Python String Formatting

Python String Formatting: F-Strings and Format Specifiers

Python string formatting embeds variables and formatted numbers inside text templates cleanly. Since Python 3.6, f-strings (f"...") are the fastest, clearest way to format strings without counting positional placeholders.

Inside an f-string, place a colon : after any expression to control decimals, commas, alignment, or percentages in one compact specifier.

Formatting Numbers, Percentages, and Alignment

You can format currency with two decimal places and thousands separators, turn ratios into percentages, and pad table columns cleanly.

product = "SSD 1TB"
price = 1249.5
margin = 0.184
units = 7

print(f"{product:<12} | ${price:,.2f} | Margin: {margin:.1%}")
print(f"Batch #{units:04d}")  # Batch #0007

In Python 3.8+, adding = after a variable name prints both the expression and its value for quick debugging: print(f"{price=}, {units=}").

When .format() Still Helps

F-strings evaluate immediately where you write them. If you load a reusable template string from a configuration file or translation table, store "Hello, {name}!" and call template.format(name=user) later.

Common Mistakes to Avoid

• Using old % formatting in new code: "%s: %d" % (a, b) fails awkwardly if you pass a tuple value and is harder to read than f-strings.
• Formatting untrusted user strings with .format(): Never let users control the template passed to .format(), because format strings can inspect object attributes.
• Forgetting {{ and }} for literal braces: Inside an f-string, double curly braces {{ and }} output literal { and } characters.

Frequently Asked Questions

Are f-strings faster than + concatenation or .format()?

Yes. CPython compiles f-strings directly into optimized bytecode instructions, making them faster than both .format() and % formatting.

Can I put function calls or math inside {...}?

Yes. Any valid Python expression works inside braces, such as f"Total: {qty * unit_price:.2f}".

What changed for f-strings in Python 3.12?

Python 3.12 lifted earlier parser limits, allowing quotes matching the outer f-string, comments, and backslashes inside {...} expressions.