[Python] Why does an rdkit function only work when I import something else?

There is something seriously strange going on here with RDKit's imports that I don't understand.

I am using some basic functionality (as per their documentation) like so:

from rdkit import Chem

# Create a molecule from a SMILES pattern (arbitrarily chosen for this example)
mol = Chem.MolFromSmiles('CCCC')

# Calculate its molecular weight
print(Chem.Descriptors.ExactMolWt(mol))


Except, this doesn't work.

AttributeError: module 'rdkit.Chem' has no attribute 'Descriptors'
​

However, if I add an import line at the top: from rdkit.Chem import Descriptors. Now it works.

58.078250319999995
​

Note, I HAVE NOT CHANGED ANY OF THE REST OF THE CODE. Only the import, which I do not use directly!

What on earth is going on here?

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