f-strings and formatting

Gluing strings together with + and str() gets untidy fast. An f-string lets you write the sentence you want and drop values into it, with fine control over how each value looks.

This lesson needs Python 3.12 or later only for the last section. Everything before it runs on 3.8 and up.

Expressions in braces

Put an f before the opening quote. Anything inside {} is evaluated as a Python expression and its result is inserted.

name = "Ada"
print(f"Hello, {name}!")
print(f"2 + 3 is {2 + 3}")
print(f"{{literal braces}}")

Hello, Ada!
2 + 3 is 5
{literal braces}

Use {{ and }} when you want literal braces in the output.

The format spec

After a colon inside the braces comes a format spec, a small language for presentation. Its general shape is [align][sign][#][0][width][,][.precision][type]. Here are the pieces you will use constantly, one group at a time.

Numbers first: thousands separators, decimal places and percentages.

price = 1234.5
ratio = 0.256
print(f"{price:,.2f}")
print(f"{ratio:.1%}")
print(f"[{42:05d}]")

1,234.50
25.6%
[00042]

,.2f means a thousands separator and 2 decimal places. .1% multiplies by 100 and adds %. 05d pads an integer with zeros to width 5.

Alignment comes next. A number after the alignment character sets the width of the field:

name = "Ada"
print(f"[{name:>8}]")
print(f"[{name:<8}]")
print(f"[{name:^8}]")

[     Ada]
[Ada     ]
[  Ada   ]

> aligns right, < left and ^ centres. Numbers align right by default and text aligns left.

For other number bases, use a type letter:

print(f"{255:x}")
print(f"{255:#b}")

ff
0b11111111

Debugging with = and !r

Adding = after an expression prints the expression text, then its value. It has worked since Python 3.8 and is the quickest way to inspect a value while you debug.

price = 1234.5
print(f"{price=}")
print(f"{2 + 3 = }")

price=1234.5
2 + 3 = 5

!r applies repr() to the value, which shows quotes around strings and makes invisible characters visible. This is usually what you want in log messages and error text.

name = "Ada"
note = "two\nlines"
print(f"{name!r}")
print(f"{note!r}")
print(f"{note}")

'Ada'
'two\nlines'
two
lines

Printing a value together with its name, as {price=} does, is called print debugging, and the loops module uses it to find bugs.

Quotes and line breaks

3.12+Requires version 3.12 or later Before Python 3.12 you could not reuse the surrounding quote character inside the braces, nor put a comment or newline in the expression. From 3.12 onwards (PEP 701) f-strings are part of the normal grammar, so all of that works.

print(f"{len("Python")} letters")

report = f"""Total: {
    2 + 3
}"""
print(report)

6 letters
Total: 5

The first line reuses the double quote inside the braces. On Python 3.11 or earlier, both examples would be a SyntaxError. Even on 3.12 and later, reusing the outer quote can make a line hard to read. Switching quote style, or computing the value on its own line first, is often clearer.

Replacement fields can also be nested inside the format spec, so that the width and precision come from variables. This is not new in 3.12, and it works on older versions as well.

width = 10
precision = 3
value = 3.14159265
print(f"{value:{width}.{precision}f}|")

     3.142|

Common pitfalls

• Forgetting the f. "Hello {name}" prints the braces literally.
• Using f-strings for text you do not control. An f-string is code in your source file, not a template you can load at run time. Use str.format() or a template library for that.
• Rounding surprises. f"{2.675:.2f}" gives 2.67 for the same reason as round() in the numbers lesson.
• Formatting a string with a number spec. f"{'abc':d}" raises ValueError. The type letter must match the value.

price = 1234.5
ratio = 0.256
print(f"${price:,.2f} is {ratio:.1%} of the budget")

$1,234.50 is 25.6% of the budget

Item           Qty    Price    Total
------------------------------------
Coffee           2     3.50     7.00
Bagel            1     2.25     2.25
Orange juice     3     4.00    12.00
------------------------------------
Total                          21.25

name1, qty1, price1 = "Coffee", 2, 3.5
name2, qty2, price2 = "Bagel", 1, 2.25
name3, qty3, price3 = "Orange juice", 3, 4.0

total1 = qty1 * price1
total2 = qty2 * price2
total3 = qty3 * price3

print(f"{'Item':<14}{'Qty':>4}{'Price':>9}{'Total':>9}")
print("-" * 36)
print(f"{name1:<14}{qty1:>4}{price1:>9.2f}{total1:>9.2f}")
print(f"{name2:<14}{qty2:>4}{price2:>9.2f}{total2:>9.2f}")
print(f"{name3:<14}{qty3:>4}{price3:>9.2f}{total3:>9.2f}")
print("-" * 36)
print(f"{'Total':<27}{total1 + total2 + total3:>9.2f}")

Item           Qty    Price    Total
------------------------------------
Coffee           2     3.50     7.00
Bagel            1     2.25     2.25
Orange juice     3     4.00    12.00
------------------------------------
Total                          21.25

Summary

• f"...{expression:spec}..." inserts values, and the spec controls alignment, width, precision and separators.
• {x=} and {x!r} are debugging helpers.
• 3.12+Requires version 3.12 or later Quotes can be reused inside the braces, and expressions can span lines.

Next: string methods.