Syntax and Indentation

The previous page noted that Python uses indentation to delimit blocks of code. This page goes into detail: what counts as a block, what PEP 8 has to say about style, how to keep your editor honest, and why this choice sparked one of programming's most famous debates.

Why Python chose significant whitespace

Most languages in the 1980s and 1990s used braces ({ ... }) or keywords (begin ... end) to mark blocks. Python took a different path: Guido van Rossum observed that programmers already indent code to show structure to humans, so why not make the compiler trust that indentation? The result is that:

1. Badly indented code will not run. A misaligned line is a syntax error, not a silent bug.
2. Every Python codebase looks consistent. There is no "my braces on the same line" vs "my braces on the next line" religious war.
3. Beginner-friendly. New programmers do not have to learn a special punctuation syntax; they just indent naturally.

This design choice is polarizing. Some find it liberating; others miss the explicit delimiters of C-like languages. Either way, it is non-negotiable in Python.

Blocks are indentation

In C, JavaScript, or Java, code inside a function or if is wrapped in { ... }. In Python it is wrapped in a deeper indentation level:

The colon (:) at the end of def, if, for, while, try, class, and so on signals "the next indented block belongs to me".

What happens when indentation is wrong

If indentation is missing where Python expects it, you get an IndentationError. If it is inconsistent, you get a TabError or a silently broken program (in older Python 2 codebases).

The four-space rule (PEP 8)

PEP 8, Python's official style guide, says:

• Use four spaces per indentation level.
• Do not mix tabs and spaces inside the same block. Python 3 raises a TabError if you try.
• Limit lines to 79 characters (or 99 in modern codebases that follow black's default of 88).
• Two blank lines between top-level functions and classes; one blank line between methods inside a class.
• One statement per line. if x: do(); other() is legal but frowned upon.

Tools that enforce this for you:

• black reformats your code in one consistent style with almost no configuration. It is the most popular Python formatter.
• ruff is a very fast linter that rolls up most of the older tools (flake8, isort, pyupgrade).
• Most editors (VS Code, PyCharm, Vim, Emacs) can run these on save.

The tabs vs spaces debate

We touched on this on the Python History page. The debate is older than Python itself and predates programming languages: it started with typewriters and word processors. The arguments in favor of each:

The strongest argument for tabs

The accessibility argument is real: a developer with low vision can configure their editor to display a tab as 8 spaces for easier reading, while a sighted colleague can set it to 2 spaces to fit more code on screen. With spaces, everyone is locked into the same visual width.

The strongest argument for spaces

Tabs break alignment of continuation lines. Consider:

result = some_function(arg1, arg2,
                       arg3, arg4)

If the file uses tabs, the alignment of arg3 depends on the tab width of your editor. On one machine it looks perfect; on another it is misaligned. Spaces render identically everywhere.

The community consensus

The community has effectively settled on four spaces. PEP 8 states:

Spaces are the preferred indentation method. Tabs should be used solely to remain consistent with code that is already indented with tabs.

For brand-new code, use four spaces. If you join a project that already uses tabs, stay consistent with what is there. Either way, configure your editor to insert one or the other automatically so you never produce mixed indentation.

Continuation lines

Python statements normally end at the end of a line. There are two ways to span multiple lines:

1. Implicit continuation inside (), [], or {}. This is the preferred style.
2. Explicit continuation with a backslash \ at end of line.

PEP 8 recommends avoiding backslash continuation where possible. Modern code wraps long lines in parentheses even when they are not strictly needed (e.g. for a long if condition).

Semicolons (please do not)

Python technically allows ; to separate statements on one line. The community consensus is do not use it. PEP 8 says "avoid it" in polite language; the real sentiment is "never do this".

The only exception is in one-liners passed to python -c:

python -c "import sys; print(sys.version)"

Even there, most people avoid ; and use newlines instead.

Real-world comparison: Python vs other languages

Here is how other popular languages handle indentation:

Python is unique in making indentation mandatory. Haskell has optional significant whitespace, but most Haskellers use braces to avoid the complexity. Python goes all-in.

Challenges

Test your knowledge

Now that the rules of the road are clear, we will start using them to build real programs, starting with variables and data types.