Elegant list of conditions in Python

Question

I'm making a unit converting function for Python for fun.

This is my code, so far:

def UnitConverter(number,*units):
 if units == ("feet","inches"):
 return number*12
 elif units == ("ft","yd"):
 return number/3

You probably get the idea of how I'm having this work.

Because I'm obsessive about elegance, good code practice, and overall flow, I want to know what you coders think about this in general, in addition to my main question: How can I efficiently check for a list of permutations in an if statement?

Example: Is there an efficient way to make this work?

def UnitConverter(number,*units):
 if units == (("feet" or "foot" or "ft."),("inches" or "in" or "in.")):
 return number*12
 elif units == ("ft","yd"):
 return number/3

If not, is there a way to restructure my program such that someone can enter the three arguments number, unit1, unit2 in a way that on the coding end, I can effectively include all alternate spellings of each unit (feet,foot,ft,etc)?

I really value everyone's opinion.

Thanks!

• abarnert: Why are you accepting *units and then repeatedly implicitly forcing it to be two values, instead of just accepting, say, fromunit, tounit as two separate parameters?
• abarnert: Also, trying to handle the cartesian product of N units with N-1 other units is going to mean a whole lot of code if N is much better than 3. Why not do this in two steps: convert from the fromunit to some canonical unit, then convert from the canonical unit to the tounit. Then you only need N*2 if conditions instead of N**2.
• Kyle Kelley: +1 to abarnert, except make it from_unit and to_unit
• jwarner112: @abarnert I'm using *units because I originally planned to return as many conversions as the user wanted, a one-to-many conversion.
• abarnert: One more suggestion here: The (BSD or GNU) units tool does almost exactly what you want, and it comes with a nifty, relatively easy-to-parse data file units.lib that can be used to build, e.g., the dictionary used in RemcoGerlich's answer.

Answer 1

I would pick a standard unit of length, let's say m. Then I would have a single dictionary that gives a factor for each other unit, and convert:

conversion_factors = {
 'foot': 0.3048, # Google search '1 foot in m'
 'yard': 0.9144,
 # etc
}
def unit_convert(number, from_unit='m', to_unit='m'):
 m = number * conversion_factor[from_unit]
 return m / conversion_factor[to_unit]

For the synonyms (feet, ft, etc) you could make a second dictionary and lookup the canonical name in that first:

conversion_factors = { ... } # as above
synonyms = {
 'feet': 'foot',
 'ft': 'foot',
 ...
}
def unit_convert(number, from_unit='m', to_unit='m'):
 from_unit = synonyms.get(from_unit, from_unit)
 to_unit = synonyms.get(to_unit, to_unit)
 # etc

...or just put them in the conversion_factors dictionary multiple times:

conversion_factors = {
 'foot': 0.3048, # Google search '1 foot in m'
 'feet': 0.3048,
 'ft': 0.3048,
 'yard': 0.9144,
 # etc
}

• abarnert: This is exactly how I'd write it... but you should add one example of how it solves the problem the OP had in the first place: multiple names for the same unit. Just add 'feet': 0.3048,.

Answer 2

Use sets.

foot_units = {"ft.", "feet", "foot"}

Then you can check ownership in the sets.

if(units[0] in foot_units):
 ...

Beyond this, make a conversion_factor dictionary that goes to a common conversion element. You can then coerce to your final after.

inches -> feet -> yards
inches -> feet -> feet

RemcoGerlich has a good solution for that step.

• abarnert: If you want to simplify this, I'd use a single 1D table to convert each unit to/from a single canonical unit, instead of a 2D table, which is going to explode combinatorially (and we all know how messy that can be) just like the OP's code.

Answer 3

Perhaps something like the following, using the in operator which checks for containment:

def UnitConverter(number,*units):
 feet = {'feet', 'foot', 'ft.'}
 inches = {'inches', 'in', 'in.'}
 yards = {'yard', 'yd', 'yd.'}
 if units[0] in feet and units[1] in inches:
 return number*12
 elif units[0] in feet and units[1] in yards:
 return number/3