Iterating over a sequence

Now we have all the tools to iterate over a sequence, so let's build on that example:

# simple.for.2.py
surnames = ['Rivest', 'Shamir', 'Adleman'] 
for position in range(len(surnames)): 
    print(position, surnames[position]) 

The preceding code adds a little bit of complexity to the game. Execution will show this result:

$ python simple.for.2.py
0 Rivest
1 Shamir
2 Adleman

Let's use the inside-out technique to break it down, OK? We start from the innermost part of what we're trying to understand, and we expand outward. So, len(surnames) is the length of the surnames list: 3. Therefore, range(len(surnames)) is actually transformed into range(3). This gives us the range [0, 3), which is basically a sequence (0, 1, 2). This means that the for loop will run three iterations. In the first one, position will take value 0, while in the second one, it will take value 1, and finally value 2 in the third and last iteration. What is (0, 1, 2), if not the possible indexing positions for the surnames list? At position 0, we find 'Rivest', at position 1, 'Shamir', and at position 2, 'Adleman'. If you are curious about what these three men created together, change print(position, surnames[position]) to print(surnames[position][0], end=''), add a final print() outside of the loop, and run the code again.

Now, this style of looping is actually much closer to languages such as Java or C++. In Python, it's quite rare to see code like this. You can just iterate over any sequence or collection, so there is no need to get the list of positions and retrieve elements out of a sequence at each iteration. It's expensive, needlessly expensive. Let's change the example into a more Pythonic form:

# simple.for.3.py
surnames = ['Rivest', 'Shamir', 'Adleman'] 
for surname in surnames: 
    print(surname) 

Now that's something! It's practically English. The for loop can iterate over the surnames list, and it gives back each element in order at each interaction. Running this code will print the three surnames, one at a time. It's much easier to read, right?

What if you wanted to print the position as well though? Or what if you actually needed it? Should you go back to the range(len(...)) form? No. You can use the enumerate built-in function, like this:

# simple.for.4.py
surnames = ['Rivest', 'Shamir', 'Adleman'] 
for position, surname in enumerate(surnames): 
    print(position, surname) 

This code is very interesting as well. Notice that enumerate gives back a two-tuple (position, surname) at each iteration, but still, it's much more readable (and more efficient) than the range(len(...)) example. You can call enumerate with a start parameter, such as enumerate(iterable, start), and it will start from start, rather than 0. Just another little thing that shows you how much thought has been given in designing Python so that it makes your life easier.

You can use a for loop to iterate over lists, tuples, and in general anything that Python calls iterable. This is a very important concept, so let's talk about it a bit more.