Python f-string
https://zetcode.com/python/fstring/
Python f-string
last modified January 9, 2023
Python f-string tutorial shows how to format strings in Python with f-string.
Python f-string
Python f-string is the newest Python syntax to do string formatting. It is available since Python 3.6. Python f-strings provide a faster, more readable, more concise, and less error prone way of formatting strings in Python.
The f-strings have the f prefix and use {} brackets to evaluate values.
Format specifiers for types, padding, or aligning are specified after the colon character; for instance: f'{price:.3}', where price is a variable name.
Python string formatting
The following example summarizes string formatting options in Python.
#!/usr/bin/python
name = 'Peter'
age = 23
print('%s is %d years old' % (name, age))
print('{} is {} years old'.format(name, age))
print(f'{name} is {age} years old')
The example formats a string using two variables.
print('%s is %d years old' % (name, age))
This is the oldest option. It uses the % operator and classic string format specifies such as %s and %d.
print('{} is {} years old'.format(name, age))
Since Python 3.0, the format function was introduced to provide advance formatting options.
print(f'{name} is {age} years old')
Python f-strings are available since Python 3.6. The string has the f prefix and uses {} to evaluate variables.
$ python formatting_string.py Peter is 23 years old Peter is 23 years old Peter is 23 years old
Python f-string expressions
We can put expressions between the {} brackets.
#!/usr/bin/python
bags = 3
apples_in_bag = 12
print(f'There are total of {bags * apples_in_bag} apples')
The example evaluates an expression inside f-string.
$ python expressions.py There are total of 36 apples
Python f-string dictionaries
We can work with dictionaries in f-strings.
#!/usr/bin/python
user = {'name': 'John Doe', 'occupation': 'gardener'}
print(f"{user['name']} is a {user['occupation']}")
The example evaluates a dictionary in an f-string.
$ python dicts.py John Doe is a gardener
Python f-string debug
Python 3.8 introduced the self-documenting expression with the = character.
#!/usr/bin/python
import math
x = 0.8
print(f'{math.cos(x) = }')
print(f'{math.sin(x) = }')
The example outputs the Sine and Cosine functions in the debug mode.
$ python debug.py math.cos(x) = 0.6967067093471654 math.sin(x) = 0.7173560908995228
Python multiline f-string
We can work with multiline strings.
#!/usr/bin/python
name = 'John Doe'
age = 32
occupation = 'gardener'
msg = (
f'Name: {name}\n'
f'Age: {age}\n'
f'Occupation: {occupation}'
)
print(msg)
The example presents a multiline f-string. The f-strings are placed between round brackets; each of the strings is preceded with the f character.
$ python multiline.py Name: John Doe Age: 32 Occupation: gardener
Python f-string calling function
We can also call functions in f-strings.
#!/usr/bin/python
def mymax(x, y):
return x if x > y else y
a = 3
b = 4
print(f'Max of {a} and {b} is {mymax(a, b)}')
The example calls a custom function in the f-string.
$ python call_fun.py Max of 3 and 4 is 4
Python f-string objects
Python f-string accepts objects as well; the objects must have either __str__ or __repr__ magic functions defined.
#!/usr/bin/python
class User:
def __init__(self, name, occupation):
self.name = name
self.occupation = occupation
def __repr__(self):
return f"{self.name} is a {self.occupation}"
u = User('John Doe', 'gardener')
print(f'{u}')
The example evaluates an object in the f-string.
$ python objects.py John Doe is a gardener
Python f-string escaping characters
The following example shows how to escape certain characters in f-strings.
#!/usr/bin/python
print(f'Python uses {{}} to evaludate variables in f-strings')
print(f'This was a \'great\' film')
To escape a curly bracket, we double the character. A single quote is escaped with a backslash character.
$ python escaping.py
Python uses {} to evaludate variables in f-strings
This was a 'great' film
Python f-string format datetime
The following example formats datetime.
#!/usr/bin/python
import datetime
now = datetime.datetime.now()
print(f'{now:%Y-%m-%d %H:%M}')
The example displays a formatted current datetime. The datetime format specifiers follow the : character.
$ python format_datetime.py 2021-08-20 15:13
Python f-string format floats
Floating point values have the f suffix. We can also specify the precision: the number of decimal places. The precision is a value that goes right after the dot character.
#!/usr/bin/python
val = 12.3
print(f'{val:.2f}')
print(f'{val:.5f}')
The example prints a formatted floating point value.
$ python format_floats.py 12.30 12.30000
The output shows the number having two and five decimal places.
Python f-string format width
The width specifier sets the width of the value. The value may be filled with spaces or other characters if the value is shorter than the specified width.
#!/usr/bin/python
for x in range(1, 11):
print(f'{x:02} {x*x:3} {x*x*x:4}')
The example prints three columns. Each of the columns has a predefined width. The first column uses 0 to fill shorter values.
$ python format_width.py 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
Python f-string justify string
By default, the strings are justified to the left. We can use the > character to justify the strings to the right. The > character follows the colon character.
#!/usr/bin/python
s1 = 'a'
s2 = 'ab'
s3 = 'abc'
s4 = 'abcd'
print(f'{s1:>10}')
print(f'{s2:>10}')
print(f'{s3:>10}')
print(f'{s4:>10}')
We have four strings of different length. We set the width of the output to ten characters. The values are justified to the right.
$ python justify.py
a
ab
abc
abcd
Python f-string numeric notations
Numbers can have various numeric notations, such as decadic or hexadecimal.
#!/usr/bin/python
a = 300
# hexadecimal
print(f"{a:x}")
# octal
print(f"{a:o}")
# scientific
print(f"{a:e}")
The example prints a value in three different notations.
$ python format_notations.py 12c 454 3.000000e+02
In this tutorial, we have worked with Python f-strings.
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