[转载] 字符串太长 pep8_Python f字符串– PEP 498

[转载] 字符串太长 pep8_Python f字符串– PEP 498 –文字字符串插值

参考链接：从Java中的字符串中删除前导零

字符串太长 pep8

Python f-strings or formatted strings are the new way to format strings. This feature was introduced in Python 3.6 under PEP-498. It’s also called literal string interpolation.

Python f字符串或格式化的字符串是格式化字符串的新方法。此功能是在PEP-498下的Python 3.6中引入的。也称为文字字符串插值。

为什么我们需要F弦？ (Why do we need f-strings?)

Python provides various ways to format a string. Let’s quickly look at them and what are the issues they have.

Python提供了多种格式化字符串的方法。让我们快速查看它们以及它们有什么问题。

% formatting – great for simple formatting but limited support for strings, ints, doubles only. We can’t use it with objects. ％格式 -非常适合简单的格式设置，但仅对string ，ints和double的支持有限。我们不能将其与对象一起使用。 Template Strings – it’s very basic. Template strings work with keyword arguments like dictionary only. We are not allowed to call any function and arguments must be strings. 模板字符串 –非常基本。模板字符串仅可与关键字参数（例如字典）一起使用。我们不允许调用任何函数，并且参数必须是字符串。 String format() – Python String format() function was introduced to overcome the issues and limited features of %-formatting and template strings. However, it’s too verbose. Let’s look at its verbosity with a simple example.>>> age = 4 * 10

>>> 'My age is {age}.'.format(age=age)

'My age is 40.' 字符串格式（）– Python字符串格式（）函数的引入是为了克服％格式和模板字符串的问题和有限的功能。但是，它太冗长了。让我们用一个简单的例子来看看它的详细程度。

Python f-strings works almost similar like format() function but removes all the verbosity that format() function has. Let’s see how easily we can format the above string using f-strings.

Python f字符串的工作原理几乎类似于format（）函数，但删除了format（）函数具有的所有冗长性。让我们看看使用f字符串格式化上述字符串的难易程度。

>>> f'My age is {age}'

'My age is 40.'

Python f-strings is introduced to have minimal syntax for string formatting. The expressions are evaluated at runtime. If you are using Python 3.6 or higher version, you should use f-strings for all your string formatting requirements.

引入Python f字符串是为了使字符串格式化的语法最少。在运行时对表达式求值。如果您使用的是Python 3.6或更高版本，则应使用f-strings满足所有字符串格式要求。

Python f字符串示例 (Python f-strings examples)

Let’s look at a simple example of f-strings.

让我们看一个简单的f字符串示例。

name = 'Pankaj'

age = 34

f_string = f'My Name is {name} and my age is {age}'

print(f_string)

print(F'My Name is {name} and my age is {age}') # f and F are same

name = 'David'

age = 40

# f_string is already evaluated and won't change now

print(f_string)

Output:

输出：

My Name is Pankaj and my age is 34

Python executes statements one by one and once f-string expressions are evaluated, they don’t change even if the expression value changes. That’s why in the above code snippets, f_string value remains same even after ‘name’ and ‘age’ variable has changed in the latter part of the program.

Python会一一执行语句，并且一旦评估了f字符串表达式，即使表达式值更改，它们也不会更改。这就是为什么在上面的代码段中，即使在程序的后半部分更改了“ name”和“ age”变量后，f_string值仍保持不变。

1.带表达式和转换的f字符串 (1. f-strings with expressions and conversions)

We can use f-strings to convert datetime to a specific format. We can also run mathematical expressions in f-strings.

我们可以使用f字符串将日期时间转换为特定格式。我们还可以在f字符串中运行数学表达式。

from datetime import datetime

name = 'David'

age = 40

d = datetime.now()

print(f'Age after five years will be {age+5}') # age = 40

print(f'Name with quotes = {name!r}') # name = David

print(f'Default Formatted Date = {d}')

print(f'Custom Formatted Date = {d:%m/%d/%Y}')

Output:

输出：

Age after five years will be 45

Name with quotes = 'David'

Default Formatted Date = 2018-10-10 11:47:12.818831

Custom Formatted Date = 10/10/2018

2. f字符串支持原始字符串 (2. f-strings support raw strings)

We can create raw strings using f-strings too.

我们也可以使用f字符串创建原始字符串。

print(f'Default Formatted Date:\n{d}')

print(fr'Default Formatted Date:\n {d}')

Output:

输出：

Default Formatted Date:

2018-10-10 11:47:12.818831

Default Formatted Date:\n 2018-10-10 11:47:12.818831

3.具有对象和属性的f字符串 (3. f-strings with objects and attributes)

We can access object attributes too in f-strings.

我们也可以在f字符串中访问对象属性。

class Employee:

id = 0

name = ''

def __init__(self, i, n):

self.id = i

self.name = n

def __str__(self):

return f'E[id={self.id}, name={self.name}]'

emp = Employee(10, 'Pankaj')

print(emp)

print(f'Employee: {emp}\nName is {emp.name} and id is {emp.id}')

Output:

输出：

E[id=10, name=Pankaj]

Employee: E[id=10, name=Pankaj]

Name is Pankaj and id is 10

4. f字符串调用函数 (4. f-strings calling functions)

We can call functions in f-strings formatting too.

我们也可以用f字符串格式调用函数。

def add(x, y):

return x + y

print(f'Sum(10,20) = {add(10, 20)}')

Output: Sum(10,20) = 30

输出： Sum(10,20) = 30

5.带空格的f字符串 (5. f-string with whitespaces)

If there are leading or trailing whitespaces in the expression, they are ignored. If the literal string part contains whitespaces then they are preserved.

如果表达式中存在前导或尾随空格，则将其忽略。如果文字字符串部分包含空格，则将保留它们。

>>> age = 4 * 20

>>> f' Age = { age } '

' Age = 80 '

6.带f字符串的Lambda表达式 (6. Lambda expressions with f-strings)

We can use lambda expressions insidef-string expressions too.

我们也可以在字符串表达式内部使用lambda表达式。

x = -20.45

print(f'Lambda Example: {(lambda x: abs(x)) (x)}')

print(f'Lambda Square Example: {(lambda x: pow(x, 2)) (5)}')

Output:

输出：