Strings

String Formatting

f-strings, format(), % formatting

Interview: Modern Python practices

Last Updated: June 12, 2026 • 7 min read

Python offers several ways to format strings, but f-strings (Python 3.6+) are the modern standard. They're faster, more readable, and support inline expressions and format specifications.

F-Strings (Python 3.6+)

Prefix with f or F before the string
Embed expressions in {curly braces}
Support format specifiers: {value:.2f}, {name:>10}
Can call functions, access dict keys, and use expressions: {len(items)}
Python 3.8+ supports {expr=} for debugging: f"{x=}" outputs "x=5"
Python 3.12+ allows f-string nesting and backslashes inside braces

Format Specification Mini-Language

{v:.2f} — 2 decimal places (3.14)
{v:.2e} — scientific notation (3.14e+00)
{v:,} — thousands separator (1,234,567)
{v:.1%} — percentage (75.0%)
{v:>10} / {v:<10} / {v:^10} — alignment
{v:08b} — binary, zero-padded (00001010)
{v:#x} — hex with prefix (0xff)

Legacy Formatting

str.format(): "{0} is {1}".format(name, age) — still used in older codebases
% operator: "%s is %d" % (name, age) — C-style, avoid in new code
Template strings: Template("$name is $age").substitute(name=name, age=age) — safe for user input

Use Cases

Building user-facing messages and reports

Formatting numerical data for display (currency, percentages)

Debug logging with f-string debug syntax (Python 3.8+)

Safe template rendering with untrusted user input

Common Mistakes

Using % formatting or .format() when f-strings are available

Forgetting that f-strings are evaluated at creation time (not lazy)

Not using format specifiers for alignment in tabular output

Using f-strings with untrusted input (use Template instead)