python字符串格式：何时使用!s转换标志

Python string format: When to use !s conversion flag

这两个字符串格式语句在python中的区别是什么：

1 2	'{0}'.format(a) '{0!s}'.format(a)

如果a是整数、列表或字典，则两者的输出相同。第一个{0}是否执行隐式str()调用？

来源

附言：关键词：感叹/砰"！S格式

文件中提到：

The conversion field causes a type coercion before formatting.
Normally, the job of formatting a value is done by the __format__()
method of the value itself. However, in some cases it is desirable to
force a type to be formatted as a string, overriding its own
definition of formatting. By converting the value to a string before
calling __format__(), the normal formatting logic is bypassed.

Two conversion flags are currently supported: '!s' which calls
str() on the value, and '!r' which calls repr().

可以(再次从文档中)举一个例子来说明区别：

1 2	>>>"repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2') "repr() shows quotes: 'test1'; str() doesn't: test2"

相关讨论

简单地说：

'{0}'.format(a)将使用a.__format__()的结果显示该值。
'{0!s}'.format(a)将使用a.__str__()的结果显示该值。
'{0!r}'.format(a)将使用a.__repr__()的结果显示该值。

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>>> class C:
... def __str__(self): return"str"
... def __repr__(self): return"repr"
... def __format__(self, format_spec): return"format as" + str(type(format_spec))
...
>>> c = C()
>>> print"{0}".format(c)
format as <type 'str'>
>>> print u"{0}".format(c)
format as <type 'unicode'>
>>> print"{0!s}".format(c)
str
>>> print"{0!r}".format(c)
repr

关于__format__的第二个参数，引用pep 3101"按类型控制格式"：

The 'format_spec' argument will be either
a string object or a unicode object, depending on the type of the
original format string. The __format__ method should test the type
of the specifiers parameter to determine whether to return a string or
unicode object. It is the responsibility of the __format__ method
to return an object of the proper type.

感谢@hjpotter92的评论和回答，请解释：

下面是一个显示差异的示例(当您重写__format__方法时)

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class MyClass:
i = 12345
def __format__(self, i):
return 'I Override'

>>> obj = MyClass()

>>> '{0}'.format(obj)
'I Override'

>>> '{0!s}'.format(obj)
'<__main__.MyClass instance at 0x021AA6C0>'