I think there is one more point worth making here. There is a suggestion that dimensional analysis can underpin a units system. Actually, the idea that all units can be broken down into a small set of fundamental units is very limiting and results in many vexing issues.
For example, consider currencies. There are currently hundreds of national currencies and thousands of cryptocurrencies. They all have the same basic fundamental unit of “value”, but value is only loosely defined. Furthermore, there is no fixed ratio between the currency and its value. It varies over time, over location, and from person to person. Consider units of a particular commodity. The example of a ream of paper was recently mentioned. A ream is 500 sheets of paper. However two reams may not be comparable. They may refer to a different size of paper or a different quality of paper. So all prices for reams of paper would have the same fundamental units of “value per each”, but both “value” and “each” are not necessarily comparable. In effect, the fundamental unit system is not complete. You also need to include information about what you are measuring. For example, “each” could represent a single item of anything. The unit is not complete until you include a description of what that anything is, and in effect, there is an unlimited number of things it could be. Now consider the issue of “unitless units”. In electrical circuit we often talk about gain, which it the ratio between the signal level at the output of a circuit relative to the signal level at the input. But you need to be specific about how you measure signal level. Typically, it would be a voltage, current, or power level. Consider a circuit where both the input and output are measured in volts. Then the gain would have units of "V/V", which is unitless in dimensional analysis. But units of "V/V" (voltage gain) is much different from units of "A/A" (current gain) or "W/W" (power gain), even though they have the same dimensions. Mixing them up results in errors and confusion. An additional complication is that sometimes logarithmic units are used. For example, decibels in voltage, or dBV, is 20*log(Vout/Vin). Again, a dimensionless quantity, but nonetheless "dBV" much different from "V/V". The same issue occurs with the arguments to trigonometric functions like sin(), cos() and tan(). Generally, we assume the arguments are given in radians, which is a dimensionless number. But it could also be given in degrees, another dimensionless number. Radians and degrees are indistinguishable from the perspective on dimensional analysis, but mixing them up results in errors. This is not to knock the idea of dimensional analysis. It is just not something that would be done in most programs that process physical quantities. Rather it is something that is largely done as a one-time check on your analysis. It is a “second opinion” on whether your hand calculation are correct, or at least plausible. So dimensional analysis packages such as pint have their place, but dimensional analysis is not something that belongs in the base language or even the standard library. However I do believe that a case can be made to allow numbers to be easily tagged with units in the base language, and then allowing those units to be accessed as an attribute of the number. Packages such as pint and QuantiPhy could then use that attribute to provide processing of units that is appropriate for the particular application. -Ken _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/A6BVGOUYE2BPO54M63KXFNDOE6FUSJ3F/ Code of Conduct: http://python.org/psf/codeofconduct/
