Writing loops

The aim of this section is to provide with several techniques to keep your code readable, concise and Pythonic when writing loops over lists and dictonaries.

Over lists

Let’s say we have the following list items = [1, 2, 3] and we want to print each individual item on a separate line. If you come from Java or C++ you will probably gravitate towards indexes and come up with a solution like this one

i = 0
while i < len(items):
    print(items[i])
    i += 1

or maybe this other one

for i in range(len(items)):
    print(items[i])

However, Python offers simplified, more readable and more efficient loops. As we mentioned before, for iterates directly over the elements of a sequence, so we can take advantage of that and forget about keeping track of any index.

for item in items:
    print(item)

But, what if we really need the index? The enumerate() function comes in handy. At each step it yields a tuple containing an automatic counter and the values obtained from the list.

for info in enumerate(items):
    print(info)

The output will be

(0, 1)
(1, 2)
(2, 3)

We can tweak our loop a little further by means of unpacking info at each iteration. What is more, enumerate() accepts an optional argument which tells from where to start the counter.

for index, item in enumerate(items, start=1):
    print(index, '->', item)

In this case, the output will be

1 -> 1
2 -> 2
3 -> 3

What if we need to loop backwards? Our first thought might be to gradually decrease the index

for i in range(len(items)-1, -1, -1):
    print(items[i])

Nonetheless, Python offers yet another useful function reversed(), which returns a reverse iterator and does not take up any more space than our previous less-elegant solution.

for item in reversed(items):
    print(item)

What if we want to loop over two lists? Python allows us to do so with the zip() function. For instance,

countries = ["France", "Italy", "Germany"]
capitals = ["Paris", "Rome", "Berlin"]

for country, capital in zip(countries, capitals):
    print('{0} is the capital of {1}.'.format(capital, country))

will produce

Paris is the capital of France.
Rome is the capital of Italy.
Berlin is the capital of Germany.

All of these techniques are analogous for strings and tuples so we encourage you to try them out for yourself.

Looping and modifying

Let’s say we have the following list l = [1, 2, 3] and, for every number greater than 2, we want to duplicate it at the end of the list and hence obtain l = [1, 2, 3, 2, 3]. Find below the code that intends to do so:

l = [1, 2, 3]
for n in l:
    if n >= 2:
        l.append(n)

The key message is to never modify a list while you are iterating over it! In this case, a good idea might be to loop over a copy of l.

Over dictionaries

The most direct way to iterate over

d1 = {'a': 1, 'b': [2, 3], 'c': 'four'}

is the following:

for k in d1:
    print(k, ':', d[k])

However, to avoid looking for each value twice we can iterate directly over the key-value pairs by means of the items() method:

for info in d1.items():
    print(info)

and obtain

('a', 1)
('b', [2, 3])
('c', 'four')

It is usual to unpack the tuple info at each iteration, just as we did with enumerate() when looping over lists:

for k, v in d1.items():
    print(k, ':', v)

Similarly, if we just want to iterate over keys or values, the elegant solution is to use the keys() and values() methods:

for k in d1.keys(): # for v in d1.values()
    print(k)

These two are in fact views, which means that they are dynamic and reflect all modifications on the dictionary:

>>> my_keys = d2.keys()
>>> my_keys
dict_keys(['b', 'f'])
>>> d2['g'] = 8
>>> my_keys
dict_keys(['b', 'f', 'g'])

As a curiosity, keys() and items() are set-like since their entries are unique and we can do something like this:

>>> d2 = {'b': True, 'f': (5, 6)}
>>> d1.keys() & d2.keys()
{'b'}

Lliçons.jutge.org
Víctor Adell
Universitat Politècnica de Catalunya, 2023