Empty query produces empty match, but at the same time is considered non-matching

[i-tub]

I found this a bit surprising: you can have an empty query that "matches" anything, producing an empty match, but at the same time it's considered non-matching:

In [31]: mol = Chem.MolFromSmiles('CCO')                                                                          

In [32]: q = Chem.MolFromSmarts('')                                                                               

In [33]: mol.HasSubstructMatch(q)                                                                                 
Out[33]: False

In [34]: mol.GetSubstructMatches(q)                                                                               
Out[34]: ((),)

In [35]: bool(mol.GetSubstructMatches(q))                                                                         
Out[35]: True

What the right behavior should be is a bit of a philosophical question, but I would argue that if GetSubstructMatches returns a truthy value, HasSubstructMatch should do so as well.

My preference would be for GetSubstructMatches should return () instead of ((),), however, so they would both be false in a Boolean context.

Configuration (please complete the following information):

• RDKit version: 2021.03.1
• OS: CentOS 7
• If you are not using conda: how did you install the RDKit? Schrödinger

[greglandrum]

Agreed. GetSubstructMatches() should return an empty tuple here.