Python §3

Python §3 is a small programming language, designed for the third chapter of the textbook Structure and Interpretation of Computer Programs, JavaScript Adaptation (SICP JS).

What names are predeclared in Python §3?

On the right, you see all predeclared names of Python §3, in alphabetical order. Click on a name to see how it is defined and used. They come in these groups:

What can you do in Python §3?

Assignment statements

Variables can be assigned to with identical syntax to variable declaration:

x = 1
print(x) // x is 1
x = x + 1
print(x)  // now x is 2

The difference from variable declaration is that in assignments, the variable must already exist, and that the assignment does not create a new variable but changes the value of the existing variable. In the example above, the first line creates a variable x and assigns it to 1. The third line changes the value of x to 2.

Read more on variable declaration and assignment in section 3.1.1 Local State Variables of the textbook.

While loops

A while loop repeatedly evaluates a predicate and if the predicate returns True, evaluates a given block. The evaluation terminates when the predicate returns False. Example:

x = 0
while x < 10:
    print(x)
    x = x + 1

will display the numbers from 0 to 9.

While loops are not covered in the textbook.

For loops

For loops repeat their body over a range of values, which starts from 0 by default. The variable associated with the for loop takes on each value in the range, and the body of the loop is evaluated once for each value. For example, the same program can be written as:

for x in range(10):
    print(x)

You can also specify a different starting point for the range, for example:

for x in range(5, 10):
    print(x)

will display the numbers from 5 to 9.

Lastly, you can specify a step size for the range, for example:

for x in range(0, 10, 2):
    print(x)

will display the even numbers from 0 to 8.

For loops are not covered in the textbook.

Lists, list access and list assignment

Lists are created using literal list expressions, as follows:

my_list = [10, 20, 30];

The constant my_list now refers to a list with three elements. The elements in such a literal list expressions have implicit keys. The first element has key 0, the second has key 1, the third has key 2 and so on.

A list can be accessed using list access expressions, with a given key:

my_list[0] + my_list[1] + my_list[2]; // 60

Like pairs, lists can be changed in Source §3. This is done using list assignment:

my_list[1] = 200;

List assignment and list access in Python §3 are restricted to integers (numbers with no fractional component) larger than or equal to 0 and less than 232-1. We call such numbers list indices.

Accessing a list at a list index that has not been assigned yet (using a literal list expression or a list assignment) will result in an error. For example, the following will result in an error:

my_list[3]; // error: list index out of range

Lists are not covered in the textbook.

You want the definitive specs?

For our development team, we are maintaining a definitive description of the language, called the Specification of Python §3. Feel free to take a peek!