What is Python's format specification mini-language?

It is the small grammar that controls how a value is rendered inside an f-string replacement field or str.format placeholder, written after a colon. The full form is [[fill]align][sign][#][0][width][grouping][.precision][type]. For example f'{x:>+08.2f}' right-aligns, forces a sign, zero-pads to width 8 with two decimals as a fixed-point float.

What is the difference between the f, e, and g type codes?

f formats a number as fixed-point with a fixed number of decimals. e uses scientific notation with an exponent. g chooses between fixed and scientific automatically based on magnitude and the precision, trimming trailing zeros. Use f for currency-style output, e for very large or small numbers, and g for compact general-purpose display.

How do I add thousands separators?

Put a comma or an underscore in the grouping position, before the precision. The comma groups by thousands with commas, as in f'{n:,}' giving 1,000,000, while the underscore uses underscores. With the comma you can combine width and precision, like f'{n:12,.2f}'. The underscore option also works with binary, octal, and hex types to group every four digits.

What does the zero before the width do?

A 0 immediately before the width enables sign-aware zero padding: the number is padded on the left with zeros to fill the width, and any sign sits outside the zeros. f'{-7:05d}' produces -0007. It is shorthand equivalent to a fill of 0 with the = alignment, which places padding between the sign and the digits.

Does this cover f-strings and str.format the same way?

Yes. F-strings and the str.format method share the identical format specification mini-language; the only difference is where the value comes from. The spec after the colon — fill, align, sign, width, precision, and type — behaves the same in f'{x:.2f}' and '{:.2f}'.format(x), so this reference and parser apply to both.

Python Format String Reference

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Python’s format specification mini-language controls how a value is rendered inside an f-string or str.format placeholder, written after a colon. This tool decodes any spec you paste and lists every fill, align, sign, and type code.

How it works

The full grammar is:

[[fill]align][sign][#][0][width][grouping][.precision][type]

Each component is optional and parsed left to right:

fill + align — a pad character and one of < (left), > (right), ^ (center), = (pad after the sign). Fill requires an explicit align character.
sign — + (always show), - (only negative, default), space (leading space for positives).
# — alternate form: 0b/0o/0x prefixes for integers, always-show decimal point for floats.
0 — sign-aware zero padding (shorthand for 0=).
width — minimum field width.
grouping — , or _ thousands separator.
.precision — digits after the decimal (floats) or max characters (strings).
type — d b o x f e g % s etc.

The parser below splits your spec into exactly these parts and explains each.

Example

f"{1234.5:>+12,.2f}"   # '   +1,234.50'
f"{255:#06x}"           # '0x00ff'
f"{0.27:.1%}"           # '27.0%'

Notes

A leading 0 enables sign-aware zero padding: f"{-7:05d}" -> -0007.
g switches between fixed and scientific automatically; n is locale-aware.
F-strings and str.format share this identical mini-language.