钩住内置的python f-string格式机制

我真的很喜欢 f-strings

我写了一个小的库-如下所述-供我自己进一步利用它们(和 put it out there for use by others ,以防对他们有帮助;编辑:我真的还没有完成这个库,所以不要太兴奋)。举一个简单的例子说明它做了什么/将要做什么:

>>> import simpleformatter as sf
>>> def format_camel_case(string):
...     """camel cases a sentence"""
...     return ''.join(s.capitalize() for s in string.split())
...
>>> @sf.formattable(camcase=format_camel_case)
... class MyStr(str): ...
...
>>> f'{MyStr("lime cordial delicious"):camcase}'
'LimeCordialDelicious'

为了简化API,并将用法扩展到内置类实例,找到一种与内置python格式化机制挂钩的方法将非常有用,这将允许内置程序的自定义格式规范:

>>> f'{"lime cordial delicious":camcase}'
'LimeCordialDelicious'

换句话说,我想重写内置的 format 函数(由f-string语法使用)——或者扩展内置的 __format__ 现有标准库类的方法--这样我就可以编写如下内容:

for x, y, z in complicated_generator:
    eat_string(f"x: {x:custom_spec1}, y: {x:custom_spec2}, z: {x:custom_spec3}")

我通过创建子类来实现这一点

我可以用 string.Formatter 应用程序编程接口:

my_formatter=MyFormatter()  # custom string.Formatter instance

format_str = "x: {x:custom_spec1}, y: {x:custom_spec2}, z: {x:custom_spec3}"

for x, y, z in complicated_generator:
    eat_string(my_formatter.format(format_str, **locals()))

另一件可以做的事是压倒一切 builtins.format

>>> import builtins
>>> builtins.format = lambda *args, **kwargs: 'womp womp'
>>> format(1,"foo")
'womp womp'

…但这不适用于f字符串:

>>> f"{1:foo}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier

目前 my API 看起来像这样(有点简化):

import simpleformatter as sf
@sf.formatter("this_specification")
def this_formatting_function(some_obj):
    return "this formatted someobj!"

@sf.formatter("that_specification")
def that_formatting_function(some_obj):
    return "that formatted someobj!"

@sf.formattable
class SomeClass: ...

之后您可以编写如下代码:

some_obj = SomeClass()
f"{some_obj:this_specification}"
f"{some_obj:that_specification}"

我希望api更像下面这样:

@sf.formatter("this_specification")
def this_formatting_function(some_obj):
    return "this formatted someobj!"

@sf.formatter("that_specification")
def that_formatting_function(some_obj):
    return "that formatted someobj!"

class SomeClass: ...  # no class decorator needed

x=1  # built-in type instance
f"{x:this_specification}"
f"{x:that_specification}"

但为了做这些事情,我们必须钻入我们的方式内置 format() 功能。我怎么能抓住那多汁的f弦呢?