f-Strings

The f prefix

Put f in front of the quotes and anything in curly braces is evaluated and dropped into the text:

name = "Ada"
age = 36
print(f"{name} is {age}")

Ada is 36

No str() calls, no +, and the sentence still reads like a sentence. This is the default way to build text in modern Python.

Any expression fits

The braces hold an expression, not just a name:

price = 19.99
count = 3
print(f"{count} items, {price * count:.2f} total")
print(f"{name.upper()}" if (name := "ada") else "")

3 items, 59.97 total
ADA

Formatting after the colon

A colon inside the braces introduces a format spec. This is where f-strings earn their keep:

value = 3.14159
print(f"{value:.2f}")
print(f"{value:10.2f}|")
print(f"{value:<10.2f}|")

3.14
      3.14|
3.14      |

.2f means two decimal places. The number before the dot is a minimum width, and < > ^ align left, right and centre.

Common specs

n = 1234.5678
print(f"{n:.2f}")
print(f"{n:,.2f}")
print(f"{n:>12.2f}|")

1234.57
1,234.57
     1234.57|

Percentages and padded integers come up constantly:

share = 0.3421
print(f"{share:.1%}")
print(f"{7:03d}")

34.2%
007

The = suffix for debugging

Put = after the expression and you get the expression itself as well as its value. It is built for print-debugging:

total = 42
items = 7
print(f"{total=}")
print(f"{total / items=}")

total=42
total / items=6.0

Braces and quotes

To print a literal brace, double it:

print(f"{{not a placeholder}}")

{not a placeholder}

Since Python 3.12 you may reuse the same quote character inside the braces, so this is now legal:

data = {"name": "Ada"}
print(f"{data["name"]}")

Ada

Multi-line f-strings

Triple quotes work the same way:

name = "Ada"
items = 3
print(f"""Order for {name}
Items: {items}""")

Order for Ada
Items: 3