PHASE 0Beginner ~28 min· topic 6 of 8

Topic 0.6

Anatomy of a Python Program

In one line

A Python program is a sequence of statements that run from top to bottom. Indentation (four spaces) is part of the grammar and shows which lines belong together, # starts a comment, and capital and small letters are different.

Think of it like this

An outline with sub-points. In written notes you push sub-points to the right under their heading, and everyone understands they belong to it. Python uses exactly that idea as grammar: lines pushed to the right belong to the line above them that ends with a colon.

Words you'll meet

New words in this topic, in plain English. Come back here whenever one feels fuzzy.

Statement: One complete instruction, usually one line.
Block: A group of indented lines that belong to the line above them ending in a colon.
Indentation: The spaces at the start of a line. Python uses it to decide which block a line is in.
Comment: A note starting with #. Python ignores it; people read it.
Case-sensitive: Capital and small letters count as different, so Total and total are different names.
Whitespace: Spaces, tabs and line breaks.
Colon: The : character. At the end of a line it announces that an indented block follows.
Keyword: A word with a fixed meaning in Python, such as if, for or else. You cannot use keywords as names.

Step by step

01Statements run top to bottom

This program has three statements. Python runs the first, then the second, then the third. The blank line and the comment are skipped.

price = 40 remembers 40 under the name price (a variable, Topic 1.1). * multiplies. Notice the spaces around = and *. They are optional and make the line easier to read.

main.pywhole filepython

price = 40

# two items at the same price
total = price * 2
print("Total:", total)

02A colon starts a block

An if statement runs some lines only when a condition is true. A condition is a question with a yes or no answer; here temperature > 30 asks "is the temperature greater than 30?". The if line ends with a colon, and the lines it controls are indented by four spaces. Those two indented lines are the block.

The last line is back at the left edge, so it is not part of the block. It runs whatever the temperature is.

main.pywhole filepython

temperature = 31

if temperature > 30:
    print("It is hot.")           # in the block
    print("Drink some water.")    # in the block
print("Have a good day.")         # after the block: always runs

A colon starts a blockdiagram

Rendering diagram…

03Indentation changes the meaning

Move one line left by four spaces and the program means something else. Below, Drink some water. is no longer inside the if, so it prints on cold days too.

No error appears, because both versions are valid Python. Indentation is not decoration: it is how you tell Python what belongs to what.

main.pywhole filepython

temperature = 12

if temperature > 30:
    print("It is hot.")
print("Drink some water.")        # moved left: now always runs
print("Have a good day.")

04Blocks inside blocks

A block can contain another line ending in a colon, which starts a deeper block, indented by four more spaces. Each level of indentation is one level of "belongs to".

In this example for day in ["Sat", "Sun"]: runs its block once for each day in the list. day == "Sun" asks "is day equal to Sun?" (two equals signs ask a question; one equals sign remembers a value).

Read the left edge of the code like a staircase: every step right goes inside something, every step left comes back out.

main.pywhole filepython

for day in ["Sat", "Sun"]:
    print(day)                    # level 1: inside the for
    if day == "Sun":
        print("  Rest day")       # level 2: inside the if, inside the for
print("Done")                     # level 0: after everything

05Comments

Everything after a # on a line is ignored. A comment can have its own line or follow code on the same line. A # inside quotes is ordinary text, not a comment.

Programmers also "comment out" a line (put # in front of it) to switch it off for a moment without deleting it.

main.pywhole filepython

# Prices include tax, so no tax is added here.
price = 40          # rupees

print("Item #1:", price)      # the # inside the quotes is just text
# print("This line is switched off")

06Long statements across lines

Inside brackets, a statement may run over as many lines as you like. Use this for long lists and long sums. A comma after the last item is allowed and makes adding another line easier. The square brackets in the second part make a list, a row of values (Phase 4).

Outside brackets, a backslash \ at the very end of a line also continues it, but brackets are the preferred way. Python also allows two statements on one line separated by ;, and you should almost never do that.

main.pywhole filepython

total = (10
         + 20
         + 30)
print(total)

names = [
    "Asha",
    "Ravi",
    "Meera",
]
print(names)

07Capital letters matter

Name and name are different names. If you store a value as Name and then ask for name, Python says it has never heard of name. Recent versions even suggest the name you probably meant.

By convention, ordinary names in Python are written in small letters with underscores between words, like total_price. Topic 1.1 covers naming.

main.pywhole filepython

Name = "Asha"
print(name)       # NameError: name 'name' is not defined. Did you mean: 'Name'?

Try it yourself

1
Move a line in and out
In the first example, set temperature = 12. Run it. Then remove the four spaces before print("Drink some water.") and run again. Explain the difference in the output in one sentence.
2
Go one level deeper
In the second example, add a line print(" Sleep late") so that it prints only on Sunday, straight after Rest day. How many spaces does it need at the start?
3
Switch a line off
In the third example, put a # in front of print(total) and run it. Then remove the # from the switched-off line and run again.

Code & diagrams

A block is the indented lines New tab

Change 31 to 12 and predict the output before running. Which line still prints?

Expected output

It is hot.
Drink some water.
Have a good day.

Blocks inside blocks New tab

Count the spaces at the start of each line: 0, 4 or 8. Each level is one more "inside".

Expected output

Sat
Sun
  Rest day
Done

Comments, blank lines and long statements New tab

Python skips comments and blank lines. Inside brackets, one statement can cover several lines.

Expected output

Item #1: 40
60
['Asha', 'Ravi', 'Meera']

How Python reads indentationdiagram

Rendering diagram…

Break it on purpose

Errors are the best teachers. Make each change, read the error, guess what went wrong, then reveal the answer.

