用 f-string 打印对象-Python合辑5-格式化字符串

当前位置:　首页>> 技术小册>> Python合辑5-格式化字符串

可以用f-string打印自定义对象。默认设置是，如果向f-string表达式传递了一个对象，它将会显示该对象 __str__ 方法的返回值。不过，也可以用显式转换操作标志来打印__repr__的值。
● !r - 使用 repr() 将值转化为文本.
● !s - 使用 str() 将值转化为文本.
```
>>> class Color:
    def __init__(self, r: float = 255, g: float = 255, b: float = 255):
        self.r = r
        self.g = g
        self.b = b

def __str__(self) -> str:
        return "A RGB color"

def __repr__(self) -> str:
        return f"Color(r={self.r}, g={self.g}, b={self.b})"

>>> c = Color(r=123, g=32, b=255)

# 如不加任何操作符, 会打印 __str__ 的值
>>> f"{c}"
'A RGB color'

# 用`obj!r` 的话会打印 __repr__ 的值
>>> f"{c!r}"
'Color(r=123, g=32, b=255)'

# 使用!s跟默认值一样
>>> f"{c!s}"
'A RGB color'
```
Python也允许通过定义不同类型使用__format__方法控制格式化结果，下面的例子会展示所有可能情况。
```
>>> class Color:
    def __init__(self, r: float = 255, g: float = 255, b: float = 255):
        self.r = r
        self.g = g
        self.b = b

def __str__(self) -> str:
        return "A RGB color"

def __repr__(self) -> str:
        return f"Color(r={self.r}, g={self.g}, b={self.b})"

>>> c = Color(r=123, g=32, b=255)

# When no option is passed, the __str__ result is printed
>>> f"{c}"
'A RGB color'

# When `obj!r` is used, the __repr__ output is printed
>>> f"{c!r}"
'Color(r=123, g=32, b=255)'

# Same as the default
>>> f"{c!s}"
'A RGB color'
Python also allows us to control the formatting on a per-type basis through the __format__ method. The following example shows how you can do all of that.

>>> class Color:
    def __init__(self, r: float = 255, g: float = 255, b: float = 255):
        self.r = r
        self.g = g
        self.b = b

def __str__(self) -> str:
        return "A RGB color"

def __repr__(self) -> str:
        return f"Color(r={self.r}, g={self.g}, b={self.b})"

def __format__(self, format_spec: str) -> str:
        if not format_spec or format_spec == "s":
            return str(self)

if format_spec == "r":
            return repr(self)

if format_spec == "v":
            return f"Color(r={self.r}, g={self.g}, b={self.b}) - A nice RGB thing."

if format_spec == "vv":
            return (
                f"Color(r={self.r}, g={self.g}, b={self.b}) "
                f"- A more verbose nice RGB thing."
            )

if format_spec == "vvv":
            return (
                f"Color(r={self.r}, g={self.g}, b={self.b}) "
                f"- A SUPER verbose nice RGB thing."
            )

raise ValueError(
            f"Unknown format code '{format_spec}' " "for object of type 'Color'"
        )

>>> c = Color(r=123, g=32, b=255)

>>> f'{c:v}'
'Color(r=123, g=32, b=255) - A nice RGB thing.'

>>> f'{c:vv}'
'Color(r=123, g=32, b=255) - A more verbose nice RGB thing.'

>>> f'{c:vvv}'
'Color(r=123, g=32, b=255) - A SUPER verbose nice RGB thing.'

>>> f'{c}'
'A RGB color'

>>> f'{c:s}'
'A RGB color'

>>> f'{c:r}'
'Color(r=123, g=32, b=255)'

>>> f'{c:j}'
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-20-1c0ee8dd74be> in <module>
----> 1 f'{c:j}'

<ipython-input-15-985c4992e957> in __format__(self, format_spec)
     29                 f"- A SUPER verbose nice RGB thing."
     30             )
---> 31         raise ValueError(
     32             f"Unknown format code '{format_spec}' " "for object of type 'Color'"
     33         )

ValueError: Unknown format code 'j' for object of type 'Color'
```

最后，还有个用来转义ASCII字符的a操作符。更多信息可参考:
docs.python.org/3/library/functions.html#as..
```
>>> utf_str = "Áeiöu"
>>> f"{utf_str!a}"
"'\\xc1ei\\xf6u'"
```