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我正在尝试在需要在维度之间进行转换的情况下使用品脱，例如。液体盎司到克。

我的转换所需的值在数据库中，并且会因各种物质而变化（例如，不同液体的不同密度），所以我使用 Context.add_transformation() 方法来动态创建我的转换。这是我的测试程序：

#!/opt/env/upgrade/bin/python
import pint

ureg = pint.UnitRegistry()
c = pint.Context('ab')

def vtom(ureg, x):
    # here I print some output just to see if this function is ever called
    # and what arguments are passed.  It is not being called.
    print "vtom(%s, %s)" % (ureg, x)

    # right now I just return the input, so it's 1:1.  I know this is 
    # not right, I am just trying to get pint to find the transform and use it.
    return x

c.add_transformation('[volume]', '[mass]', vtom)
ureg.add_context(c)

PQ = ureg.Quantity

# this works fine    
a = PQ(1 * ureg.oz)
print '%s = %s' % (a, a.to('gram'))

b = PQ(10 * ureg.floz)
c = PQ(10 * ureg.gram)

# both these fail, see exception output below
print '%s = %s' % (c, c.to('floz'))
print '%s = %s' % (b, b.to('gram'))

输出：

1 ounce = 28.349523125 gram
Traceback (most recent call last):
  File "./p.py", line 26, in <module>
    print '%s = %s' % (b, b.to('gram'))
  File "/opt/env/upgrade/local/lib/python2.7/site-packages/pint/quantity.py", line 332, in to
    magnitude = self._convert_magnitude_not_inplace(other, *contexts, **ctx_kwargs)
  File "/opt/env/upgrade/local/lib/python2.7/site-packages/pint/quantity.py", line 300, in _convert_magnitude_not_inplace
    return self._REGISTRY.convert(self._magnitude, self._units, other)
  File "/opt/env/upgrade/local/lib/python2.7/site-packages/pint/registry.py", line 686, in convert
    return self._convert(value, src, dst, inplace)
  File "/opt/env/upgrade/local/lib/python2.7/site-packages/pint/registry.py", line 1214, in _convert
    return super(ContextRegistry, self)._convert(value, src, dst, inplace)
  File "/opt/env/upgrade/local/lib/python2.7/site-packages/pint/registry.py", line 937, in _convert
    return super(NonMultiplicativeRegistry, self)._convert(value, src, dst, inplace)
  File "/opt/env/upgrade/local/lib/python2.7/site-packages/pint/registry.py", line 708, in _convert
    raise DimensionalityError(src, dst, src_dim, dst_dim)
pint.errors.DimensionalityError: Cannot convert from 'fluid_ounce' ([length] ** 3) to 'gram' ([mass])

似乎品脱没有找到我的转变。在units文本文件（pint自带的默认文件）中，定义了'[volume] = [length] ** 3'，所以应该可以走图找到'[mass]'..左右我想...

谢谢！

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1 回答 1

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看起来您需要指定调用时需要使用.to的上下文，因为您的上下文称为“ab”，您应该编写c.to('floz', 'ab'). 这解决了为什么你的函数vtom没有被调用的问题。

看起来您还必须在两个方向上定义转换：

#!/opt/env/upgrade/bin/python
import pint

ureg = pint.UnitRegistry()
c = pint.Context('ab')

def vtom(ureg, x):
    print ("vtom(%s, %s)" % (ureg, x))
    density = 0.9 * ureg.gram / ureg.ml
    return x/density

def vtom2(ureg, x):
    print ("vtom2(%s, %s)" % (ureg, x))
    density = 0.9 * ureg.gram / ureg.ml
    return x*density

c.add_transformation('[volume]', '[mass]', vtom2)
c.add_transformation('[mass]', '[volume]', vtom)
ureg.add_context(c)
#ureg.enable_contexts('ab')
PQ = ureg.Quantity

# this works fine    
a = PQ(1 * ureg.oz)
print ('%s = %s' % (a, a.to('gram')))

b = PQ(10 * ureg.floz)
c = PQ(10 * ureg.gram)

# both these fail, see exception output below
print ('%s = %s' % (c, c.to('floz', 'ab')))
print ('%s = %s' % (b, b.to('gram','ab')))

（我想每个方向都需要不同的功能）。

输出

1 ounce = 28.349523125 gram
vtom(<pint.registry.UnitRegistry object at 0x0390CDB0>, 10 gram)
10 gram = 0.37571136335381117 fluid_ounce
vtom2(<pint.registry.UnitRegistry object at 0x0390CDB0>, 10 fluid_ounce)
10 fluid_ounce = 266.16176606249996 gram

编辑： 正如下面评论中提到的，我认为原始错误很可能是因为库对返回的值进行了一些维度分析以检查它是否有意义，如果没有，则会引发错误。例如，质量可以是密度乘以体积，但不能是体积乘以标量，因为那只是更大/更小的体积，而不是质量。

于 2018-06-14T23:02:34.490 回答