Values and Variables

Values and Variables#

A value is something that can be stored in a computer and manipulated by a computer program. In this tutorial, values are referred to as objects, and the terms are used interchangeably.

Note: In other programming languages, a distinction is made between primitive data types (which can only store values) and objects (which also have attributes and methods). For example, the world class has an attribute size and a method add_background.

Python follows a simple philosophy: everything is an object — which is why we generally use the term value.

Each value has a data type, which you can check using the type() function. The following program:

from miniworlds import *
import random
world = World(100, 100)

print(type("Hello World"))
print(type(Line((10, 10), (100, 100))))
print(type(17))

world.run()

…produces this output on the console:

<class 'str'>
<class 'actors.shapes.Line'>
<class 'int'>

Variables#

To access stored objects later, you need to save where they are. Technically, the memory address of an object is stored, but in Python, we use names to refer to objects.

line = Line((10, 10), (100, 100))

This stores the line in the variable named line. You can now use line to access and modify the object, for example:

line.fill_color = (255, 0, 0)

You can also do calculations with variables, such as:

a = 3
b = 4
print(a + b)

Assignments#

Assignments use the = symbol:

c = a + b

This means:

  • First, the expression on the right is evaluated.

  • Then, the result is stored in the variable on the left.

After this code:

a = 3
b = 4
c = a + b

  • a has the value 3

  • b has the value 4

  • c has the value 7

You can also update object attributes, like moving a circle left or right using the keyboard:

from miniworlds import *
world = World(100, 100)

circle = Circle((50, 50), 20)

@world.register
def on_key_pressed_a(self):
    circle.x = circle.x - 1

@world.register
def on_key_pressed_d(self):
    circle.x = circle.x + 1

world.run()

The line circle.x = circle.x + 1 means:

  • Compute circle.x + 1

  • Store the result back into circle.x

Note: = does not mean mathematical equality. Instead, the right-hand expression is evaluated and the result is assigned to the left-hand side.

Some programming languages use a different symbol to avoid confusion for beginners.

Usage#

You can use variables wherever you would use a number or string — as long as the data type is correct:

a = 3
b = 4
line = Line((a, b), (5, 6))

This works because (a, b) is a tuple — exactly what Line expects.

But this will raise an error:

a = 3
b = 4
line = Line(a, (5, 6))

Line expects a tuple as the first argument, but a is an integer. The error will be:

miniworlds.exceptions.miniworlds_exception.ActorArgumentShouldBeTuple: First argument to create a Actor [position] should be a Tuple.

Reading error messages often helps identify mistakes.

Scope#

In larger programs — especially when working in teams — variable names and their scope are important to prevent conflicts.

A variable has different scopes depending on where it’s defined:

  • A variable inside a function has local scope — it’s only visible within that function.

  • A variable outside any function is global — visible throughout the program.

⚠️ Important: If you want to modify a global variable inside a function, you must use the global keyword.

This works:#

from miniworlds import *
world = World(100, 100)

a = 3

@world.register
def on_key_pressed_a(self):
    print(a)

world.run()

Output: 3

But this does NOT work:#

from miniworlds import *
world = World(100, 100)

a = 3

@world.register
def on_key_pressed_a(self):
    a = a + 1
    print(a)

world.run()

This causes an error because a is treated as a local variable, but hasn’t been initialized yet.

This works again using global:#

from miniworlds import *
world = World(100, 100)

a = 3

@world.register
def on_key_pressed_a(self):
    global a
    a = a + 1
    print(a)

world.run()
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