!r 在 str() 和 repr() 中做什么？
What does !r do in str() and repr()?
根据 Python 2.7.12 documentation:
!s (apply str()) and !r (apply repr()) can be used to convert
the value before it is formatted.
>>> import math
>>> print 'The value of PI is approximately {}.'.format(math.pi)
The value of PI is approximately 3.14159265359.
>>> print 'The value of PI is approximately {!r}.'.format(math.pi)
The value of PI is approximately 3.141592653589793.
有趣的是，转换后的值是repr()的输出，而不是str().
>>> str(math.pi)
'3.14159265359'
>>> repr(math.pi)
'3.141592653589793'
那么这里的"convert the value"是什么意思呢？使其更难阅读？
为了将 中的某些内容格式化为字符串，必须首先创建该内容的字符串表示形式。 “转换值”基本上是在谈论如何构造字符串表示形式。在 python 中，有两个相当自然的选择来获取某物的字符串表示形式……str 和 repr。 str 通常更人性化，repr 通常更精确。也许 official documentation 是寻找差异的最佳位置：
object.__repr__(self)
Called by the repr() built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an “informal” string representation of instances of that class is required.
This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.
object.__str__(self)
Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.
This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression: a more convenient or concise representation can be used.
The default implementation defined by the built-in type object calls object.__repr__().
在str.format中，!s选择使用str格式化对象，而!r选择repr格式化值。
字符串的区别很容易看出（因为字符串的 repr 将包含外引号）。:
>>> 'foo {}'.format('bar')
'foo bar'
>>> 'foo {!r}'.format('bar')
"foo 'bar'"
这两种方法之间的区别在很大程度上取决于所格式化的对象。对于许多对象（例如那些不重写 __str__ 方法的对象），格式化输出没有区别。
我调用了 str(object) 和函数 -format() 和 print() 来计算有人称之为“非正式”的。
完美打印对象的字符串表示。 return 值也必须是字符串对象。
根据 Python 2.7.12 documentation:
!s(applystr()) and!r(applyrepr()) can be used to convert the value before it is formatted.>>> import math >>> print 'The value of PI is approximately {}.'.format(math.pi) The value of PI is approximately 3.14159265359. >>> print 'The value of PI is approximately {!r}.'.format(math.pi) The value of PI is approximately 3.141592653589793.
有趣的是，转换后的值是repr()的输出，而不是str().
>>> str(math.pi)
'3.14159265359'
>>> repr(math.pi)
'3.141592653589793'
那么这里的"convert the value"是什么意思呢？使其更难阅读？
为了将 中的某些内容格式化为字符串，必须首先创建该内容的字符串表示形式。 “转换值”基本上是在谈论如何构造字符串表示形式。在 python 中，有两个相当自然的选择来获取某物的字符串表示形式……str 和 repr。 str 通常更人性化，repr 通常更精确。也许 official documentation 是寻找差异的最佳位置：
object.__repr__(self)
Called by the
repr()built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form<...some useful description...>should be returned. The return value must be a string object. If a class defines__repr__()but not__str__(), then__repr__()is also used when an “informal” string representation of instances of that class is required.This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.
object.__str__(self)
Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.
This method differs from
object.__repr__()in that there is no expectation that__str__()return a valid Python expression: a more convenient or concise representation can be used.The default implementation defined by the built-in type object calls
object.__repr__().
在str.format中，!s选择使用str格式化对象，而!r选择repr格式化值。
字符串的区别很容易看出（因为字符串的 repr 将包含外引号）。:
>>> 'foo {}'.format('bar')
'foo bar'
>>> 'foo {!r}'.format('bar')
"foo 'bar'"
这两种方法之间的区别在很大程度上取决于所格式化的对象。对于许多对象（例如那些不重写 __str__ 方法的对象），格式化输出没有区别。
我调用了 str(object) 和函数 -format() 和 print() 来计算有人称之为“非正式”的。
完美打印对象的字符串表示。 return 值也必须是字符串对象。