转载：Python f-string - formatting strings in Python with f-string (zetcode.com)

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

$ 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.
Python f-string expressions
#!/usr/bin/python
>>>bags = 3
>>>apples_in_bag = 12
>>>print(f'There are total of {bags * apples_in_bag} apples')
There are total of 36 apples



Python f-string dictionaries
We can work with dictionaries in f-strings.
dicts.py
#!/usr/bin/python
user = {'name': 'John Doe', 'occupation': 'gardener'}
print(f"{user['name']} is a {user['occupation']}")

John Doe is a gardener


Python f-string debug
Python 3.8 introduced the self-documenting expression with the = character.
debug.py
#!/usr/bin/python
import math
x = 0.8
print(f'{math.cos(x) = }')
print(f'{math.sin(x) = }')

math.cos(x) = 0.6967067093471654
math.sin(x) = 0.7173560908995228



Python multiline f-string
We can work with multiline strings.
multiline.py
#!/usr/bin/python
name = 'John Doe'
age = 32
occupation = 'gardener'
msg = (
    f'Name: {name}\n'
    f'Age: {age}\n'
    f'Occupation: {occupation}'
)
print(msg)

Name: John Doe
Age: 32
Occupation: gardener


Python f-string calling function
We can also call functions in f-strings.

call_function.py
#!/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)}')

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.
objects.py
#!/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}')

$ 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.

escaping.py
#!/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.

format_datetime.py
#!/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.

format_floats.py
#!/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.

format_width.py
#!/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.

justify.py
#!/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.

format_notations.py
#!/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.