How Python Reads and Runs Code

Learn how Python follows instructions in order, evaluates expressions, and makes results visible with print(). Trace a short program line by line and keep its source code separate from its output.

For the small programs in this chapter, Python begins with the first line and works downward. It runs one instruction before moving to the next.

This gives us an important distinction:

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

If the code says print(2 + 3), Python adds the numbers before displaying 5. If the code contains something Python cannot understand or run, Python reports an error. The error gives us information about where execution stopped.

The built-in function print() displays a value as text. A function is a named piece of code that performs a task. In this book, displayed output appears below the code that produced it.

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.

Ready to run.

We can trace the program without running it:

Line	Instruction	New output
1	display `first`	`first`
2	display `second`	`second`
3	display `third`	`third`

The output has the same order as the instructions because the three calls run from top to bottom.

If you edit "first" to another word and run the program again, the output changes with the code. Python runs the program that exists at that moment, not an earlier version or an unstated intention.

Expressions Produce Values

An expression is code that produces a value. This arithmetic expression produces the number 5:

2 + 3

Evaluating an expression means working out its value. In a script, Python does not automatically display that value. You usually pass it to print() when you want to see it:

print(2 + 3)

This distinction matters later. A program may compute a value correctly without displaying it. The program can print the value, plot it, save it, or pass it to another step.

The following program evaluates 2 + 3 twice. The first result is not sent to the output. The second result is passed to print(), so it becomes visible.

A value and visible output

Run the program, then change both additions and compare what appears.

Ready to run.

Printing More Than One Value

print() can receive more than one value:

print("loss:", 0.42)

Python first evaluates each value inside the parentheses. By default, print() places a space between them, so the output is:

loss: 0.42

You will see this pattern in later experiments. A script computes something, then prints selected values so you can inspect what happened.

Code Is Not Output

Code and output play different roles.

Code:

print(2 + 3)

Output:

The output does not contain print, the parentheses, or the expression 2 + 3. Those are parts of the source code. The output contains the value that the program displayed after evaluating the expression.

Exercise: Predict the complete output

What does this program display?

Output

start, then 5, then donestart, then 2 + 3, then donedone, then 5, then start

Select one choice, then check.

HintSeparate code from output

Follow the three print() calls from top to bottom. The middle call prints the value produced by 2 + 3, not the expression's source text.

SolutionEvaluate, then print

The first call prints start. The second evaluates 2 + 3 and prints 5. The third prints done. Each call ends its output with a new line.

A Useful Habit

When you read a small program, keep the code and its output separate. Predict the output from the code, run the program, and compare the actual output with your prediction. A difference between them points to something in the code or in your understanding that is worth inspecting.

The next lesson shows how to check whether a Python runtime is available on your computer and how to install one if it is not.