Programming Fundamentals › Programming Basics

String Interpolation

Embedding values directly inside a string literal.

Also known as: string formatting, template literal, f-string

String interpolation puts values directly inside a string, instead of joining pieces with +.

name, count = "Ana", 3
f"Hello {name}, you have {count} messages"       # f-string (Python)

`Hello ${name}, you have ${count} messages`      // template literal (JavaScript)

Compare with concatenation:

"Hello " + name + ", you have " + str(count) + " messages"

The interpolated version is easier to read, and in Python you don’t need str() for each number.

Expressions and formatting

f"Total: {price * qty:.2f}"        # 2 decimal places
f"{name!r}"                         # the repr, handy for debugging

`Total: ${(price * qty).toFixed(2)}`

Other styles

• Python: "Hi {}".format(name), and the older "Hi %s" % name.
• Java: String.format("Hi %s", name).
• Languages with no built-in interpolation use a formatting function.

Cautions

• Don’t interpolate untrusted input into SQL, HTML or shell commands. Use parameterized queries and escaping, or you invite injection (SQL injection, XSS).
• Don’t log secrets by interpolating whole objects.
• Translatable text: building sentences from pieces breaks in other languages. Use a message template with named placeholders (internationalization).
• Dates and numbers have locale-specific formats. Use formatting functions, not manual string work.
• In logging, prefer the logger’s own formatting (log.info("order %s", order_id)) so formatting is skipped when the level is off.