Python f-Strings

3f-Strings: explained step by step

An f-string is a string that can hold Python expressions. Put the letter f in front of the opening quote, write any value or calculation inside {}, and Python replaces each pair of braces with the result. It is the cleanest way to mix text and values, and it is what you will use for almost every message, report and log line you print. If you are new to text values, start with Python strings and come back.

How f-string interpolation works

Everything inside the braces is a normal Python expression, evaluated at the moment the line runs. That means variables, arithmetic, function calls and method calls all work.

name = "Asha"
items = 3
price = 4.5

print(f"{name} bought {items} items")
print(f"Total: {items * price}")
print(f"Shouting: {name.upper()}, letters: {len(name)}")
print(f"Is it a bulk order? {items >= 10}")

Unlike joining with +, you never convert numbers yourself: the f-string turns each result into text for you. That avoids the classic TypeError from adding a string to an int. For more text tools such as upper(), see Python string methods.

Format specifiers: control how a value looks

After the expression you can add a colon and a format spec. The expression says what to show, the spec says how. The most useful ones for numbers are:

price = 1234.5
ratio = 0.256
order_no = 42

print(f"{price:.2f}")      # 2 decimal places
print(f"{price:,.2f}")     # thousands separator
print(f"{ratio:.1%}")      # percentage (multiplies by 100)
print(f"{order_no:05d}")   # pad with zeros to 5 digits
print(f"{255:b} {255:x} {255:X} {255:o}")   # binary, hex, octal
print(f"{12345.678:e}")    # scientific notation

Read .2f as "2 digits after the point, fixed notation". Rounding happens in the display only; the variable itself is unchanged. For rounding and other number tools see Python numbers.

Align and pad text with width

A number in the spec sets the width. Add <, > or ^ to left-align, right-align or centre inside that width, and put a character before the alignment symbol to use it as padding. This is how you build tidy columns without any extra libraries.

rows = [("Tea", 3.5), ("Coffee", 12), ("Cake", 4.25)]

for name, price in rows:
    print(f"{name:<8}|{price:>8.2f}|")

print(f"{'TOTAL':*^17}")
print(f"{'left':-<10}{'right':->10}")

Strings default to left alignment and numbers to right alignment, so you only need the symbol when you want the opposite.

Width and precision from variables

Format specs can contain their own braces, so the width does not have to be a fixed number. This is handy when column sizes depend on your data.

names = ["Asha", "Benjamin", "Li"]
width = max(len(n) for n in names) + 2
digits = 1

for n in names:
    print(f"[{n:<{width}}]")
print(f"{3.14159:{width}.{digits}f}")

Debug with the = specifier

Add = after an expression and the f-string prints the expression text, an equals sign and the value. It saves you from typing every label by hand while you debug.

total = 19.99
count = 3
print(f"{total=}")
print(f"{count * total=:.2f}")
print(f"{total = }")

Quotes, braces and multi-line f-strings

Since Python 3.12 you may reuse the same quote type inside the braces, and backslashes are allowed there too. In older versions you had to use different quotes. Mixing quotes is still the more readable choice.

To print a literal brace, double it: {{ and }}. For a long message, wrap adjacent f-strings in parentheses; each piece needs its own f.

user = {"name": "Ben", "plan": "pro"}
print(f"{user['name']} is on {user["plan"]}")
print(f"Braces: {{plan}} means {user['plan']}")

message = (
    f"Hello {user['name']},\n"
    f"your plan is {user['plan']}."
)
print(message)

Looking up dictionary values this way pairs well with Python dictionaries.

The !r conversion and formatting dates

!r shows the repr() of a value, which includes quotes and escape characters. It is the quickest way to spot stray spaces or newlines in text. Date and time objects accept strftime codes directly in the spec, so you do not need a separate call (see the datetime module).

from datetime import date

name = " Asha\n"
print(f"{name!r}")
launch = date(2026, 3, 9)
print(f"{launch:%d %B %Y}")
print(f"{launch:%A, %Y-%m-%d}")

f-strings compared with format() and %

All three styles give the same result. f-strings win on readability because the value sits right where it appears in the text.

name, score = "Asha", 91.456
f"{name} scored {score:.1f}"             # f-string (best)
"{} scored {:.1f}".format(name, score)   # str.format()
"%s scored %.1f" % (name, score)         # old % style

Use str.format() only when the template is stored separately from the values, such as text loaded from a file. If a spec fails with a ValueError (for example :.2f on a string), see Python ValueError, and for reading values from users see input and output.