F-Strings

Building a string out of variables used to mean chaining + and str() calls, or reaching for .format(). Since Python 3.6, f-strings do it more directly, and they're now the standard way to format strings in Python.

The basics

Prefix a string with f and embed any expression in {}:

name = "Ada"
age = 25
print(f"{name} is {age} years old.")
# Ada is 25 years old.

Compare that to the concatenation approach it replaced:

print(name + " is " + str(age) + " years old.")

The f-string version needs no manual str() conversion — Python converts whatever's inside the braces automatically — and it reads in the same order you'd say it out loud.

Any expression works inside the braces

It's not limited to plain variable names:

price = 19.99
quantity = 3
print(f"Total: {price * quantity}")
# Total: 59.97

items = ["eggs", "milk", "bread"]
print(f"You have {len(items)} items")
# You have 3 items

You can even call functions or access dictionary keys and object attributes directly inside the braces.

Formatting numbers

A colon after the expression introduces a format spec, controlling things like decimal places, padding, and thousands separators:

price = 19.9
print(f"${price:.2f}")       # $19.90 -- 2 decimal places
print(f"{1000000:,}")        # 1,000,000 -- thousands separator
print(f"{0.856:.1%}")        # 85.6% -- percentage
print(f"{42:05d}")           # 00042 -- zero-padded to 5 digits

.2f means "fixed-point with 2 decimal places" — invaluable for displaying currency without manual rounding logic.

Debugging with =

A lesser-known but genuinely useful trick: adding = inside the braces prints both the expression and its value, handy for quick debugging without writing a separate print("x:", x):

x = 10
print(f"{x=}")
# x=10

Multi-line f-strings

Triple-quoted strings can be f-strings too:

name = "Ada"
score = 95
report = f"""
Name: {name}
Score: {score}
Grade: {"Pass" if score >= 60 else "Fail"}
"""
print(report)

That ternary-style expression ("Pass" if score >= 60 else "Fail") works inside an f-string exactly as it would anywhere else in Python — a preview of the conditional expressions covered in the next section on control flow.

F-strings are the default choice for string formatting in modern Python. You'll see them in nearly every example for the rest of this course.