Prevent duplicate objects in python

Question

I have class definition like...

class companyInCountry:
    def __init__(self,name,country):
        self.name = name
        self.country = country
        self.amountOwed = defaultdict(int)

And I'm looping through a table that let's say has 6 rows...

COMPANY     COUNTRY  GROSS  NET
companyA    UK       50     40
companyA    DE       20     15
companyA    UK       10     5
companyA    FR       20     10
companyB    DE       35     25
companyB    DE       10     5

What I want at the end of looping through this table is to end up with many company/territory specific objects, e.g.

object1.name = companyA
object1.territory = UK
object1.amountOwed['GROSS'] = 60
object1.amountOwed['NET'] = 45

But what I'm struggling to visualise is the best way to prevent objects being created that have duplicate company/country combinations (e.g. that would happen for the first time on row 3 in my data). Is there some data type or declaration I can include inside my init def that will ignore duplicates? Or do I need to manually check for the existence of similar objects before calling companyInCountry(name,country) to initialise a new instance?

snakecharmerb · Accepted Answer · 2020-08-14 16:01:08Z

The simplest way to do this would be to maintain a set of (company, country) tuples which can be consulted before creating a new object. If the pair already exists, skip it, otherwise create the object and add the new pair to the set. Something like

pairs = set()
for row in table:
    if (row.company, row.country) in pairs:
        continue
    pairs.add((row.company, row.country))
    company = CompanyInCountry(row.company, row.country)
    # do something with company

If you want a more object-oriented solution, delegate creation of companies to a collection class that performs the necessary checks before creation.


class CompanyCollection:
    def __init__(self):
        # A list to hold the companies - could also be a dict.
        self._companies = []
        self._keys = set()

    def add_company(self, row):
        key = (row.company, row.country)
        if key in self._keys:
            return
        self._companies.append(CompanyInCountry(*key))
        return

    # Define methods for accessing the companies,
    # or whatever you want

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