BUG/API: allowing scalars in Categorical constructor

[jbrockmendel]

I expected this to raise

>> pd.Categorical("A")
['A']
Categories (1, object): ['A']

This gets listified inside the call to maybe_infer_to_datetimelike in Categorical.__init__

[simonjayhawkins]

the docs for pd.Categorical do indeed suggest values should be list-like. and the string is NOT being interpreted as a list.

>>> pd.Categorical("ABC")
['ABC']
Categories (1, object): ['ABC']
>>>

this is a long standing behaviour, same result on 0.25.3

[jbrockmendel]

Deep inside tests.arrays.categorical.test_constructors.TestCategoricalConstructors.test_constructor is

        # Scalars should be converted to lists
        cat = Categorical(1)
        assert len(cat.categories) == 1
        assert cat.categories[0] == 1
        assert len(cat.codes) == 1
        assert cat.codes[0] == 0