Python’s execution model

In this section, we would like to introduce you to some important concepts, such as scope, names, and namespaces. You can read all about Python’s execution model in the official language reference (https://docs.python.org/3/reference/executionmodel.html), of course, but we would argue that it is quite technical and abstract, so let us give you a less formal explanation first.

Names and namespaces

Say you are looking for a book, so you go to the library and ask someone for it. They tell you something like Second Floor, Section X, Row Three. So, you go up the stairs, look for Section X, and so on. It would be very different to enter a library where all the books are piled together in random order in one big room. No floors, no sections, no rows, no order. Fetching a book would be extremely hard.

When we write code, we have the same issue: we have to try and organize it so that it will be easy for someone who has no prior knowledge about it to find what they are looking for. When software is structured correctly, it also promotes code reuse. Furthermore, disorganized software is more likely to contain scattered pieces of duplicated logic.

As a first example, let us take a book. We refer to a book by its title; in Python lingo, that would be a name. Python names are the closest abstraction to what other languages call variables. Names refer to objects and are introduced by name-binding operations. Let us see a quick example (again, notice that anything that follows a # is a comment):

>>> n = 3  # integer number
>>> address = "221b Baker Street, NW1 6XE, London"  # Sherlock Holmes' address
>>> employee = {
...     'age': 45,
...     'role': 'CTO',
...     'SSN': 'AB1234567',
... }
>>> # let us print them
>>> n
3
>>> address
'221b Baker Street, NW1 6XE, London'
>>> employee
{'age': 45, 'role': 'CTO', 'SSN': 'AB1234567'}
>>> other_name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'other_name' is not defined
>>>

Remember that each Python object has an identity, a type, and a value. We defined three objects in the preceding code; let us now examine their types and values:

• An integer number n (type: int, value: 3)
• A string address (type: str, value: Sherlock Holmes’ address)
• A dictionary employee (type: dict, value: a dictionary object with three key/value pairs)

Fear not, we know we have not covered what a dictionary is. We will see, in Chapter 2, Built-In Data Types, that it is the king of Python data structures.

Have you noticed that the prompt changed from >>> to ... when we typed in the definition of employee? That is because the definition spans multiple lines.

So, what are n, address, and employee? They are names, and they can be used to retrieve data from within our code. They need to be kept somewhere so that whenever we need to retrieve those objects, we can use their names to fetch them. We need some space to hold them, hence: namespaces!

A namespace is a mapping from names to objects. Examples are the set of built-in names (containing functions that are always accessible in any Python program), the global names in a module, and the local names in a function. Even the set of attributes of an object can be considered a namespace.

The beauty of namespaces is that they allow you to define and organize names with clarity, without overlapping or interference. For example, the namespace associated with the book we were looking for in the library could be used to import the book itself, like this:

from library.second_floor.section_x.row_three import book 

We start from the library namespace, and by means of the dot (.) operator, we walk into that namespace. Within this namespace, we look for second_floor and, again, we walk into it with the . operator. We then walk into section_x, and finally, within the last namespace, row_three, we find the name we were looking for: book.

Walking through a namespace will be clearer when dealing with real code examples. For now, just keep in mind that namespaces are places where names are associated with objects.

There is another concept, closely related to that of a namespace, which we would like to mention briefly: scope.

Scopes

According to Python’s documentation:

“A scope is a textual region of a Python program, where a namespace is directly accessible.”

Directly accessible means that, when looking for an unqualified reference to a name, Python tries to find it in the namespace.

Scopes are determined statically, but at runtime, they are used dynamically. This means that by inspecting the source code, you can tell what the scope of an object is. There are four different scopes that Python makes accessible (not necessarily all of them are present at the same time, though):

• The local scope, which is the innermost one and contains the local names.
• The enclosing scope; that is, the scope of any enclosing function. It contains non-local names and non-global names.
• The global scope contains the global names.
• The built-in scope contains the built-in names. Python comes with a set of functions that you can use in an off-the-shelf fashion, such as print, all, abs, and so on. They live in the built-in scope.

