子类string.Formatter [英] subclass string.Formatter
问题描述
在此处加上备注:如何定义新的字符串格式器,我尝试对string.Formatter
进行子类化.这是我所做的.不幸的是,我似乎在此过程中将其破坏了
Following a remark here: How to define a new string formatter, I tried subclassing string.Formatter
. Here is what I've done. Unfortunately I seem to have broken it in the process
import string
from math import floor, log10
class CustFormatter(string.Formatter):
"Defines special formatting"
def __init__(self):
super(CustFormatter, self).__init__()
def powerise10(self, x):
if x == 0: return 0, 0
Neg = x < 0
if Neg: x = -x
a = 1.0 * x / 10**(floor(log10(x)))
b = int(floor(log10(x)))
if Neg: a = -a
return a, b
def eng(self, x):
a, b = self.powerise10(x)
if -3 < b < 3: return "%.4g" % x
a = a * 10**(b%3)
b = b - b%3
return "%.4g*10^%s" % (a, b)
def format_field(self, value, format_string):
# handle an invalid format
if format_string == "i":
return self.eng(value)
else:
return super(CustFormatter,self).format_field(value, format_string)
fmt = CustFormatter()
print('{}'.format(0.055412))
print(fmt.format("{0:i} ", 55654654231654))
print(fmt.format("{} ", 0.00254641))
就像在最后一行中一样,我没有按位置引用变量,所以得到了KeyError
.显然,它期望在原始类中是可选的键,但是我不明白为什么,而且我不确定自己做错了什么.
As if as in the last line, I don't refer to the variables by position, I get a KeyError
. It is obviously expecting a key which is optional in the original class but I don't understand why and I am not sure what I've done wrong.
推荐答案
str.format
does auto numbering, while string.Formatter
does not.
修改__init__
并覆盖get_value
即可解决问题.
Modifying __init__
and overriding get_value
will do the trick.
def __init__(self):
super(CustFormatter, self).__init__()
self.last_number = 0
def get_value(self, key, args, kwargs):
if key == '':
key = self.last_number
self.last_number += 1
return super(CustFormatter, self).get_value(key, args, kwargs)
顺便说一句,上面的代码并不严格模仿str.format
行为. str.format
抱怨我们是否将自动编号与手动编号混合使用,但是上面没有.
BTW, above code does not strictly mimic str.format
behavior. str.format
complains if we mix auto numbering with manual number, but above does not.
>>> '{} {1}'.format(1, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: cannot switch from automatic field numbering to manual field specification
>>> '{0} {}'.format(1, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: cannot switch from manual field specification to automatic field numbering
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