Python Code Formatter

Black is the most widely adopted opinionated Python code formatter. It enforces a single consistent style — double quotes, 88-character line length, specific spacing rules — so your team never debates formatting again. Black is intentionally uncompromising: you don't configure it, you just run it. The result is diffs that only contain meaningful changes, not whitespace noise. It's the de-facto standard in modern Python projects and is recommended by the Python Packaging Authority.

PEP 8 is Python's official style guide, authored by Guido van Rossum. It specifies conventions for indentation (4 spaces), line length (79 chars recommended, 99 for code), blank lines, imports, whitespace around operators, and more. Black is broadly PEP 8 compliant but makes some stricter decisions — for example, using 88-character lines (wider than PEP 8's 79) and always using double quotes. Think of Black as an opinionated, automatically enforced subset of PEP 8.

No. All formatting happens entirely in your browser using JavaScript. Your code never leaves your device. The formatter implements Black-inspired rules in pure JS: quote normalization, operator spacing, comma spacing, indentation normalization, and blank-line rules. This makes it safe for sensitive or proprietary code.

Black chose double quotes as the standard because they are more common in other languages (Java, JavaScript, Go, C#) and because single-quoted strings often need escaping when they contain contractions like "it's" or "don't". With double quotes as the default, you only need to fall back to single quotes when the string itself contains a double quote — which is rarer. This reduces the number of backslash escapes in your code.

When diff mode is active, lines in the formatted output that differ from the original input are highlighted with a yellow background. This lets you quickly see exactly what the formatter changed without reading through the entire file. It's especially useful when you paste someone else's code and want to understand the scope of changes before applying them.

Black's default maximum line length is 88 characters (compared to PEP 8's 79). Lines longer than this limit are highlighted with a red right-border indicator. Black itself would automatically wrap these lines — for example by splitting function arguments across multiple lines or using parentheses for implicit string continuation. This browser-based formatter flags them visually so you can identify and manually refactor them, since safe automatic line-wrapping requires full AST parsing.