A Sample Python Program

A Sample Python Program#

Note

Source: Adapted from the C# edition (data/sampleprogram.rst). The room-painting scenario and line-by-line walkthrough mirror the C# original; the code is a Python translation.

As a start, consider a small problem and a program to solve it. Suppose you paint the walls of rooms in one color and the ceiling in another, and you want to calculate the size of the areas to cover with paint. For simplicity, ignore doors. What data do you need to start with? Clearly the dimensions of the room. Suppose we consider modern houses where the height of the room is predictably 8 feet, so the new starting data is just the length and width of the room.

You need to:

  1. Obtain the length and width from the user.

  2. Calculate the wall area and ceiling area.

  3. Let the user know the results.

This is a common programming pattern: data in, calculate results, output results. In this case the calculations in the middle are straightforward.

Here is what the program looks like when it runs, with the user typing 20.5 and 10:

Calculation of Room Paint Requirements
Enter room length: 20.5
Enter room width: 10
The wall area is 488.0 square feet.
The ceiling area is 205.0 square feet.

Here is the program:

HEIGHT = 8

print("Calculation of Room Paint Requirements")
length = float(input("Enter room length: "))
width = float(input("Enter room width: "))

wall_area = 2 * (length + width) * HEIGHT
ceiling_area = length * width

print(f"The wall area is {wall_area} square feet.")
print(f"The ceiling area is {ceiling_area} square feet.")

This section gives an overview of a working program, even if some explanations do not make total sense yet. Do not worry if you do not understand everything here — these concepts are each explained in detail in later sections.

Line by line#

HEIGHT = 8

This is an assignment statement. It creates a variable named HEIGHT and gives it the value 8. We use all capital letters by convention to suggest that this value will not change — it is a constant. Python has no special syntax for constants; all-caps naming is just a widely followed convention.

print("Calculation of Room Paint Requirements")

print is a built-in Python function that writes output to the screen. Its argument — the value inside the parentheses — is a string, a sequence of characters enclosed in quotes. The string is displayed without the quotes.

length = float(input("Enter room length: "))

This line does several things at once, working from the inside out:

  1. input("Enter room length: ") displays the prompt string and then waits for the user to type something and press Enter. It always returns a string — for example, "20.5".

  2. float(...) converts that string to a floating-point number: 20.5.

  3. The result is assigned to the variable length.

Unlike C#, there is no need to declare length first or specify its type.

wall_area = 2 * (length + width) * HEIGHT
ceiling_area = length * width

These are arithmetic assignment statements. The expression on the right-hand side is evaluated first, then the result is stored in the variable on the left. The * symbol means multiplication. In the sample run:

2 * (20.5 + 10) * 8  →  488.0

print(f"The wall area is {wall_area} square feet.")

The f before the opening quote makes this an f-string (formatted string). Inside the curly braces, {wall_area} is replaced by the current value of that variable. So with wall_area = 488.0, the output is:

The wall area is 488.0 square feet.

Whitespace#

The blank lines in the program help the human reader separate sections visually; Python ignores them. Indentation also matters in Python, but for a program without functions or loops (like this one), the statements are simply at the leftmost column. We will discuss indentation fully when we reach functions and loops.