Introduction to f string in python and its common usage

Preface

A friend saw that f'{}' appeared in the article related to the Faker library that I wrote before. He asked me what it meant?

Actually, this uses f-string as a formatting method. For this reason, we briefly list f-string and its common usage for your friends to use as a reference.

1. What is f-string?

Currently, Python f-string is the latest syntax to perform string formatting. Available since Python 3.6.

Python f-strings provide a faster, more readable, concise, and less error-prone way of formatting strings in Python. f Strings are prefixed with f and the value is evaluated using {} brackets. Specify format specifiers for type, padding, or alignment after the colon. For example: f'{price:.3f}', where price is the variable name and .3f indicates the precision (retaining three decimal places).

2. Common usage of formatting

There are three common formatting methods in Python, namely:

1. % symbol format specifier

2. format() function

3. f-string

The following is an example. The results are all "[Zhang San] is [18] years old this year", but different formats are used. Method:

name = "张三"
age = 18
# 最原始提供的字符串替换方法，使用了 % 运算符和经典字符串格式指定，如 %s %d 等
print("【%s】今年【%d】岁" % (name, age))  # 【张三】今年【18】岁
# Python 3.0 新增了 format() 函数，可以提供高级的格式化选项
print("【{}】今年【{}】岁".format(name, age))  # 【张三】今年【18】岁
# Python 3.6 f-string出现，使得格式化方法更加灵活，字符串前缀为 f，并使用 {} 评估值
print(f"【{name}】今年【{age}】岁")  # 【张三】今年【18】岁

3. Summary of common usages of f-string

The following are examples of some common usages of f-string

1. Receive expressions in f-string

num = 12
price = 6
# f-string 中可以接收表达式
print(f'【{num}】个苹果，每个【{price}】元，一共要花【{num * price}】元')
# 【12】个苹果，每个【6】元，一共要花【72】元

2. f-string can take values ​​from the dictionary

user = {"name": "Ace", "job": "teacher"}
print(f'【{user["name"]}】的工作是【{user["job"]}】')
# 【Ace】的工作是【teacher】

3. f-string formats multi-line data

name = "李四"
age = 28
job = "码农"
msg = (
    f'Name: {name}\n'
    f'Age: {age}\n'
    f'Job: {job}'
)  # 注意 msg 使用了 () 进行包裹
print(msg)
# Name: 李四
# Age: 28
# Job: 码农

4. Call functions in f-string

def my_max(x, y):
    """
    三目运算比较两个数字大小
    :param x: int x
    :param y: int y
    :return: x 和 y 中较大的数字
    """
    return x if x > y else y
a = 3
b = 4
print(f'【{a}】和【{b}】中较大的是【{my_max(a, b)}】')
# 【3】和【4】中较大的是【4】

5. f-string escape usage

print(f"Python 使用 {{}} 来计算f-string中的变量")
# Python 使用 {} 来计算f-string中的变量
print(f'你真的很\'厉害\'')  # 注意：单引号中想继续使用单引号，那就需要进行转义
# 你真的很'厉害'

6. f-string formatted floating point number

val = 11
# 通过 : 后跟 浮点数标识 ，可以实现格式化浮点数
print(f'{val:.3f}')  # 11.000
print(f'{val:.4f}')  # 11.0000

7. f-string formatted width

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

8. f-string aligned string

s1 = 'a'
s2 = 'ab'
s3 = 'abc'
s4 = 'abcd'
# 将输出的宽度设置为十个字符。 使用 > 符号，让输出结果右对齐。
print(f'{s1:>10}')
print(f'{s2:>10}')
print(f'{s3:>10}')
print(f'{s4:>10}')
#          a
#         ab
#        abc
#       abcd
# 实际上，只要大于最大的字符串长度，就可以实现右对齐，感兴趣可以试下小于最大字符串长度会有什么表现

9. f-string formatting time

import datetime
now = datetime.datetime.now()
print(f'{now:%Y-%m-%d %H:%M}')
# 2023-04-02 21:07

10. f-string receives object, note: the object must define __str__( ) or __repr__() function

class User:
    def __init__(self, name, job):
        self.name = name
        self.job = job
    def __repr__(self):
        return f"{self.name} is a {self.job}"
u = User('Ace', 'teacher')
print(f'{u}')
# Ace is a teacher

Note: Only supported by Python 3.6 or above!

Note: Only supported by Python 3.6 or above!

Note: Only supported by Python 3.6 or above!

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