What Python Runs

Python runs code. Code is text written in a form Python understands.

For now, think of Python as a careful reader. It starts at the top, reads one instruction, runs it, and then moves to the next instruction.

Keep this distinction in view:

Python does not run your intention. It runs the exact code it receives.

If the code says print(2 + 3), Python computes 5 and displays it. If the code has a spelling mistake, Python reports an error. This is not a personal judgment; it is Python telling you which instruction it could not run.

The word print means "show this on the screen." It is one of the first tools you use to make a program visible.

Code Is a Sequence of Instructions

Consider this program:

Python runs the lines in order:

first
second
third

Line order

Run the program, then edit the words and run it again.

Runs locally with Python in your browser.

Ready to run.

The output appears in the same order because each line runs after the previous line finishes.

If you edit "first" to another word and run again, Python will show the new word. It does not remember what you meant before. It runs the code that is there now.

Expressions Produce Values

Some pieces of code produce a result:

2 + 3

The result is 5. In a script, Python does not automatically show that result. You usually use print() when you want to see it:

print(2 + 3)

This distinction matters later. A program may compute something correctly, but you will not see it unless the program prints it, plots it, saves it, or passes it to another step.

Instructions Do Work

An instruction tells Python to do something. This line is an instruction:

print("loss:", 0.42)

It tells Python to call print() with two pieces of information. The printed output is:

loss: 0.42

You will see this pattern constantly in experiments. A script computes something, then prints enough evidence for you to inspect what happened.

Code Is Not Output

Do not confuse the code with the output.

Code:

print(2 + 3)

Output:

5

The output does not contain print. The output is what happened after Python ran the instruction.

Exercise: Predict the output

In what order does this program print the letters?

Order

Select one choice, then check.

HintFollow the lines

Start at the top. Each print() finishes before Python moves to the next line.

SolutionPreserve execution order

Python first runs print("A"), then runs print("B"). Each call prints its own line, so the output is A followed by B.

The First Job

Python runs exact instructions. Your first job as a programmer is to make those instructions visible: code on one side, output on the other. When those two match your expectation, you have learned something real.