In the Python Console type:

In [1]:
def add(x, y):
    """Simple function returns the sum of the arguments"""
    return x + y
In [2]:
def multiply(x, y):
    """
    Simple function that returns the product of the
    two arguments
    """
    return x * y
In [3]:
a = [1, 2, 3, 4, 5]
b = [6, 7, 8, 9, 10]

result = []

for i, j in zip(a, b):
    r = add(i, j)
    result.append(r)

print(result)
[7, 9, 11, 13, 15]

The above code has looped over every pair of numbers in the lists a and b, and has called the function add for each pair. Each result is appended to the list result, which is printed at the end of the loop.

Applying the same function to every item in a list (or pair of lists) of data is really common. For example, in a molecular simulation, you may want to loop over a list of every molecule and call a calculate_energy function for each one. In a fluid dynamics simulation, you may want to loop over a list of grid points and call a solve_gridpoint function for each one. This pattern, of calling the same function for each element of a list (or set of lists) of data, is called mapping. In the above example, we have mapped the function add onto the lists a and b, giving us result.

The above code mapped the function add. How about if we wanted to map our diff or multiply functions? One option would be to copy out this code again. A better solution would be to use functional programming to write our own mapping function.

In [4]:
def mapper(func, arg1, arg2):
    """
    This will map the function 'func' to each pair
    of arguments in the list 'arg1' and 'arg2', returning
    the result
    """

    res = []

    for i, j in zip(arg1, arg2):
        r = func(i, j)
        res.append(r)

    return res
In [5]:
mapper(add, a, b)
Out[5]:
[7, 9, 11, 13, 15]

Now type

In [6]:
mapper(multiply, a, b)
Out[6]:
[6, 14, 24, 36, 50]

Can you see how this works?

The mapper function takes as its first argument the function to be mapped. The other arguments are the two lists of data for the mapping. The part zip(arg1, arg2) takes the two arguments and returns an interator which can go through them both at the same time. As soon as one of them runs out of elements, it will stop. The mapper function then loops through each of these pairs of data, calling func for each pair, and storing the result in the list res. This is then returned at the end.

Because the mapper function calls the mapped function using the argument func, it can map any function that is passed to it, as long as that function accepts two arguments. For example, let us now create a completely different function to map:

In [7]:
import math

def calc_distance(point1, point2):
    """
    Function to calculate and return the distance between
    two points
    """

    dx2 = (point1[0] - point2[0]) ** 2
    dy2 = (point1[1] - point2[1]) ** 2

    return math.sqrt(dx2 + dy2)

This has created a function that calculates the distance between two points. Let’s now create two lists of points and use mapper to control the calculation of distances between points:

In [8]:
points1 = [(1.0, 1.0), (2.0, 2.0), (3.0, 3.0)]
points2 = [(4.0, 4.0), (5.0, 5.0), (6.0, 6.0)]

mapper(calc_distance, points1, points2)
Out[8]:
[4.242640687119285, 4.242640687119285, 4.242640687119285]

As long as the function and the data are compatible, the mapper function will work.

Standard Map

Mapping is so common and useful that it is built in as a standard Python function, called map. For example:

In [9]:
map(calc_distance, points1, points2)
Out[9]:
<map at 0x7fca684fa6d0>

This is perhaps a little unexpected as Python hasn’t actually given us the answer. Instead, the built-in map function has returned an object which is ready and waiting to perform the calculation you’ve asked, but won't actually run those calculations unitl you request the result. This can be useful because by evaluating the map “lazily”, you can avoid unnecessary computation. The technical term for the thing that has been returned is an iterator. You can use this object in a for loop just fine but you can only loop over it once.

If you want to force Python to evaluate the map and give you the answers, you can turn it into a list usig the list function:

In [10]:
distances = map(calc_distance, points1, points2)
list(distances)
Out[10]:
[4.242640687119285, 4.242640687119285, 4.242640687119285]

You should see that your calc_distances function has been mapped to all of the pairs of points.

There is one more niggle to be aware of, and that is that an iterator object can only be used one before it is "exhausted". If we try to get the result of a map in two places we see the following:

In [11]:
distances = map(calc_distance, points1, points2)
print(list(distances))
print(list(distances))
[4.242640687119285, 4.242640687119285, 4.242640687119285]
[]

You see that the second time we converted distances to a list it created an empty list. If you need to use the result of a map in multiple places, you should convert it to a list once, and then use that list result:

In [12]:
distances = map(calc_distance, points1, points2)
distance_list = list(distances)
print(distance_list)
print(distance_list)
[4.242640687119285, 4.242640687119285, 4.242640687119285]
[4.242640687119285, 4.242640687119285, 4.242640687119285]

The standard map function behaves very similarly to your hand-written mapper function, returing an iterator containing the result of applying your function to each item of data.

One advantage of map is that it knows how to handle multiple arguments. For example, let’s create a function that only maps a single argument:

In [13]:
def square(x):
    """
    Simple function to return the square of
    the passed argument
    """
    return x * x

Now, let’s try to use your handwritten mapper function to map square onto a list of numbers:

In [14]:
numbers = [1, 2, 3, 4, 5]

result = mapper(square, numbers)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_16620/1145100616.py in <module>
      1 numbers = [1, 2, 3, 4, 5]
      2 
----> 3 result = mapper(square, numbers)

TypeError: mapper() missing 1 required positional argument: 'arg2'

This raises an exception since we wrote our mapper function so that it mapped functions that expected two arguments. That meant that our mapper function needs three arguments; the mapped function plus two lists of arguments.

The standard map function can handle different numbers of arguments:

In [15]:
result = map(square, numbers)

list(result)
Out[15]:
[1, 4, 9, 16, 25]

The standard map function can work with mapping functions that accept any number of arguments. If the mapping function accepts n arguments, then you must pass n+1 arguments to map, i.e. the mapped function, plus n lists of arguments:

In [16]:
def find_smallest(arg1, arg2, arg3):
    """
    Function used to return the smallest value out 
    of 'arg1', 'arg2' and 'arg3'
    """

    return min(arg1, arg2, arg3)

a = [1, 2, 3, 4, 5]
b = [5, 4, 3, 2, 1]
c = [1, 2, 1, 2, 1]

result = map(find_smallest, a, b, c)

list(result)
Out[16]:
[1, 2, 1, 2, 1]

Is this output what you expect?

Exercise

Download and unpack the file shakespeare.tar.bz2, e.g. type into a Python Console:

import urllib.request
import tarfile

urllib.request.urlretrieve("https://milliams.com/courses/parallel_python/shakespeare.tar.bz2", "shakespeare.tar.bz2")
with tarfile.open("shakespeare.tar.bz2") as tar:
    tar.extractall()

This has created a directory called shakespeare that contains the full text of many of Shakespeare’s plays.

Your task is to write a Python script, called countlines.py, that will count the total number of lines in each of these Shakespeare plays, e.g. that can be run in the Terminal with:

python countlines.py shakespeare

To do this, first write a function that counts the number of lines in a file.

Then, use the standard map function to count the number of lines in each Shakespeare play, printing the result as a list.

As a reminder, you can open a single file and count the lines with:

with open(filename) as f:
    lines_in_file =  len(f.readlines())

If you get stuck or want some inspiration, a possible answer is given here.