Python f-strings and formatting: 12 patterns you will reuse

Tags: Python, Programming, Beginner

Introduction – what, why, and who this is for

Python’s f‑strings (formatted string literals) were introduced in Python 3.6 and quickly became the go‑to way to build readable, efficient strings. They let you embed variables, run expressions, and apply formatting inside a string literal, all without the clutter of % or str.format().

If you’re a beginner who’s tired of concatenating with + or calling format(), or an intermediate coder looking for clean, repeatable patterns, this tutorial is for you. We’ll walk through 12 practical f‑string recipes, show the exact output before and after applying the pattern, and give you a ready‑to‑copy code block for each. No fluff—just runnable examples you can paste into your own projects.

Part 1 – Basic interpolation

# before
name = "Alice"
greeting = "Hello, " + name + "!"
print(greeting)   # Hello, Alice!

# after
greeting = f"Hello, {name}!"
print(greeting)   # Hello, Alice!

Why it matters: No more + juggling, and the variable name is visible right where it appears.

Part 2 – Numeric formatting

2️⃣ Fixed‑point numbers (2 decimal places)

price = 9.5
# before
print("Price: $" + format(price, ".2f"))
# after
print(f"Price: ${price:.2f}")

Output

Price: $9.50

3️⃣ Thousands separator

population = 12345678
print(f"Population: {population:,}")

Output

Population: 12,345,678

4️⃣ Binary / hex / octal

value = 42
print(f"binary: {value:b}, hex: {value:#x}, oct: {value:#o}")

Output

binary: 101010, hex: 0x2a, oct: 0o52

Part 3 – Dates and times

5️⃣ ISO‑8601 date

from datetime import date
today = date.today()
print(f"Today is {today:%Y-%m-%d}")

Output

Today is 2026-10-09

6️⃣ 12‑hour clock with AM/PM

from datetime import datetime
now = datetime.now()
print(f"Current time: {now:%I:%M %p}")

Output

Current time: 03:27 PM

7️⃣ Custom weekday name

print(f"Weekday: {now:%A}")

Output

Weekday: Monday

Part 4 – Alignment and padding

name = "Bob"
num = 7
word = "hi"

print(f"{name:<20}")   # Bob                 
print(f"{num:0>5}")    # 00007
print(f"{word:*^10}")  # ****hi****

Part 5 – Expressions inside f‑strings

11️⃣ Inline calculations

a, b = 5, 3
print(f"{a} + {b} = {a + b}")

Output

5 + 3 = 8

12️⃣ Debug‑friendly output (= specifier)

Python 3.8 added the = conversion flag, which prints both the expression and its value.

x = 12
print(f"{x=}, {x*2=}")

Output

x=12, x*2=24

Hands‑on example – a tiny report generator

Below is a complete, runnable script that combines all 12 patterns into a single, readable report. Copy it, run python report.py, and you’ll see the formatted output immediately.

# report.py
from datetime import datetime, date

# 1 – basic interpolation
user = "Eve"
greeting = f"Hello, {user}!"

# 2 – fixed‑point price
price = 49.99
price_line = f"Price: ${price:.2f}"

# 3 – thousands separator
visits = 1520345
visits_line = f"Total visits: {visits:,}"

# 4 – binary/hex/octal
mask = 0b101010
mask_line = f"Mask (bin/hex/oct): {mask:b}/{mask:#x}/{mask:#o}"

# 5‑7 – dates and times
today = date.today()
now = datetime.now()
date_line = f"Date (ISO): {today:%Y-%m-%d}"
time_line = f"Time (12‑hr): {now:%I:%M %p}"
weekday_line = f"Weekday: {now:%A}"

# 8‑10 – alignment & padding
left = f"{user:<15}"
right = f"{price:0>8}"
center = f"{'Report':*^30}"

# 11 – inline calculation
total = price * visits
calc_line = f"{price:.2f} × {visits:,} = ${total:,.2f}"

# 12 – debug output
debug_line = f"{visits=}, {price=}, {total=:,.2f}"

# Assemble report
report = f"""
{center}
{greeting}
{price_line}
{visits_line}
{mask_line}
{date_line}
{time_line}
{weekday_line}
Left‑align: {left}
Zero‑pad:   {right}
{calc_line}
{debug_line}
"""

print(report)

Sample output

*************Report*************
Hello, Eve!
Price: $49.99
Total visits: 1,520,345
Mask (bin/hex/oct): 101010/0x2a/0o52
Date (ISO): 2026-10-09
Time (12‑hr): 03:27 PM
Weekday: Monday
Left‑align: Eve             
Zero‑pad:   00049.99
49.99 × 1,520,345 = $75,997,185.55
visits=1520345, price=49.99, total=75,997,185.55

All 12 patterns are demonstrated, and you can edit any line to fit your own data.

Common mistakes / troubleshooting

• Forgot the leading f – "{name}" won’t interpolate. Add f before the opening quote.
• Mismatched braces – Every { must have a matching }. Python raises SyntaxError: f-string: unmatched '{'. Count them or use an editor’s bracket‑matching feature.
• Using a colon inside the expression – Only the format spec after the first colon belongs to the f‑string. Write {value:.2f} not {value : .2f} (extra spaces break the spec).
• Mixing bytes and f‑strings – bf"..." is not allowed. Convert to str first or use .decode().
• Running on Python < 3.6 – f‑strings don’t exist. Upgrade or fall back to str.format().

Try it yourself

Create a list of product names and prices, then print a table where:

1. Names are left‑aligned in a 12‑character column.
2. Prices show two decimals and a thousands separator.
3. Each line ends with the expression price * 1.07 (adding 7 % tax) shown using the = debug flag.

products = [("Notebook", 1999.5), ("Pen", 2.3), ("Backpack", 49.99)]
# Write your f‑string loop below

What’s next

• Advanced formatting: custom numeric formats, locale‑aware output.
• Multiline f‑strings: using textwrap.dedent for clean templates.
• Performance tip: f‑strings vs. %/format() in tight loops.
• Logging with f‑strings: structured logs without extra libraries.

Happy formatting!

Bookmark this page for quick copy‑paste of the 12 patterns.