Formatting output

Introduction

f-strings can do more than drop values into text. A format code after a colon controls width, alignment and decimal places, which is how programs print neat tables and receipts.

A correct answer shown badly is hard to use. Format codes let a program print tables, receipts and reports that people can read at a glance, and they only take a few characters to write.

How it works

Number formats

pi = 3.14159265
big = 1234567.891
print(f"{pi:.2f}")
print(f"{big:,.2f}")
print(f"{0.256:.1%}")
print(f"{42:05d}")

Any expression

name = "Nadia"
marks = [72, 85, 90]
print(f"{name} scored {sum(marks)} in total, {sum(marks) / len(marks):.1f} on average")
print(f"{name.upper()} has {len(name)} letters")

The braces can hold any expression, including function calls and calculations. Keep them short; if an expression gets long, compute it on its own line first.

Syntax

f"{name:<10}"      # left-align in 10 characters
f"{qty:>4}"        # right-align in 4
f"{price:>10.2f}"  # right-align in 10, with 2 decimals
f"{big:,}"         # thousands separators
f"{ratio:.1%}"     # a percentage

Example

items = [("Rice", 2, 68.5), ("Dal", 1, 120), ("Oil", 3, 175.25)]
for name, qty, price in items:
    print(f"{name:<6}{qty:>3}{qty * price:>10.2f}")

Inside the braces, name:<6 means "left-align in 6 characters", qty:>3 means "right-align in 3", and :>10.2f means "right-align in 10, with 2 decimal places". Every row uses the same widths, so the columns line up in the output panel.

Line by line

1. items holds three tuples, each with a name, a quantity and a price.
2. The for line takes each tuple apart into name, qty and price.
3. {name:<6} shows the name left-aligned in 6 characters.
4. {qty:>3} shows the quantity right-aligned in 3.
5. {qty * price:>10.2f} works out the cost, then right-aligns it in 10 with 2 decimals.

Expected result

Rice    2    137.00
Dal     1    120.00
Oil     3    525.75

In the real world

Bank statements, invoices and scientific tables all rely on columns that line up. Command-line tools format their output in the same way, so that people can compare values down a column.

Common mistakes

• Forgetting the f before the quote: the braces are then printed literally.
• Using .2f on text: format codes like f only work on numbers.
• Expecting :5 to cut long text. Width is a minimum; longer values simply take more room.

Best practices

• Use the same widths on every row of a table.
• Do long calculations on their own line, then format the result.
• Choose the width from the longest value you expect.

Summary

A format code after a colon sets the width, alignment, decimals, separators and percentages of a value. .2f gives two decimals, < and > align, and width is only a minimum.

Practice

Print a three-line price list where item names are left-aligned in 12 characters and prices are right-aligned in 8 with two decimals.

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