Python Format Specifications

Intermediate

Python already has f-string coverage in the Strings lesson. This lesson goes one step deeper: it teaches the format_spec part that controls how a value is presented, and the built-in format() function that applies a format specification directly.

Start with format()

The built-in format(value, format_spec) asks the value to produce its formatted representation using the supplied format specification. With an empty specification, the result normally matches str(value).

Control width and alignment

A width reserves space for the formatted value. Alignment can be left, centered, or right. If you do not choose an alignment, the default depends on the value type; for text, left alignment is common, while numbers are normally right-aligned.

Choose a fill character

A fill character can make unused width visible. It is written together with an alignment option, so the first character supplies the fill and the second supplies the alignment.

Format numbers precisely

Numeric format specifications can control decimal precision and presentation. For example, .2f asks for two digits after the decimal point, while , requests grouping separators for supported numeric types.

Build a specification from values

A format specification can itself contain a value when it is used inside a formatted string. This is useful when the width or precision is part of the program state.

Conversion is different from formatting

In a formatted string, !r, !s, and !a choose a conversion before the format specification is applied. The conversion answers “which representation should be formatted?”; the format specification answers “how should that representation or value be presented?”

Keep formatting separate from f-string syntax

The Strings lesson owns how f-strings insert expressions into text. This lesson owns the formatting specification itself and the format() built-in. The same specification syntax can be used from f-strings, str.format(), and format(), so learning the specification once pays off in several contexts.

Quick check

result = ?