Syntax

Indentation is not decoration

Most languages mark a block of code with braces and indent it so humans can read it. Python drops the braces and uses the indentation itself. What is indented under a line belongs to that line.

temperature = 31

if temperature > 30:
    print("Hot")
    print("Still inside the if")

print("Always runs")

Hot
Still inside the if
Always runs

Move that last print four spaces to the right and it becomes part of the if. Nothing else about the program changes. That is the whole idea.

The colon opens a block

A line that starts a block ends with a colon, and the next line is indented. You will see this with if, for, while, def, class, with and try:

def shout(word):
    return word.upper() + "!"

print(shout("hey"))

HEY!

How much to indent

Four spaces per level. This is a convention rather than a rule — the language accepts any consistent amount — but it is the one nearly all Python code uses, and your editor will do it for you.

One statement per line

Python does not need semicolons. A newline ends a statement:

first = "Ada"
last = "Lovelace"
print(first, last)

Ada Lovelace

A long line can be broken inside brackets of any kind, and the indentation inside them is free:

names = [
    "Ada",
    "Grace",
    "Katherine",
]
print(len(names))

3

That trailing comma after the last item is legal and normal. It keeps the diff small when you add another name later.

Case matters

name, Name and NAME are three different things. So are print and Print — and only the first exists:

value = 10
Value = 20
print(value, Value)

10 20

Empty blocks need pass

A block cannot be empty, but sometimes you want a placeholder. pass is a statement that does nothing, and exists exactly for this:

def not_written_yet():
    pass

not_written_yet()
print("no error")

no error