Very often, we want to apply a function to an iterable without using a for loop. We saw an example in the python reduce function but the python reduce function doesn’t really fit want we want to do because the reduce function successively accumulates the results. We want the function to be applied to each element of the iterable and be stored separately. To do that, we would use a python map. It is just the right function for the job. In this blog post, I will show you how to use a python map and also its subsequent, a python starmap, along with usage.
What is a python map.
A python map is just a function that takes iterables and applies a function to the elements of the iterables. If there are more than one iterable, it applies the function to the corresponding elements of the iterable in succession. If the iterables are of different lengths, it stops at the shortest iterable. The result of a python map function is an iterator object. That means to get out the results you have to apply another function to the object. Most times, you would cast it to a list or set.
The syntax of the python map function is map(function, iterable, ...) where function is the function you want to apply to the items of the iterable and iterable is the object which contains the items.
Visit this link if you want to refresh yourself on iterators, and this other one on python iterables. They are important concepts in python. I explained them in depth.
Now, let’s take some examples.
We’ll show examples using a single python iterable and then when more than one python iterable is used. First using a single python iterable.
Supposing we have a list of numbers and we have a function that raises a given number by a power of 3. We could use python map to apply the function to each of the items in the list of numbers.
You will notice from the code above that I used the python map function to apply power_func to each of the items of the num_list in line 5. The first time we printed out the object, items_raised, what we get is a map object. The map object is an iterator. So, to get out the elements in the iterator we cast to a list in line 8 and it then extracted each of the items raised.
Now let’s show an example with more than one iterable. This time, we will use two iterables and add their items together.
What we did above is to provide two iterables, num_list1 and num_list2, to the python map function, and then use the function, adder. What map does is take the items at corresponding indices and pass them to adder which adds them together and then provides the result to the map iterator. Then using list function, we extract each of the elements in the map iterator which is then printed out in line 7.
There is also another case I want you to consider if using more than one iterable and the iterables are not of equal length. What a python map does is that it applies the function to the items of the iterable successively until it comes to the end of the shorter length iterable and then it stops. Let’s take an example, this time letting num_list2 be longer than num_list1.
You can see this time that it stops short at the items at index 4 in both lists and ignores the rest of the items in num_list2 because num_list2 is longer.
One thing you need to know is that the python map function falls short when the items of the iterable is a tuple. This is because map takes each of the items as a single element and was not meant to work with tuples. But not to worry, we have another descendant of map, the python starmap function, that helps us to deal with tuples as elements.
What is the python starmap function?
Just like the python map function, the python starmap function returns an iterator based on the operation of a provided function to the elements of an iterable but this time it works when the elements are tuples. Since you know the basic concepts behind python maps, it also applies to python starmaps. The python starmap function is included with the itertools module. So to use it, you first need to import it from the itertools module.
The syntax for starmap function is itertools.starmap(function, iterable) where function is the function carrying out the operation on each of the elements of the iterable. The elements of the iterable are arranged in tuples.
As an illustration, let’s take an iterable of tuples for example.
As you can see from above, in line 1 we first imported the python starmap function from itertools. Then we defined a function, power, that takes in two arguments and raises the first argument to the power of the second argument at lines 3 and 4. Then at line 7 we used the starmap function to apply the power function to each of the elements of the iterable, this time a list, which are tuples. Python starmap unpacks the tuple when sending them to the power function such that the first item in the tuple becomes bound to x and the second item of the tuple becomes bound to y, and then the function is applied to them such that x is raised to power y and the result is added to the starmap object. Then in line 8 we call list function on the starmap object which is an iterator to extract each of the items in the iterator. Then finally, we print them out.
We can use an iterable with a tuple that has any number of items as long as the function to which they would be applied can accept that number of arguments when the tuple is unpacked by the python starmap function. Let’s take another example of a starmap being used with an iterable that has a tuple of three items.
In the code above, the functioin, is_pythagoras, is based on the Pythagoras rule that the square of two numbers in a triangle is equal to the square of the longest side. What is_pythagoras function does is take three positional arguments and checks for the Pythagoras rule in the arguments. If it obeys the pythagors rule, it returns True but if not it returns False. Then in line 9 we created a list of tuples that is structured with 3 items representing the sides of a triangle, with the third item being the longest side. Then we applied the triangles list and the is_pythagoras function together in the starmap function to check which side obeys the Pythagoras rule. You will notice that line 10 produces a list having either True or False as entries. Then in line 11 to 14, we checked which of the entries in the list has the True value and then printed the corresponding entry from the triangles list of tuples as the tuple obeying the Pythagoras rule.
I hope you enjoyed yourself as I did. These functions show you the powerful abilities of python. Use it to your enjoyment. To keep receiving updates from me on how to use python, you can subscribe to my blog. Thanks for reading.
Happy pythoning.