Python f-string format specifier

Python f-string format specifier has an f prefix and uses {} brackets to evaluate values. Where Format specifiers for types, padding, or aligning are specified after the colon character.

Python f-string format specifier

Simple example code string formatting in Python.

name = 'John'
age = 25

print('%s is %d years old' % (name, age))
print('{} is {} years old'.format(name, age))
print(f'{name} is {age} years old')

Output:

f-string expressions

You can put expressions between the {} brackets.

bags = 3
apples_in_bag = 12

print(f'There are total of {bags * apples_in_bag} apples')

Output: There are a total of 36 apples\

f-string dictionaries

user = {'name': 'John Doe', 'occupation': 'gardener'}

print(f"{user['name']} is a {user['occupation']}")

f-string debug

import math

x = 0.8

print(f'{math.cos(x) = }')
print(f'{math.sin(x) = }')

Multiline f-string

name = 'John Doe'
age = 32
occupation = 'gardener'

msg = (
    f'Name: {name}\n'
    f'Age: {age}\n'
    f'Occupation: {occupation}'
)

print(msg)

f-string format width

for x in range(1, 11):
    print(f'{x:02} {x*x:3} {x*x*x:4}')

Output:

01   1    1
02   4    8
03   9   27
04  16   64
05  25  125
06  36  216
07  49  343
08  64  512
09  81  729
10 100 1000

f-string justify string

s1 = 'a'
s2 = 'ab'
s3 = 'abc'
s4 = 'abcd'

print(f'{s1:>10}')
print(f'{s2:>10}')
print(f'{s3:>10}')
print(f'{s4:>10}')

Output:

         a
        ab
       abc
      abcd

You can also use format specifiers within the curly braces to control the formatting of your expressions. Here are some common format specifiers you might use with f-strings:

Specifier	Description	Example	Output
`<`	Left-align the string within the available space.	`f"{'Alice':<10}"`	`Alice`
`>`	Right-align the string within the available space.	`f"{'Alice':>10}"`	`Alice`
`^`	Center-align the string within the available space.	`f"{'Alice':^10}"`	`Alice`
`d`	Decimal integer.	`f"{123:d}"`	`123`
`b`	Binary format.	`f"{123:b}"`	`1111011`
`o`	Octal format.	`f"{123:o}"`	`173`
`x`	Hexadecimal format (lowercase).	`f"{123:x}"`	`7b`
`X`	Hexadecimal format (uppercase).	`f"{123:X}"`	`7B`
`,`	Use comma as a thousand separator.	`f"{1234567:,}"`	`1,234,567`
`_`	Use underscore as a thousand separator.	`f"{1234567:_}"`	`1_234_567`
`e`	Exponential notation (lowercase).	`f"{1234567.89:e}"`	`1.234568e+06`
`E`	Exponential notation (uppercase).	`f"{1234567.89:E}"`	`1.234568E+06`
`f`	Fixed-point notation.	`f"{1234.56789:f}"`	`1234.567890`
`g`	General format.	`f"{1234.56789:g}"`	`1234.57`
`%`	Percentage.	`f"{0.1234:%}"`	`12.340000%`
`.nf`	Fixed-point notation with `n` digits after the decimal point.	`f"{1234.56789:.2f}"`	`1234.57`
`.ne`	Exponential notation with `n` digits after the decimal point.	`f"{1234.56789:.2e}"`	`1.23e+03`
`.ng`	General format with `n` significant digits.	`f"{1234.56789:.2g}"`	`1.2e+03`
`w`	Minimum width of the field.	`f"{123.456:10}"`	`123.456`
`.p`	Precision for floating-point numbers or maximum length for strings.	`f"{123.456789:.2f}"`	`123.46`
`%Y-%m-%d %H:%M:%S`	Date and time formatting using `datetime` module.	`f"{datetime.now():%Y-%m-%d %H:%M:%S}"`	`2024-06-10 14:23:45`

Here are some additional examples combining different specifiers:

Example	Description	Output
`f"{'Bob':<10} Age: {25:<5}"`	Left-align string and integer.	`Bob Age: 25`
`f"Balance: ${1234.56789:,.2f}"`	Comma separator and 2 decimal places.	`Balance: $1,234.57`
`f"Hex: {25:x}, Binary: {25:b}, Octal: {25:o}"`	Different number formats.	`Hex: 19, Binary: 11001, Octal: 31`
`f"Current time: {datetime.now():%Y-%m-%d %H:%M:%S}"`	Date and time formatting.	`Current time: 2024-06-10 14:23:45`

Do comment if you have any doubts or suggestions on this Python string format topic.

Note: IDE: PyCharm 2021.3.3 (Community Edition)
Windows 10
Python 3.10.1
All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.

Python f-string format specifier