Rethinking ``to_flat_index`` on flat ``Index``

[toobaz]

With reference to #22866, and to a comment I made ( #22866 (comment) ) which I thought was paranoid and maybe is not.

Three considerations:

1. to_flat_index is the only method (the only operation in pandas, actually) I can think of which gives a different result when called on a flat Index and when called on an equivalent 1-level MultiIndex: you obtain the Index itself (each item being assumingly a scalar) in the first case and an Index of length 1 tuples in the second case
2. if one needs, for any reason, an Index of length-one scalar, there is no simple way to obtain it. Or if you prefer: if we add some sep or analogous arguments (e.g. fmt) to to_flat_index, then it is not going to work on a flat Index (at least with the current implementation). In general, to_flat_index is a method which makes sense also because it can be combined with, for instance .map. When doing so, it is good to know for sure that the arguments received by the callable are always tuples.
3. Index.to_flat_index() as it is now is idempotent and pretty useless: it makes sense for compatibility... but then compatibility is a priority!

These all lead me to conclude: shouldn't pd.Index.to_flat_index() return an Index of length-1 tuples?

@WillAyd

Code Sample, a copy-pastable example if possible

In [2]: pd.Index([1, 2, 3,]).to_flat_index()
Out[2]: Int64Index([1, 2, 3], dtype='int64')

In [3]: pd.MultiIndex.from_arrays([pd.Index([1, 2, 3,])]).to_flat_index()
Out[3]: Index([(1,), (2,), (3,)], dtype='object')

Expected Output

Out[2] should be equal to Out[3]

Output of pd.show_versions()

Details

INSTALLED VERSIONS

commit: 454ecfc
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-8-amd64
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+995.g454ecfc61
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.28.4
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.2.2.post1634.dev0+ge8120cf6d
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1
gcsfs: None

[WillAyd]

Hmm not sure I agree. Might just be my personal usage bias but why would someone opt for a 1-D MultiIndex in the first place?

I ultimately foresee most people using sep or the fmt argument you proposed with this method rather than creating an Index of tuples at the end of the day

[toobaz]

Hmm not sure I agree. Might just be my personal usage bias but why would someone opt for a 1-D MultiIndex in the first place?

I don't have in mind a killer application, but my answer to your question is "for the same reason why people would want to call pd.Index.to_flat_index(): compatibility"...

[toobaz]

I ultimately foresee most people using sep or the fmt argument you proposed with this method rather than creating an Index of tuples at the end of the day

... and explaining that .to_flat_index(fmt=callable) is just syntactic sugar for .to_flat_index().map(lambda x : callable(*x)) is I think a nice and concise description - which is currently false for pd.Index.

[h-vetinari]

[...] only method [...] I can think of which gives a different result when called on a flat Index and when called on an equivalent 1-level MultiIndex

FWIW, .str.cat works for Index, but not a 1-level MultiIndex, which I guess qualifies as a different result. ;-)

There's probably many more broken methods for 1-level MultiIndex in the .str accessor (the .str-constructor passes for 1-level MultiIndex, but the methods behind it are completely untested for this case). My current idea/plan is to disable .str for MultiIndex in #23167 (which is in the process of being split into several PRs).

>>> pd.Index(['a', 'b', 'c']).str.cat()
'abc'
>>> pd.MultiIndex.from_arrays([pd.Index(['a', 'b', 'c'])]).str.cat()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\ProgramData\Miniconda3\envs\pf-dev\lib\site-packages\pandas\core\strings.py", line 2275, in cat
    data = Series(self._orig, index=self._orig)
  File "C:\ProgramData\Miniconda3\envs\pf-dev\lib\site-packages\pandas\core\series.py", line 191, in __init__
    raise NotImplementedError("initializing a Series from a "
NotImplementedError: initializing a Series from a MultiIndex is not supported

[toobaz]

FWIW, .str.cat works for Index, but not a 1-level MultiIndex, which I guess qualifies as a different result. ;-)

Right, thanks! But this is clearly a bug... (because at least some other .str methods work...)

My current idea/plan is to disable .str for MultiIndex in #23167

Sorry, naive question, but what is the problem with just running .str on the result of self.get_level_values(0)?

More in general, differences between pd.Index and 1-level pd.MultiIndex could be of two kinds:

• not implemented features of one or the other
• consequences of design decisions

I think there are no differences of the second kind. If we have, and plan to keep, differences of the first kind (and not considering them as "bug"), then I wonder whether we should ban 1-level pd.MultiIndex altogether.

(Notice that if we did that, I would even more convinced of the current issue, where we pretend that pd.Index behaves like a 1-level pd.MultiIndex)

[h-vetinari]

Sorry, naive question, but what is the problem with just running .str on the result of self.get_level_values(0)?

Fair point, will most likely use that. Just didn't bother investigating further when I saw those were mostly broken. Only question then would be the following: Currently, Index.str.<method> always returns an Index. It'd be easy to return the same for 1-level MultiIndex, but would that change (from MI->Index) be confusing/surprising from a user-POV? I guess this goes towards your second point.

[...] then I wonder whether we should ban 1-level pd.MultiIndex altogether.

I admit I don't have the full implications in view, but would be sympathetic (+0.25) to always turn 1-level MI to regular Index where necessary.

(Notice that if we did that, I would even more convinced of the current issue, where we pretend that pd.Index behaves like a 1-level pd.MultiIndex)

I may be missing something, but if that ban should be enforced, then this issue goes away, no?
It would be much more strange (IMO) to have:

In [2]: pd.Index([1, 2, 3,]).to_flat_index()
Out[2]: Index([(1,), (2,), (3,)], dtype='object')

[toobaz]

I may be missing something, but if that ban should be enforced, then this issue goes away, no?
It would be much more strange (IMO) to have:

On the contrary, I would think it is even more important. to_flat_index always produces an index of tuples, except on Index. As of now, we can say (e.g. to a user whose callable expects an iterable argument) "there is the 1-level MultiIndex for that". If we decide to replace all 1-level MultiIndexes with flat Indexes, then we really want Index to behave, on any "assumingly multilevel operation", like a 1-level MultiIndex does now.

[h-vetinari]

then we really want Index to behave, on any "assumingly multilevel operation", like a 1-level MultiIndex does now.

This sounds like an argument against abolishing 1-level MultiIndex to me -- what I mean is that IMO, "assumingly multilevel operation" do not have to work on an Index at all (per se).

[toobaz]

This sounds like an argument against abolishing 1-level MultiIndex to me -- what I mean is that IMO, "assumingly _multi_level operation" do not have to work on an Index at all (per se).

That we do tend to "backport" (all?) MultiIndex methods to flat Index is a fact, right? And I think that's a good thing. That's what I had in mind with "assumingly multilevel"...