天文单位等效-使用类和SI前缀的干涉测量基线

Question

作为以下Astropy单位等效物-干涉测量基线。我想问一下如何使用SI前缀作为自定义单位。到目前为止，根据所附链接中的建议，我从astropy Quantity中创建了一个子类，然后重写了.to方法。

这是我的代码：

class uWavelength(un.Quantity):
def __new__(cls, value, freq=None, dtype=None, copy=True, **kwargs):
unit = un.Unit(un.def_unit('lambdas', format={'format': r'lambda'}, prefixes=True))
self = super().__new__(cls, value=value, unit=unit, dtype=dtype, copy=copy, **kwargs)
self.freq = freq
if self.freq is not None:
self.equivalencies = self.lambdas_equivalencies()
return self
@property
def freq(self):
return self._freq
@freq.setter
def freq(self, val):
if val is not None:
self._equivalencies = self.lambdas_equivalencies(restfreq=val)
self._freq = val
@property
def equivalencies(self):
return self._equivalencies
@equivalencies.setter
def equivalencies(self, val):
self._equivalencies = val
def lambdas_equivalencies(self, restfreq=None):
if self.freq is not None:
restfreq_hz = self.freq.to(un.Hz, equivalencies=un.spectral())
elif restfreq is not None:
restfreq_hz = restfreq.to(un.Hz, equivalencies=un.spectral())
else:
sys.exit("Frequency not provided")
eq = [
(self.unit, un.s, lambda x: x / restfreq_hz, lambda x: x * restfreq_hz),
(self.unit, un.m, lambda x: x / restfreq_hz * co.c.to(un.m / un.s).value,
lambda x: x / co.c.to(un.m / un.s).value * restfreq_hz),
(un.m, un.s, lambda x: x / co.c.to(un.m / un.s).value, lambda x: x * co.c.to(un.m / un.s).value),
]
return eq
def to(self, unit, restfreq=None, copy=True):
equiv = []
if restfreq is None:
equiv = self.equivalencies
else:
equiv = self.lambdas_equivalencies(restfreq=restfreq)
unit = un.Unit(unit)
if copy:
# Avoid using to_value to ensure that we make a copy. We also
# don't want to slow down this method (esp. the scalar case).
value = self._to_value(unit, equiv)
else:
# to_value only copies if necessary
value = self.to_value(unit, equiv)
return self._new_view(value, unit)class uWavelength(un.Quantity):
def __new__(cls, value, freq=None, dtype=None, copy=True, **kwargs):
unit = un.Unit(un.def_unit('lambdas', format={'format': r'lambda'}, prefixes=True))
self = super().__new__(cls, value=value, unit=unit, dtype=dtype, copy=copy, **kwargs)
self.freq = freq
if self.freq is not None:
self.equivalencies = self.lambdas_equivalencies()
return self
@property
def freq(self):
return self._freq
@freq.setter
def freq(self, val):
if val is not None:
self._equivalencies = self.lambdas_equivalencies(restfreq=val)
self._freq = val
@property
def equivalencies(self):
return self._equivalencies
@equivalencies.setter
def equivalencies(self, val):
self._equivalencies = val
def lambdas_equivalencies(self, restfreq=None):
if self.freq is not None:
restfreq_hz = self.freq.to(un.Hz, equivalencies=un.spectral())
elif restfreq is not None:
restfreq_hz = restfreq.to(un.Hz, equivalencies=un.spectral())
else:
sys.exit("Frequency not provided")
eq = [
(self.unit, un.s, lambda x: x / restfreq_hz, lambda x: x * restfreq_hz),
(self.unit, un.m, lambda x: x / restfreq_hz * co.c.to(un.m / un.s).value,
lambda x: x / co.c.to(un.m / un.s).value * restfreq_hz),
(un.m, un.s, lambda x: x / co.c.to(un.m / un.s).value, lambda x: x * co.c.to(un.m / un.s).value),
]
return eq
def to(self, unit, restfreq=None, copy=True):
equiv = []
if restfreq is None:
equiv = self.equivalencies
else:
equiv = self.lambdas_equivalencies(restfreq=restfreq)
unit = un.Unit(unit)
if copy:
# Avoid using to_value to ensure that we make a copy. We also
# don't want to slow down this method (esp. the scalar case).
value = self._to_value(unit, equiv)
else:
# to_value only copies if necessary
value = self.to_value(unit, equiv)
return self._new_view(value, unit)

然而，当我使用类时，我只能使用单位lambda，但我想使用klambda或mega lambda等。根据astropy，这可以通过使用参数prefixes=True来完成，但这似乎不起作用。

Answer 1

我认为您实际上不应该在类的__new__中定义单元，因为当您实例化uWavelength时，它不允许您实际设置单元。

相反，把这个放在你的课堂之外：

lambdas = u.def_unit('lambdas', format={'format': r'lambda'})

我认为文档并没有真正明确的一点是，当你使用prefixes=True时，除非你还提供了namespace=参数，否则它不会真正起到任何作用，即使这样，文档也没有非常清楚地说明如何使用名称空间。我认为最好采用Astrogrog的建议，明确声明所需的前缀单位，如：

klambdas = u.def_unit('kilolambdas', represents=u.CompositeUnit(1e3, [lambdas], [1]), format={'format' : r'klambda'})```
unless you *really* need every imaginable SI prefix, you could try:

lambdas=u.def_unit('lambdas'，format={'format'：r'\lambda'}，前缀=True，namespace=globals(((

and it will inject every prefixed unit into your module namespace.
Then, since you want your default unit for `uWavelength` to be `lambdas`, then both to reduce confusion, and also add some documentation of this fact (through the signature of `__new__`) specify:
```python
class uWavelength(un.Quantity):
def __new__(cls, value, freq=None, unit=lambdas, dtype=None, copy=True, **kwargs):

此外，如果你愿意，你可以添加一个支票，比如：

unit = u.Unit(unit)
assert unit.is_equivalent(lambdas), 'unit must be equivalent to lambdas'