Units conversion in Python

Question

SymPy is a great tool for doing units conversions in Python:

>>> from sympy.physics import units
>>> 12. * units.inch / units.m
0.304800000000000

You can easily roll your own:

>>> units.BTU = 1055.05585 * units.J
>>> units.BTU
1055.05585*m**2*kg/s**2

However, I cannot implement this into my application unless I can convert degrees C (absolute) to K to degrees F to degrees R, or any combo thereof.

I thought maybe something like this would work:

units.degC = <<somefunc of units.K>>

But clearly that is the wrong path to go down. Any suggestions for cleanly implementing "offset"-type units conversions in SymPy?

Note: I'm open to trying other units conversion modules, but don't know of any besides Unum, and found it to be cumbersome.

Edit: OK, it is now clear that what I want to do is first determine if the two quantities to be compared are in the same coordinate system. (like time units reference to different epochs or time zones or dB to straight amplitude), make the appropriate transformation, then make the conversion. Are there any general coordinate system management tools? That would be great.

I would make the assumption that °F and °C always refer to Δ°F Δ°C within an expression but refer to absolute when standing alone. I was just wondering if there was a way to make units.degF a function and slap a decorator property() on it to deal with those two conditions.

But for now, I'll set units.C == units.K and try to make it very clear in the documentation to use functions convertCtoK(...) and convertFtoR(...) when dealing with absolute units. (Just kidding. No I won't.)

Answer 1

The Unum documentation has a pretty good writeup on why this is hard:

Unum is unable to handle reliably conversions between °Celsius and Kelvin. The issue is referred as the 'false origin problem' : the 0°Celsius is defined as 273.15 K. This is really a special and annoying case, since in general the value 0 is unaffected by unit conversion, e.g. 0 [m] = 0 [miles] = ... . Here, the conversion Kelvin/°Celsius is characterized by a factor 1 and an offset of 273.15 K. The offset is not feasible in the current version of Unum.

Moreover it will presumably never be integrated in a future version because there is also a conceptual problem : the offset should be applied if the quantity represents an absolute temperature, but it shouldn't if the quantity represents a difference of temperatures. For instance, a raise of temperature of 1° Celsius is equivalent to a raise of 1 K. It is impossible to guess what is in the user mind, whether it's an absolute or a relative temperature. The question of absolute vs relative quantities is unimportant for other units since the answer does not impact the conversion rule. Unum is unable to make the distinction between the two cases.

It's pretty easy to conceptually see the problems with trying to represent absolute temperature conversion symbolically. With any normal relative unit, (x unit) * 2 == (x * 2) unit—unit math is commutative. With absolute temperatures, that breaks down—it's difficult to do anything more complex than straight temperature conversions with no other unit dimensions. You're probably best off keeping all calculations in Kelvin, and converting to and from other temperature units only at the entry and exit points of your code.