为什么Python调用__str__而不是返回long值

我有一个简单的类,它延长了接受带有值修饰符的字符串(即’10m’将是1024 * 1024 * 10)

我有__str__函数打印传入的原始值(即如果’10m’传入,返回’10m’)

问题是当我打电话时,例如：

>>>  printf("%d" % Size('10m'))

我得到以下内容

SystemError: ../Objects/stringobject.c:4044: bad argument to internal function

显然,如果我打印“％s”,我会得到’10米’

所以问题是,因为我是子类化long,为什么类应该在获取long值时调用__str__.

顺便说一下,多一点测试显示％x和％f将打印整数值,这让我更加困惑.我也尝试添加__format__,但似乎只在调用“…”.format()时调用.

编辑#1,这是代码：

class Size(long):
    '''Represents a size reflected bytes.  Subclass of long.
     Size passed in must be in the formats terabytes
    '''

    units = { 'b':1,'s':512,'k':1024,'m':1024 ** 2,'g':1024 ** 3,'t':1024 ** 4 }

    def __new__(cls,value):
        '''Creates a Size object with the specified value.

        Value can be a number or a string (optionally prefixed with '0x' or
        postfixed with a type character).  If using hex,the final character
        will be treated as part of the value if it is a hex digit,regardless
        of whether it is a valid unit character.

        Examples:
           Size(50)
           Size("0x100s") # 256 sectors
           Size("64")
           Size("512k")
           Size("0x1b") # this is 1b bytes,not 1 byte
        '''
        self = _new_unit_number(value,cls.units,long,cls)
        return self

    def __init__(self,value):
        self._orig_value = value

    def __str__(self):
        print "calling str"
        return str(self._orig_value)  # Convert to str in case the object was created w/an int

    def __format__(self,format_spec):
        print "calling format"
        print format_spec
        try:
            value = format(str(self),format_spec)
        except ValueError:
            value = format(int(self),format_spec)
        return value

def _new_unit_number(value,unit_list,num_type,cls):
    '''Converts a string of numbers followed by a unit character to the
    requested numeric type (int or long for example).
    '''
    base = 10
    start = 0
    digits = string.digits
    try:
        if value[0:2] == '0x':
            start = 2
            base = 16
            digits = string.hexdigits

        if value[-1] in digits:
            return num_type.__new__(cls,value[start:],base)
        else:
            try:
                # Use a regex to split the parts of the unit
                regex_string = '(\d+[%s])' % (''.join(unit_list.keys()))
                parts = [x for x in re.split(regex_string,value[start:]) if x]

                if len(parts) == 1:
                    return num_type.__new__(cls,num_type(value[start:-1],base) * unit_list[value[-1]])
                else:
                    # Total up each part
                    # There's probably a better way to do this.
                    # This converts each unit to its base type,stores it in total,# only to be converted back to the base type. 
                    total = 0
                    for part in parts:
                        total += num_type(part[start:-1],base) * unit_list[part[-1]]

                    # Finally return the requested unit
                    return num_type.__new__(cls,total)
            except KeyError:
                raise ValueError("Invalid %s unit identifier: %s"
                    % (cls.__name__,unit_list[value[-1]]))

    # not a string or empty,see if we can still use the class's constructor
    except (TypeError,IndexError):
        return num_type.__new__(cls,value)

最佳答案

不是答案,但评论太久了.

我发现这个问题非常有趣.我尝试使用以下方法复制行为：

#! /usr/bin/python2.7

class Size (long):
    def __new__ (cls,arg):
        if arg and type (arg) == str:
            if arg [-1] == 'm':
                return super (Size,cls).__new__ (cls,long (arg [:-1] ) * 2 ** 20)
        return super (Size,arg)

    def __init__ (self,arg):
        self.s = arg

    def __str__ (self):
        return self.s

a = Size ('12m')
print (a)
print ('%s' % a)
#The following fails horribly
print ('%d' % a)

OP描述的行为.但现在有趣的是：当我从int继承而不是从long继承时,它运行顺利：

class Size (int):
    def __new__ (cls,int (arg [:-1] ) * 2 ** 20)
        return super (Size,arg):
        self.s = arg

    def __str__ (self):
        return self.s

也就是说,它在python2中工作正常,但在python3中失败.奇怪,奇怪.