9.6 String Formatting

Three Ways to Format

All three produce identical output; f-strings are the recommended default for new code.

% Formatting

The oldest style, borrowed from C’s printf. %s substitutes a string, %d an integer, %f a float. Still seen in older codebases and some logging configs, but largely superseded.

>>> "Server %s is at %s%% capacity" % ("web01", 87)     # %% escapes a literal %
'Server web01 is at 87% capacity'

str.format()

Replaces {} placeholders with arguments, by position or by name. More readable than %-formatting, and still common where the template string itself is loaded from a file or config.

>>> "Server {name} is at {pct}% capacity".format(name="web01", pct=87)
'Server web01 is at 87% capacity'

f-Strings

Embed expressions directly inside {} within the string literal itself, prefixed with f. The fastest and most readable option, and the recommended default for new code since Python 3.6.

>>> name, pct = "web01", 87
>>> f"Server {name} is at {pct}% capacity"
'Server web01 is at 87% capacity'

Alignment and Padding

A format spec’s <, >, and ^ characters left-align, right-align, and center-align a value within a fixed width — the basis for neatly columned console output or reports. Padding fills unused width with a character, spaces by default.

>>> f"{'Alice':<10}|{'30':<5}"     # left-align, width 10 and 5
'Alice     |30   '
>>> f"{7:03d}"                     # zero-pad an integer to width 3
'007'

Number, Currency, and Date Formatting

Format specs insert thousands separators, convert a fraction to a percentage, or apply strftime-style date codes automatically.

>>> f"{1234567:,}"           # thousands separator
'1,234,567'
>>> f"{0.4567:.2%}"          # percentage
'45.67%'
>>> f"${1234.5:,.2f}"        # currency: precision + separator + symbol
'$1,234.50'

>>> import datetime
>>> d = datetime.datetime(2026, 7, 13, 10, 22, 5)
>>> f"{d:%Y-%m-%d %H:%M:%S}"
'2026-07-13 10:22:05'

Logging Format Example

Combining date formatting with a static prefix/suffix is exactly how most log lines are built:

>>> f"[{d:%Y-%m-%d %H:%M:%S}] INFO Service started"
'[2026-07-13 10:22:05] INFO Service started'

Quick Interview Answer

“Python has three formatting styles: %-formatting (legacy, printf-style, error-prone with many arguments), .format() (readable, supports positional and named fields, good when the template itself is loaded as data), and f-strings (fastest, most readable, expressions evaluated inline — the default for new code in 3.6+). Format specs share the same mini-language across all of them for alignment (</>/^), padding, thousands separators, percentages, and strftime-style date codes.”

Common Mistakes

• Using an f-string on a template string loaded from a config file or user input — f-strings evaluate arbitrary expressions at parse time, so the template itself must be a literal in the source code; use .format() or string.Template when the template is data.
• Forgetting %% is required to output a literal % in %-style formatting, since a bare % is interpreted as a format specifier.
• Mixing positional and named .format() arguments incorrectly, or miscounting %-style positional placeholders against the tuple of values, causing a TypeError.