Is there a convention for indicating a quantity's units in Python code?

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Please note that I'm coming at this from the point of view of human readability rather than informing the program itself of the units in use.

I'm working on a project at the moment with strong links to mechanical engineering, and one of the issues that has come up a couple of times is that it's not clear from my code what units a given quantity - usually a constant, but the criticism could be extended to some variables as well - is measured in.

At the moment, I'm doing this:

length = 28
# ^^^ measured in mm ^^^

But this feels hacky and a bit ugly. I could do this:

length = 28 # <<< measured in mm

That looks better, but I had it drummed into me at university that inline comments are Satanic, and it's a prejudice I'm having trouble letting go of. (Although, having said that, I was only taught C and Java in university, where the pitfalls of inline comments are more obvious. Are inline comments acceptable in Python?)

Is there some useful convention dictating how to indicate a quantity's units in a clear and elegant fashion? Or am I on my own on this one?

comments python
2 Answers

On top of inline comments which are fine;

Option 1 : Variable name

You can set a variable name, yet it may fault if you calculate and change it later - so it better suits constants.

LENGTH_IN_METERS = 3

Option 2: pint

Pint is a nice library that lets you keep up with the variable measurement unit.

e.g.

import pint
ureg = pint.UnitRegistry()
3 * ureg.meter + 4 * ureg.cm
<Quantity(3.04, 'meter')>

Option 3: python-measurement

Easily use and manipulate unit-aware measurement objects in Python.

e.g.

Converting other measures into kilometers:

from measurement.measures import Distance
distance = Distance(mi=10).km
user_141488640

Inline comments are certainly acceptable in Python, but I'm not sure there is a single accepted way for annotating units. You might want to attach the information to the variable name, e.g. length_mm = 28. Or build a proper class or namedtuple, e.g.

>>> from collections import namedtuple
>>> Length = namedtuple('Length', ['value', 'unit'])
>>> ln = Length(28, 'mm')
>>> ln
Length(value=28, unit='mm')
>>> ln.value
28
>>> ln.unit
'mm'
user_62601700
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