Python — String Formatting

f-strings — the modern standard

Prefix with f, drop values into {}:

name = "Ada"
age = 36

print(f"Hello {name}, you are {age}")

Braces run ANY expression:

f"Next year: {age + 1}"
f"Loud: {name.upper()}"
f"Status: {'adult' if age >= 18 else 'minor'}"

The mini-language after the colon

pi = 3.14159265

f"{pi:.2f}"        # "3.14"     2 decimal places
f"{1234567:,}"     # "1,234,567" thousands separators
f"{0.256:.1%}"     # "25.6%"     percentages!
f"{42:05d}"        # "00042"     zero-padded width 5

Specifier	Effect
.Nf	N decimals
:,	comma grouping
:.1%	percent
05d	pad to width 5 with zeros
>10 / <10 / ^10	align right/left/center in width 10

Alignment tables

for item, price in [("Keyboard", 80), ("Cable", 9.5)]:
    print(f"{item:<12}{price:>8.2f}")
# Keyboard        80.00
# Cable            9.50

< > ^ plus a width — instant clean columns.

Debugging shortcut (3.8+)

f"{name=}, {age=}"
# name='Ada', age=36    ← variable name AND value, one line

Legendary for quick console diagnosis.

Multiline & escaping

// python:
msg = f"""User {name}
Total: {price * qty:.2f}"""

print(f"{{literal braces}}")    # double them

Older styles — recognize, don't start

"Hello %s" % name                       # printf-era
"{} is {}".format(name, age)            # pre-3.6 workhorse
"Hi %(name)s" % {"name": "Ada"}         # dict mapping

You'll meet all three in tutorials and legacy code; new code uses f-strings.

Formatting dates inside f-strings

from datetime import datetime

f"Today is {datetime.now():%A, %B %d}"   # methods AND format specs compose
f"Log at {datetime.now():%H:%M:%S}"

Mini Practice

1. Receipt printer: three items aligned to two columns.
2. Percentage formatter for a 0–1 score.
3. Zero-padded clock from h/m/s ints.
4. {var=} debug five variables at once.
5. Convert one %s-style legacy string to an f-string.

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