如何在python项目中构造代码； 类或模块？

Question

I am trying to write a primes module in python. One thing I would like to be able to write is

>>> primes.primesLessThan(12)
[2, 3, 5, 7, 11]

However, I would also like to be able to write

>>> primes.primesLessThan.Sundaram(12)
[2, 3, 5, 7, 11]

to force it to use the Sieve of Sundaram. My original idea was to make primesLessThan a class with several static methods, but since __init__ can't return anything, this didn't let me achieve the first example. Would this be better done as a seperate module that primes imports or is there something else I missed?

Answer 1

As a rule of thumb, if you have a class without any instance variables, an empty init method and just a bunch of static methods, then its probably going to be simpler to organize it as a module instead.

#sieves module
def Sundaram(n):
  return [2,3,5,7]

def Eratosthenes(n):
  return [2,3,5,7]

And then you can use the functions from the module

import primes.sieves
primes.sieves.Sundaram(12)

Finally, python functions are first class and can be passed around in function parameter or stored in data structures. This means that if you ever need to write some code that depends on an algorithm choice, you can just pass that as a parameter.

def test_first_primes(algorithm):
   return algorithm(10) == [2,3,5,7]

print (test_first_primes(Sundaram))
print (test_first_primes(Eratosthenes))

Answer 2

Two ways I can think of, to get these kinds of semantics.

• Make primes a class, and then make primesLessThan a property. It would also be a class, which implements __iter__ etc. to simulate a list, while also having some subfunctions. primesLessThan would be a constructor to that class, with the argument having a default to allow passing through.

• Make primes itself support __getitem__ / __iter__ /etc. You can still use properties (with default), but make primesLessThan just set some internal variable in the class, and then return self. This lets you do them in any order ie primes.Sundaram.primesLessThan(12) would work the same way as primes.primesLessThan.Sundaram(12), though, that looks strange to me.

Either one of these are going to be a bit weird on the return values... you can create something that acts like a list, but it obviously won't be. You can have repr show it like a list, and you'll be able to iterate over like a list (ie for prime in primes.Sundaram(12) ), but it can't return an actual list for obvious reasons....