BUG (string): contruction of Series / Index fails from dict keys when "str" dtype is specified explicitly

[jorisvandenbossche]

When not specifying a dtype (inferring the type), construction of Index or Series from dict keys goes fine:

>>> pd.options.future.infer_string = True
>>> d = {"a": 1, "b": 2}
>>> pd.Index(d.keys())
Index(['a', 'b'], dtype='str')

But if you explicitly specify the dtype, then it fails:

>>> pd.Index(d.keys(), dtype="str")
...

File ~/scipy/repos/pandas/pandas/core/arrays/string_arrow.py:206, in ArrowStringArray._from_sequence(cls, scalars, dtype, copy)
    203     return cls(pc.cast(scalars, pa.large_string()))
    205 # convert non-na-likes to str
--> 206 result = lib.ensure_string_array(scalars, copy=copy)
    207 return cls(pa.array(result, type=pa.large_string(), from_pandas=True))

File lib.pyx:727, in pandas._libs.lib.ensure_string_array()

File lib.pyx:822, in pandas._libs.lib.ensure_string_array()

ValueError: Buffer has wrong number of dimensions (expected 1, got 0)

The reason is that at that point we pass the data directly to the dtype's array _from_sequence instead of first pre-processing the data into a numpy array, and _from_sequence calling ensure_string_array directly doesn't seem to be able to handle dict keys (although we do call np.asarray(..) inside ensure_string_array, so not entirely sure what is going wrong)

[tasfia8]

Hi Joris! If I fix this, I could send you a PR. Would you be able to merge my PR then or give suggestions on my PR so it can be merged? I have a school assignment deadline of working on an open source good first issue where the owner will at the end merge my PR. I was wondering if you can assign me this and help me? I am a 4th year Computer Engineering major.

[tasfia8]

Also, would you be able to tell me what files I should look at for this so I can start? Do I fork the main branch?

[KevsterAmp]

Hi @tasfia8 kindly check the contributing docs: https://pandas.pydata.org/docs/development/contributing.html. For guidance regarding github issue assignment, proper format of PRs, etc...

I recommend you to work on an issue with a label good first issue since those issues mainly work on simple fixes that are good for first time contributors

[tasfia8]

I have already started working on this, would you be able to assign me this? I think I can do it and I have read the contributing files thank you.

[KevsterAmp]

@tasfia8 - issue assignment can be found on the contributing docs

[tasfia8]

take

[tasfia8]

@jorisvandenbossche
I think I have figured it out, just wanted to show both of @KevsterAmp and you before I make a PR. I will issue a PR soon and let you know. I get this as output now, is this what you are expecting? I have additional test cases as well and it passes all existing test cases as well.
Output:

The issue was that dict_keys was passed directly to the StringDtype's _from_sequence method, which could not handle non-array-like inputs like dict_keys.