How do you format strings in Python?

Question

Accepted Answer

Python has several ways to build strings with dynamic values. The modern, recommended approach is the **f-string** (formatted string literal), introduced in Python 3.6.

## f-strings — the modern default

```python
name = "Ann"
age = 30

msg = f"{name} is {age} years old"   # "Ann is 30 years old"

# any expression works inside the braces
f"{name.upper()} in {age * 12} months"   # "ANN in 360 months"
f"Total: {price * quantity:.2f}"          # format spec: 2 decimals
```

The `f` prefix lets you embed expressions directly in `{}` — concise, readable, and fast. This is the preferred method in modern Python.

## Format specifiers — control the output

```python
f"{3.14159:.2f}"     # "3.14"      — 2 decimal places
f"{1000000:,}"        # "1,000,000" — thousands separator
f"{0.85:.1%}"         # "85.0%"     — percentage
f"{42:05d}"           # "00042"     — pad with zeros to width 5
f"{'hi':>10}"         # "        hi" — right-align in width 10
f"{'hi':^10}"         # "    hi    " — center
```

The `:spec` after the value controls precision, padding, alignment, and number formatting — powerful for clean output.

## Debugging shortcut (3.8+)

```python
f"{name=}"            # "name='Ann'" — prints both the expression AND value
```

The `=` is handy for quick debugging — it shows the variable name and its value.

## Older methods (you'll still see them)

```python
"{} is {}".format(name, age)     # str.format() — pre-f-string standard
"%s is %d" % (name, age)          # %-formatting — old C-style (avoid in new code)
name + " is " + str(age)          # concatenation — verbose, error-prone
```

`.format()` is still common in older code; `%`-formatting is legacy; raw concatenation is discouraged (verbose and requires manual `str()` conversion).

## Why it matters

String formatting is needed constantly — building messages, logs, output, queries.

F-strings are the modern, readable, and efficient standard, and knowing their format specifiers (precision, padding, alignment, separators) lets you produce clean, professional output without verbose code.

Recognizing the older `.format()` and `%` styles helps when reading existing codebases.

Using f-strings well is a small but pervasive part of writing clear, idiomatic Python.