Reading Python Syntax

Note

Source: Adapted from the C# edition (data/typography.rst). Python-specific elements (REPL >>> prompt, colon-and-indent blocks, reading error messages) are original additions.

Throughout this book, we describe Python syntax using templates — patterns showing the structure of a statement with the variable parts in italics or indicated by placeholder names.

For example, the template for an assignment statement is:

variable = expression

This means: write any valid variable name, then =, then any expression. The actual code might be:

area = length * width

Indentation is Syntax

The most important thing to understand about reading Python syntax is that indentation matters. In languages like C# or Java, blocks of code are delimited by braces {}. In Python, a block is indicated by indentation — all statements in the same block are indented by the same amount.

Compare a C# if statement:

// C# — not Python
if (x > 0) {
    Console.WriteLine("positive");
}

With the Python equivalent:

if x > 0:
    print("positive")

Key points:

• The condition is followed by a colon :.

• No parentheses are required around the condition (though they are allowed).

• The body is indented — the standard is 4 spaces.

• There are no braces.

Colons Introduce Blocks

Whenever you see a :, a block follows. The block is indented relative to the line with the colon:

def greet(name):        # colon after function header
    print(f"Hello, {name}!")   # indented body

for i in range(5):      # colon after for header
    print(i)            # indented body

if x > 0:               # colon after if condition
    print("positive")   # indented body

Reading Shell Output

When we show an interactive shell session, we use >>> for the prompt and ... for continuation lines. Output appears on lines with no prompt:

>>> x = 10
>>> if x > 5:
...     print("big")
...
big

Lines beginning with $ show terminal commands run outside Python:

$ python3 painting.py
Calculation of Room Paint Requirements
Enter room length:

Reading Error Messages

When your program has an error, Python shows a traceback pointing to where the problem occurred:

Traceback (most recent call last):
  File "painting.py", line 3, in <module>
    length = float(input("Enter room length: "))
ValueError: could not convert string to float: 'abc'

Read from the bottom up: the last line tells you what went wrong; the lines above show where in the code it happened.