Learning Python

As we already said, everything in Python is an object. But what really happens when you type an instruction like age = 42 in a Python module?

So, what happens is that an object is created. It gets an id, the type is set to int (integer number), and the value to 42. A name age is placed in the global namespace, pointing to that object. Therefore, whenever we are in the global namespace, after the execution of that line, we can retrieve that object by simply accessing it through its name: age.

If you were to move house, you would put all the knives, forks, and spoons in a box and label it cutlery. Can you see it's exactly the same concept? Here's a screenshot of how it may look like (you may have to tweak the settings to get to the same view...

A first fundamental distinction that Python makes on data is about whether or not the value of an object changes. If the value can change, the object is called mutable, while if the value cannot change, the object is called immutable.

It is very important that you understand the distinction between mutable and immutable because it affects the code you write, so here's a question:

>>> age = 42
>>> age
42
>>> age = 43  #A
>>> age
43

In the preceding code, on the line #A, have I changed the value of age? Well, no. But now it's 43 (I hear you say...). Yes, it's 43, but 42 was an integer number, of the type int, which is immutable. So, what happened is really that on the first line, age is a name that is set to point to an int object, whose value is 42. When we type age = 43, what happens is that another object is created, of the type int and value 43 (also, the id will be different), and the name age is set to point...

Let's start by exploring Python's built-in data types for numbers. Python was designed by a man with a master's degree in mathematics and computer science, so it's only logical that it has amazing support for numbers.

Numbers are immutable objects.

>>> a = 12
>>> b = 3
>>> a + b  # addition
15
>>> b - a  # subtraction
-9
>>> a // b  # integer division
4
>>> a / b  # true division
4.0
>>> a * b  # multiplication
36
>>> b ** a  # power operator
531441
>>> 2 ** 1024  # a very big number, Python handles it gracefully
17976931348623159077293051907890247336179769789423065727343008115...

Let's start with immutable sequences: strings, tuples, and bytes.

>>> # 4 ways to make a string
>>> str1 =...

Mutable sequences differ from their immutable sisters in that they can be changed after creation. There are two mutable sequence types in Python: lists and byte arrays. I said before that the dictionary is the king of data structures in Python. I guess this makes the list its rightful queen.

>>> []  # empty list
[]
>>> list()  # same as []
[]
>>> [1, 2, 3]  # as with tuples, items are comma separated
[1, 2, 3]
>>> [x + 5 for x in [2, 3, 4]]  # Python is magic
[7, 8, 9]
>>> list((1, 3, 5, 7, 9))  # list from a tuple
[1, 3, 5, 7, 9]
>>> list('hello')  # list from a string
['h',...

Python also provides two set types, set and frozenset. The set type is mutable, while frozenset is immutable. They are unordered collections of immutable objects.

Hashability is a characteristic that allows an object to be used as a set member as well as a key for a dictionary, as we'll see very soon.

Objects that compare equally must have the same hash value. Sets are very commonly used to test for membership, so let's introduce the in operator in the following example:

>>> small_primes = set()  # empty set
>>> small_primes.add(2)  # adding one element at a time
>>> small_primes.add(3)
>>> small_primes.add(5)
>>> small_primes
{2, 3, 5}
>>> small_primes.add(1)  # Look what I've done, 1 is not a prime!
>>> small_primes
{1, 2, 3, 5}
>>> small_primes.remove(1)  # so let's remove it
>>> 3 in small_primes  # membership...

Of all the built-in Python data types, the dictionary is probably the most interesting one. It's the only standard mapping type, and it is the backbone of every Python object.

A dictionary maps keys to values. Keys need to be hashable objects, while values can be of any arbitrary type. Dictionaries are mutable objects.

There are quite a few different ways to create a dictionary, so let me give you a simple example of how to create a dictionary equal to {'A': 1, 'Z': -1} in five different ways:

>>> a = dict(A=1, Z=-1)
>>> b = {'A': 1, 'Z': -1}
>>> c = dict(zip(['A', 'Z'], [1, -1]))
>>> d = dict([('A', 1), ('Z', -1)])
>>> e = dict({'Z': -1, 'A': 1})
>>> a == b == c == d == e  # are they all the same?
True  # indeed!

Have you noticed those double equals? Assignment is done with one equal, while to check whether an object is the same as another one (or 5 in one go, in this case), we use double equals. There is also...

When Python general purpose built-in containers (tuple, list, set, and dict) aren't enough, we can find specialized container data types in the collections module. They are:

We don't have the room to cover all of them, but you can find plenty of examples in the official documentation...

That's it. Now you have seen a very good portion of the data structures that you will use in Python. I encourage you to take a dive into the Python documentation and experiment further with each and every data type we've seen in this chapter. It's worth it, believe me. Everything you'll write will be about handling data, so make sure your knowledge about it is rock solid.

Before we leap into the next chapter, I'd like to make some final considerations about different aspects that to my mind are important and not to be neglected.

>>> a = 1000000
>>> b = 1000000
>>> id(a) == id(b)
False

In this chapter, we've explored the built-in data types of Python. We've seen how many they are and how much can be achieved by just using them in different combinations.

We've seen number types, sequences, sets, mappings, collections, we've seen that everything is an object, we've learned the difference between mutable and immutable, and we've also learned about slicing and indexing (and, proudly, negative indexing as well).

We've presented simple examples, but there's much more that you can learn about this subject, so stick your nose into the official documentation and explore.

Most of all, I encourage you to try out all the exercises by yourself, get your fingers using that code, build some muscle memory, and experiment, experiment, experiment. Learn what happens when you divide by zero, when you combine different number types into a single expression, when you manage strings. Play with all data types. Exercise them, break them, discover all their methods, enjoy them and learn them...