Python Reference
str.format()
Replace {} placeholders with values. The classic string formatting method.
Last updated: October 2026
Definition: str.format(*args, **kwargs) replaces {} placeholders in a string with the provided arguments. It supports positional and keyword arguments, plus format specifications for alignment, padding, and type conversion.
🎬 Live Example
Five common format patterns. Click any button to see the output.
💡 F-strings (3.6+) are the modern preferred way. format() is still useful for templates.
Basic Syntax
"template {}".format(value)
"template {0} {1}".format(a, b)
"template {name}".format(name="Alice")
# Returns: new str
# Does NOT modify the original🎯 Common Patterns
| Pattern | Example | Result |
|---|---|---|
| Auto-index | "{} {}".format(1, 2) | '1 2' |
| Manual index | "{1} {0}".format('a', 'b') | 'b a' |
| Keyword | "{x}".format(x=5) | '5' |
| Float | "{:.2f}".format(3.14159) | '3.14' |
| Alignment | "{:>10}".format('hi') | ' hi' |
Positional Arguments
"Hello, {}!".format("World")
# 'Hello, World!'
"{} + {} = {}".format(1, 2, 3)
# '1 + 2 = 3'
"{1} {0}".format("a", "b")
# 'b a'Keyword Arguments
"{name} is {age}".format(name="Alice", age=30)
# 'Alice is 30'
# Reuse values
"{0} and {0}".format("X")
# 'X and X'
"{greeting}, {name}!".format(greeting="Hi", name="Bob")
# 'Hi, Bob!'Number Formatting
"{:.2f}".format(3.14159) # '3.14'
"{:,.2f}".format(1234567.89) # '1,234,567.89'
"{:d}".format(42) # '42'
"{:>10}".format("hi") # ' hi'
"{:<10}".format("hi") # 'hi '
"{:^10}".format("hi") # ' hi 'format vs f-string
name = "Alice"
# format()
"Hello, {}".format(name)
# f-string (preferred, 3.6+)
f"Hello, {name}"
# f-strings are faster and more readable
# Use format() for stored templatesRule of thumb: Use f-strings for inline interpolation, format() for reusable templates.
Real-World: Email Template
template = """Dear {name},
Your order #{order_id} has shipped.
Total: ${total:.2f}
Thanks,
{company}
"""
message = template.format(
name="Alice",
order_id=12345,
total=99.99,
company="Acme Inc"
)🎯 format vs f-string vs %
| Method | Version | Use when |
|---|---|---|
f-string | 3.6+ | Most inline formatting (preferred) |
format | 2.6+ | Stored/reusable templates |
% | Old | Legacy code only |
⚠️ Common Mistakes
- Mixing auto and manual indexing.
"{} {0}".format(1, 2)raises ValueError. - Forgetting the colon in format specs.
"{.2f}"is wrong — use"{:.2f}". - Using format when f-strings are clearer. Prefer f-strings for inline cases.
- Assuming format modifies the string. Returns a new string; original unchanged.
- Mismatching placeholder count. Extra args are ignored; missing args raise KeyError/IndexError.
❓ Frequently Asked Questions
What does format() do in Python?
str.format() replaces placeholders {} in a string with the provided arguments. It supports positional and keyword arguments, plus format specifications for alignment, padding, and type conversion.
What is the difference between format and f-strings?
F-strings (Python 3.6+) are more concise and faster for inline interpolation. format() is still useful when the template is defined separately from the values or when you need to reuse the same template multiple times.
How do I use positional arguments in format?
Use empty braces {} for automatic order, or numbered braces {0} {1} to reference arguments by index. '{1} {0}'.format('a', 'b') returns 'b a'.
How do I use keyword arguments in format?
Pass keyword arguments and reference them in braces: '{name} is {age}'.format(name='Alice', age=30). This is useful for templates where the order of arguments is unclear.
Does format modify the original string?
No. Strings are immutable in Python. format returns a new string.