Break #1

Indent a line for no reason

Put two spaces in front of the second line of a simple program.

main.pywhole filepython

print("start")
  print("oops")

terminal

$ python main.py

── what you'll see ──

File "main.py", line 2

print("oops")

IndentationError: unexpected indent

Break #2

Come back to an indentation that doesn't exist

Indent a block by four spaces, then indent the next line by only two.

main.pywhole filepython

if True:
    print("a")
  print("b")

terminal

$ python main.py

── what you'll see ──

File "main.py", line 3

print("b")

IndentationError: unindent does not match any outer indentation level

Break #3

Forget the colon

Write an if line without the colon at the end.

main.pywhole filepython

if 5 > 3
    print("yes")

terminal

$ python main.py

── what you'll see ──

File "main.py", line 1

if 5 > 3

SyntaxError: expected ':'

Myth vs fact

Myth

Indentation in Python is just for neatness.

Fact

It is grammar. Changing the indentation changes which lines belong to an if or a loop, and so changes what the program does.

Myth

Tabs and spaces are the same thing.

Fact

They are different characters that can look identical. Mixing them in one block gives TabError: inconsistent use of tabs and spaces in indentation. Use four spaces everywhere.

Myth

More comments always make code better.

Fact

A comment that repeats the code (# add 1 to x) is noise and goes stale. Comment the reasons and the surprises; let clear names explain the rest.

Myth

Python needs a main function like Java or C.

Fact

A Python file runs from its first line. The if __name__ == "__main__": pattern you will see later (Topic 6.2) is a convention for files that are also imported, not a requirement.

Pro corner

Extra depth for experienced readers. New to this? Skip it for now and come back later.

▸
The tokenizer turns indentation into explicit INDENT and DEDENT tokens before the parser sees the code. That is why an indentation mistake is reported as a syntax-level error before any line of the file runs.
▸
Inside brackets, indentation is not significant at all, which is why multi-line lists, calls and expressions can be laid out freely. Formatters such as Ruff (Topic 11.6) apply one consistent layout so that teams never argue about it.
▸
A lone string at the top of a file, function or class is a docstring, not a comment: it is stored on the object as __doc__ and shown by help(). Triple-quoted strings used to "comment out" code are real expressions that Python evaluates and discards.
▸
The first line of a script on macOS and Linux is often #!/usr/bin/env python3, a "shebang" that lets the file be run directly. To Python it is just a comment.

Remember this

1
A program is made of statements, normally one per line. There is no semicolon at the end of a line. You also don't need a wrapping main function (languages such as Java and C require one). Python starts at the first line of the file and works downward.
2
Indentation is syntax. A line ending in a colon (:), such as an if (makes a decision) or a for (repeats lines), starts a block: the group of lines it controls. Phase 2 teaches both. The block is every following line that is indented further. The block ends at the first line that returns to the earlier indentation. Other languages mark blocks with { and }; Python uses the spaces themselves.
3
The standard is four spaces per level. Every line in a block must be indented by exactly the same amount. Don't mix tabs and spaces: they can look identical on screen and Python rejects the mix. Any code editor set up for Python inserts four spaces when you press the Tab key.
4
A comment starts with # and runs to the end of the line. Python ignores it completely; it is a note for human readers. Good comments explain why, not what. Blank lines are also ignored and are used to separate steps visually.
5
Python is case-sensitive: name, Name and NAME are three different names, and print works while Print does not. Spaces inside a line mostly don't matter (2+3 and 2 + 3 are the same); spaces at the start of a line always matter.
6
A statement can continue over several lines when it is inside brackets: ( ), [ ] or { }. Python knows the statement isn't finished until the bracket closes. This is the normal way to write a long list or a long calculation readably.

Explain it without notes

How does Python know which lines belong to an if?

What is the difference between a syntax mistake in indentation and a logic mistake in indentation?

What is a comment for, and what makes a good one?

How can one statement be spread over several lines?

Practice

Write a program that sets marks = 82. Only if marks >= 50, it prints Well done! and You passed. (two indented lines under an if). Then it always prints Report finished.

This program has two indentation mistakes. Line 1 is for n in [1, 2, 3]:, which repeats its block once for each number. Line 2 is print(n) with no spaces in front. Line 3 is print("Done") with four spaces in front. Fix the spaces so the program prints 1, 2, 3 and then Done once.

Write a list of five fruit names over five lines inside square brackets, then print the list.

Trade-offs

↔
Indentation as syntax. The code you see is the code Python runs: layout cannot lie about structure, and programs look alike across teams. The cost is that copying code from a web page or chat can silently damage the indentation, and you must fix it before it runs.
↔
Comments vs clear code. Comments explain intent but can go out of date. Clear names and small functions explain themselves and cannot go stale. Prefer clear code, and comment the why.

Done when you can

Done when you can point to the block of an if or for and say where it ends.
Done when you can predict how moving a line left or right changes a program.
Done when you can explain the three errors: unexpected indent, unindent does not match, expected ':'.
Done when you can write a comment and say what a good comment contains.
Done when you can split a long list over several lines.

Command Palette

Words you'll meet

Step by step

01Statements run top to bottom

02A colon starts a block

03Indentation changes the meaning

04Blocks inside blocks

05Comments

06Long statements across lines

07Capital letters matter

Try it yourself

Move a line in and out

Go one level deeper

Switch a line off

Code & diagrams

Break it on purpose

Indent a line for no reason

Come back to an indentation that doesn't exist

Forget the colon

Myth vs fact

Pro corner

Remember this

Explain it without notes

Practice

Trade-offs

Done when you can