Python String Formatting

The formatting operations described here (% operator) exhibit a variety of quirks that lead to a number of common errors [...]. Using the newer formatted string literals [...] helps avoid these errors. These alternatives also provide more powerful, flexible and extensible approaches to formatting text.

% operator

Prefer String Literals

For new code, using str.format, or formatted string literals (Python 3.6+) over the % operator is strongly recommended.

# % operator: old-style string formatting (not recommended for new code)
name = 'Pete'
'Hello %s' % name  # %s = string placeholder

"Hello Pete"

We can use the %d format specifier to convert an int value to a string:

num = 5
'I have %d apples' % num

"I have 5 apples"

str.format

Python 3 introduced a new way to do string formatting that was later back-ported to Python 2.7. This makes the syntax for string formatting more regular.

# str.format() method: modern string formatting (Python 2.7+)
name = 'John'
age = 20

"Hello I'm {}, my age is {}".format(name, age)  # {} = placeholder

"Hello I'm John, my age is 20"

"Hello I'm {0}, my age is {1}".format(name, age)

"Hello I'm John, my age is 20"

Formatted String Literals or f-Strings

If your are using Python 3.6+, string f-Strings are the recommended way to format strings.

From the Python 3 documentation

A formatted string literal or f-string is a string literal that is prefixed with `f` or `F`. These strings may contain replacement fields, which are expressions delimited by curly braces {}. While other string literals always have a constant value, formatted strings are really expressions evaluated at run time.

# f-string: recommended way to format strings (Python 3.6+)
name = 'Elizabeth'
f'Hello {name}!'  # f prefix allows expressions in {}

'Hello Elizabeth!'

Quiz

What prefix is used for f-strings in Python?

A. fmt

B. f or F

C. format

D. str

It is even possible to do inline arithmetic with it:

# f-strings support expressions: can include calculations inside {}
a = 5
b = 10
f'Five plus ten is {a + b} and not {2 * (a + b)}.'  # Evaluates expressions

'Five plus ten is 15 and not 30.'

Multiline f-Strings

name = 'Robert'
messages = 12
(
f'Hi, {name}. '
f'You have {messages} unread messages'
)

'Hi, Robert. You have 12 unread messages'

The `=` specifier

This will print the expression and its value:

# = specifier: prints both variable name and value (Python 3.8+)
from datetime import datetime
now = datetime.now().strftime("%b/%d/%Y - %H:%M:%S")
f'date and time: {now=}'  # Prints "now='Nov/14/2022 - 20:50:01'"

"date and time: now='Nov/14/2022 - 20:50:01'"

Adding spaces or characters

name = 'Robert'
f"{name.upper() = :-^20}"

'name.upper() = -------ROBERT-------'

f"{name.upper() = :^20}"

'name.upper() =        ROBERT       '

f"{name.upper() = :20}"

'name.upper() = ROBERT              '

Formatting Digits

Adding thousands separator

a = 10000000
f"{a:,}"

'10,000,000'

Rounding

a = 3.1415926
f"{a:.2f}"

'3.14'

Quiz

What does f"{a:.2f}" do?

A. Rounds the number to the nearest integer

B. Formats as a percentage

C. Formats the number as a float with 2 decimal places

D. Converts to scientific notation

Showing as Percentage

a = 0.816562
f"{a:.2%}"

'81.66%'

Number formatting table

Number	Format	Output	description
3.1415926	{:.2f}	3.14	Format float 2 decimal places
3.1415926	{:+.2f}	+3.14	Format float 2 decimal places with sign
-1	{:+.2f}	-1.00	Format float 2 decimal places with sign
2.71828	{:.0f}	3	Format float with no decimal places
4	{:0>2d}	04	Pad number with zeros (left padding, width 2)
4	{:x<4d}	4xxx	Pad number with x’s (right padding, width 4)
10	{:x<4d}	10xx	Pad number with x’s (right padding, width 4)
1000000	{:,}	1,000,000	Number format with comma separator
0.35	{:.2%}	35.00%	Format percentage
1000000000	{:.2e}	1.00e+09	Exponent notation
11	{:11d}	11	Right-aligned (default, width 10)
11	{:<11d}	11	Left-aligned (width 10)
11	{:^11d}	11	Center aligned (width 10)

Template Strings

A simpler and less powerful mechanism, but it is recommended when handling strings generated by users. Due to their reduced complexity, template strings are a safer choice.

from string import Template
name = 'Elizabeth'
t = Template('Hey $name!')
t.substitute(name=name)

'Hey Elizabeth!'