The rule is the following: when we refer to a name, Python starts looking for it in the current namespace. If the name is not found, Python continues the search in the enclosing scope, and this continues until the built-in scope is searched. If a name has still not been found after searching the built-in scope, then Python raises a NameError exception, which basically means that the name has not been defined (as seen in the preceding example).

The order in which the namespaces are scanned when looking for a name is therefore local, enclosing, global, built-in (LEGB).

This is all theoretical, so let us see an example. To demonstrate local and enclosing namespaces, we will have to define a few functions. Do not worry if you are not familiar with their syntax for the moment—we will cover that in Chapter 4, Functions, the Building Blocks of Code. Just remember that in the following code, when you see def, it means we are defining a function:

# scopes1.py
# Local versus Global
# we define a function, called local
def local():
    age = 7
    print(age)
# we define age within the global scope
age = 5
# we call, or `execute` the function local
local()
print(age)

In the preceding example, we define the same name, age, in both the global and local scopes. When we execute this program with the following command (have you activated your virtual environment?):

$ python scopes1.py

We see two numbers printed on the console: 7 and 5.

What happens is that the Python interpreter parses the file, top to bottom. First, it finds a couple of comment lines, which are skipped, then it parses the definition of the function local. When called, this function will do two things: it will set a name for an object representing the number 7 and will print it. The Python interpreter keeps going, and it finds another name binding. This time, the binding happens in the global scope and the value is 5. On the next line, there is a call to the local function. At this point, Python executes the function, so this time, the binding age = 7 happens in the local scope and is printed. Finally, there is a call to the print function, which is executed and will now print 5.

One particularly important thing to note is that the part of the code that belongs to the definition of the local function is indented by four spaces on the right. Python, in fact, defines scopes by indenting the code. You walk into a scope by indenting, and walk out of it by dedenting. Some coders use two spaces, others three, but the suggested number of spaces to use is four. It is a good measure to maximize readability. We will talk more about all the conventions you should embrace when writing Python code later.

In other languages, such as Java, C#, and C++, scopes are created by writing code within a pair of curly braces: { … }. Therefore, in Python, indenting code corresponds to opening a curly brace, while dedenting code corresponds to closing a curly brace.

What would happen if we removed that age = 7 line? Remember the LEGB rule. Python would start looking for age in the local scope (function local), and, not finding it, it would go to the next enclosing scope. The next one, in this case, is the global one. Therefore, we would see the number 5 printed twice on the console. Let us see what the code would look like in this case:

# scopes2.py
# Local versus Global
def local():
    # age does not belong to the scope defined by the local
    # function so Python will keep looking into the next enclosing
    # scope. age is finally found in the global scope.
    print(age, 'printing from the local scope')
age = 5
print(age, 'printing from the global scope')
local()

Running scopes2.py will print this:

$ python scopes2.py
5 printing from the global scope
5 printing from the local scope

As expected, Python prints age the first time, then when the local function is called, age is not found in its scope, so Python looks for it following the LEGB chain until age is found in the global scope. Let us see an example with an extra layer, the enclosing scope:

# scopes3.py
# Local, Enclosing and Global
def enclosing_func():
    age = 13
    def local():
        # age does not belong to the scope defined by the local
        # function so Python will keep looking into the next
        # enclosing scope. This time age is found in the enclosing
        # scope
        print(age, 'printing from the local scope')
    # calling the function local
    local()
age = 5
print(age, 'printing from the global scope')
enclosing_func()

Running scopes3.py will print on the console:

$ python scopes3.py
5, 'printing from the global scope'
13, 'printing from the local scope'

As you can see, the print instruction from the local function is referring to age as before. age is still not defined within the function itself, so Python starts walking scopes following the LEGB order. This time, age is found in the enclosing scope.

Do not worry if this is still not perfectly clear for now. It will become clearer as we go through the examples in the book. The Classes section of the Python tutorial (https://docs.python.org/3/tutorial/classes.html) has an interesting paragraph about scopes and namespaces. Be sure you read it to gain a deeper understanding of the subject.