Python __repr__和无 [英] Python __repr__ and None

问题描述

我是Python的新手，目前我需要为SqlAlchemy类添加__repr__. 我有一个可以接受Null值的整数列，而SqlAlchemy将其转换为None. 例如:

I'm quite new to Python and currently I need to have a __repr__ for a SqlAlchemy class. I have an integer column that can accept Null value and SqlAlchemy converts it to None. For example:

class Stats(Base):
   __tablename__ = "stats"
   description = Column(String(2000))
   mystat = Column(Integer, nullable=True)

当SqlAlchemy返回None时，在__repr__函数中表示"mystat"字段的正确方法是什么?

What is the correct way to represent the "mystat" field in the __repr__ function when SqlAlchemy returns None?

推荐答案

__repr__应该返回描述对象的字符串.如果可能的话，它应该是一个有效的Python表达式，其结果等于一个相等的对象.对于诸如int或str的内置类型，这是正确的:

The __repr__ should return a string that describes the object. If possible, it should be a valid Python expression that evaluates to an equal object. This is true for built-in types like int or str:

>>> x = 'foo'
>>> eval(repr(x)) == x
True

如果这不可能，则应为唯一描述对象的'<...>'字符串.默认的__repr__是此示例:

If that's not possible, it should be a '<...>' string uniquely describing the object. The default __repr__ is an example of this:

>>> class Foo(object):
        pass
>>>
>>> repr(Foo())
'<__main__.Foo object at 0x02A74E50>'

它使用内存中对象的地址唯一标识它.当然，地址不会告诉我们有关对象的太多信息，因此重写__repr__并返回描述对象状态的字符串很有用.

It uses the object's address in memory to uniquely identify it. Of course address doesn't tell us much about the object so it's useful to override __repr__ and return a string describing the object's state.

对象的状态由它包含的其他对象定义，因此在您的对象中包含它们的repr是有意义的.这正是list或dict的作用:

The object's state is defined by other objects it contains so it makes sense to include their repr in yours. This is exactly what list or dict do:

>>> repr(['bar', Foo()])
"['bar', <__main__.Foo object at 0x02A74710>]"

在您的情况下，状态位于您的Column属性中，因此您想使用它们的repr.您可以为此使用%r格式，它会插入参数的repr():

In your case, the state is in your Column properties so you want to use their repr. You can use the %r formatting for this, it inserts a repr() of the argument:

def __repr__(self):
    return '<Stats: description=%r, mystat=%r>' % (self.description, self.mystat)

使用新格式的等效项:

def __repr__(self):
    return '<Stats: description={0.description!r}, mystat={0.mystat!r}>'.format(self)

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