fix #53 by importing Exchange from bw2data.backends

[muelleram]

No description provided.

[muelleram]

#53

[tngTUDOR]

I'm not sure if using "Exchange" in fact was intended here, or if isis the best solution. This requires adding one more class to the module's dependencies.

things suggesting [to me] against using Exchange:

• The function's suggested return type is ExchangeDataset and not Exchange.
• the iterator is supposed to provide a list of EchangeDataset.
• ExchangeDataset class members input_code and output_code are tuples strings, and lack the "database" part of a full Exchange input tuple (which is less informational when printed in a the exception.
• the new dependency does come from a package already imported (not the schemas module, but the proxies)

the opposite

• The raise statement uses Exchange class member names (input and output)
• Exchange class members input and output are tuples like: ('database', 'code'). These are nicer to see "printed" in the error message

[tngTUDOR]

I'm not sure if using "Exchange" in fact was intended here, or if isis the best solution. This requires adding one more class to the module's dependencies.

things suggesting [to me] against using Exchange:

• The function's suggested return type is ExchangeDataset and not Exchange.
• the iterator is supposed to provide a list of EchangeDataset.
• ExchangeDataset class members input_code and output_code are tuples strings, and lack the "database" part of a full Exchange input tuple (which is less informational when printed in a the exception.
• the new dependency does come from a package already imported (not the schemas module, but the proxies)

the opposite

• The raise statement uses Exchange class member names (input and output)
• Exchange class members input and output are tuples like: ('database', 'code'). These are nicer to see "printed" in the error message

I think, Adding Exchange does not bring additional burden in terms of deps, and it helps dev work.

[tngTUDOR]

@muelleram I think the way the pull request looks now, it could be merged.
However, it would be good to have a "test" to make sure that the "MultipleEdges" condition is tested. Do you think you can add this to the PR ?

[muelleram]

@tngTUDOR, thanks, yes, I can write a test for that.

[muelleram]

@tngTUDOR, I added a test. Please check and feel free to merge if you think the PR looks good now.

[tngTUDOR]

Test added along the fix.