Question

str='test'
example={'test':'value',}
return str in example and example[str] or None

why the seemingly redundant extra test for key str in example?

Answer 1

In this specific example, the check is to first make sure that ‘test’ is actually a valid key in the example dict, otherwise you would get a KeyError exception. Then the logic proceeds to check the key and either return it, or a None if the value of example[str] evals to False

It would be a lot easier if this example simply did:

str='test'
example={'test':'value',}
return example.get(str, None) or None

Update

Even simpler, since the extra param to get() is not needed:

return example.get(str) or None

Update 2: Breaking down the truth tests and boolean operations from the OP (based on comments)

example = {
    'test' : 'value', 
    'test2': 0, 
    'test3': [],
}
test = lambda k: k in example and example[k] or None
print test('test')
# value
print test('test2')
# None
print test('test3')
# None