The original SP-API getMarketplaceParticipations operation takes no parameters. The regional endpoint (e.g. https://sellingpartnerapi-na.amazon.com) plus credentials are sufficient to obtain the full list of marketplaces where the seller is present.

Before the proposed change, we had to store the originally authorized marketplace ID alongside credentials to make the call work. Say, a seller is present in Japan, but not in Singapore in the Far East region. Then the following code would run correctly:

from sp_api.api import Sellers
from sp_api.base import Marketplaces

sellers = Sellers(marketplace=Marketplaces.JP, credentials=credentials)
result = sellers.get_marketplace_participation()
print(result.payload)

Whereas the same code with marketplace=Marketplaces.SG, would raise an exception:

sp_api.base.exceptions.SellingApiForbiddenException: 
[{'message': 'Access to requested resource is denied.', 'code': 'Unauthorized', 'details': ''}]

The code change allows specifying any marketplace from a particular region to obtain the list of countries where the seller is present.