Statements, Lines, and Why Indentation Is Not Decoration

Part I · Module 1 · Chapter 1.10 · Statements, Lines, and Why Indentation Is Not Decoration

1. One Statement, One Line

A statement is one complete instruction. In Python, one statement normally occupies one line, and the end of the line ends the statement.

print("Compile")
print("Simulate")
print("Report")

Compile
Simulate
Report

Three statements, three lines, carried out in the order written — the model from Chapter 1.1.

There is no ; at the end. If you have written C or SystemVerilog, that absence will feel wrong for about a day. Python uses the line ending itself as the separator, so a semicolon would be redundant.

2. The Rule for Everything You Can Currently Write

Here it is:

Every statement you write today must start at the very beginning of the line, with no spaces in front of it.

That is the whole rule for now. These three lines all start at column one — right at the left edge — and that is what Python expects.

print("Compile")
print("Simulate")

3. What Happens If You Break It

Add a single space in front of the second line:

print("Compile")
 print("Simulate")

Run it and you get:

  File "hello.py", line 2
    print("Simulate")
IndentationError: unexpected indent

Read what it gave you. The file, the line number, the line itself, and the name of the problem: IndentationError: unexpected indent. "Unexpected indent" means this line is pushed in and I have no reason for it to be.

Notice also that Compile did not print, even though line 1 is perfectly correct and comes first. That is Chapter 1.4 again: this is a problem with the shape of the file, so it is caught at compile time and nothing runs at all.

4. Spaces Inside a Line Are Fine

The rule is about the start of a line. Spaces elsewhere are ignored:

print( "Compile" )
print("Errors:",   0)

Compile
Errors: 0

Both are valid. Neither is good style — the usual convention is print("Compile") and a single space after each comma — but Python does not object.

So it is specifically the leading whitespace that carries meaning. Everything after the first non-space character is just spacing.

5. Why the Language Works This Way — Preview

This section explains why, using syntax you have not been taught. You are not expected to write any of this, and nothing here is examined. Skip to §6 if you would rather wait; nothing later in Module 1 depends on it.

In most languages, a group of lines that belong together is marked with symbols — braces in C, begin and end in SystemVerilog:

if (errors == 0) begin
    print_result();
    close_log();
end

Python has no such markers. Instead, the indentation itself does that job:

if errors == 0:
    print("Clean")
    print("Closing log")
print("Done")

The two indented lines belong to the if. The last line, back at the left edge, does not — it runs regardless.

That is the payoff. Indentation is not decoration in Python because it is doing the job that braces do elsewhere. Python simply requires the layout that other languages merely recommend.

Until Module 9, none of your lines have a reason to be indented. Keep everything at the left edge and you cannot hit this.

6. Tabs and Spaces

One practical warning.

Indentation can be made of spaces or tab characters, and mixing them in one file causes errors that are genuinely hard to see, because both look identical on screen.

The universal Python convention is four spaces, never tabs. Every code editor can be told to insert four spaces when you press Tab, and most do so already for .py files. If you are setting up an editor, that is the setting to check.

You will not need to indent anything until Module 9. Configure it now anyway, and the problem never arises.

7. Blank Lines

Blank lines are ignored entirely. Use them freely to separate parts of a file:

print("Compile")

print("Simulate")

print("Report")

Compile
Simulate
Report

The blank lines did nothing to the output — that is a print() with nothing in it, from Chapter 1.8 §4. They exist purely to make the file easier to read, and that is a good enough reason.

8. Predict, Run, Observe

Predict the exact output of each, then run them.

(a)

print("one")
print("two")

(b)

print("one")

print("two")

(c) — this one will fail. Predict what you will see, not just that it breaks:

print("one")
  print("two")

For (c), the useful prediction is whether one appears before the error. Chapter 1.4 §4 has the answer, and this is a good moment to confirm you believe it.

9. Exercise

Exercise 1 — cause it and read it. Take a working two-line file and put one space in front of the second line. Run it. Write down: the line number reported, the name of the error, and whether any output appeared first.

Exercise 2 — fix it blind. Have a colleague (or your past self, ten minutes later) add a stray space somewhere in a five-line file without telling you where. Run it, and use only the error message to find the line. This is the actual skill.

Exercise 3 — what is ignored. Predict which of these run without error: (a) print( "a" ) (b) print("a") (c) print("a") with trailing spaces (d) a blank line between two prints.

Exercise 4 — check your editor. Open your editor's settings and find out whether pressing Tab inserts a tab character or spaces. If it inserts a tab, change it to four spaces. Write down which it was — you have just prevented a bug you would otherwise have met in Module 9.

10. Summary

• One statement per line, and no semicolon at the end.
• Everything you can currently write starts at the left edge, with no leading spaces.
• A stray leading space is an IndentationError, not a style issue — and because it is a shape problem, nothing in the file runs, including correct lines above it.
• Only leading whitespace carries meaning. Spaces inside a line are ignored.
• Indentation does the job braces do elsewhere — grouping lines under a header. That is previewed here and taught properly in Module 9.
• Four spaces, never tabs. Set your editor now; the problem then never occurs.
• Blank lines are ignored and are worth using for readability.

One chapter left in Module 1, and it is the one that makes everything else recoverable: what to do when Python stops and prints a message at you.