convert numeric column to dedicated `pd.StringDtype()`

[vadella]

Code Sample, a copy-pastable example if possible

pd.Series(range(5, 10), dtype="Int64").astype("string")

raises TypeError: data type not understood

while

pd.Series(range(5, 10)).astype("string")

raises ValueError: StringArray requires a sequence of strings or missing values.

If you first do astype(str):

pd.Series(range(5, 10)).astype(str).astype("string")

and

pd.Series(range(5, 10), dtype="Int64").astype(str).astype("string")

work as expected:

0    5
1    6
2    7
3    8
4    9
dtype: string

While astype(object) raises in both cases ValueError: StringArray requires a sequence of strings or missing values.

Problem description

I can understand the ValueError, since you don't feed strings to the StringArray. Best for me would be if the astype("string") converts it to strings, or if the astype(str) would return a StringArray, but in any case, I would expect both pd.Series(range(5, 10), dtype="Int64").astype("string") and pd.Series(range(5, 10)).astype("string") to raise the same error.

Expected Output

0    5
1    6
2    7
3    8
4    9
dtype: string

or

ValueError: StringArray requires a sequence of strings or missing values.

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]

[TomAugspurger]

Overlaps with #22384, which is trying to solve this problem in general.

In the meantime, we can add support for this in IntegerArray.astype.

[Dr-Irv]

@TomAugspurger Was skimming issues and saw this one.

I'm wondering if Series.astype('string') should be treated as a special case independent of the underlying dtype of the underlying Series. That's because the following code should always work assuming s is a Series:

pd.Series([str(x) if not pd.isna(x) else pd.NA for x in s], dtype="string")

So since we know that the underlying objects in the EA have to support str(), there is a straightforward way of doing that conversion.

If you agree, I can look into doing a PR

[TomAugspurger]

I don't think we should preempt the array from having a chance to perform the conversion. And code-wise, I don't think we'd want a special case for this in NDFrame.astype.

…

On Fri, Jan 24, 2020 at 10:19 AM Irv Lustig ***@***.***> wrote: @TomAugspurger <https://github.com/TomAugspurger> Was skimming issues and saw this one. I'm wondering if Series.astype('string') should be treated as a special case independent of the underlying dtype of the underlying Series. That's because the following code should always work assuming s is a Series: pd.Series([str(x) if not pd.isna(x) else pd.NA for x in s], dtype="string") So since we know that the underlying objects in the EA have to support str(), there is a straightforward way of doing that conversion. If you agree, I can look into doing a PR — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#31204?email_source=notifications&email_token=AAKAOIWWCMHG5VDJ2ALQ6KTQ7MIHNA5CNFSM4KKFMCQ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEJ3JVCQ#issuecomment-578198154>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAKAOIWKL7DYNRJPOCG5XFLQ7MIHNANCNFSM4KKFMCQQ> .

[Dr-Irv]

And code-wise, I don't think we'd want a special case for this in NDFrame.astype.

Well, I'm suggesting that we do want a special case just for StringDtype in NDFrame.astype. It seems natural that the 2 operations below should produce the same result, modulo having an object vs string dtype, for a given Series s (and returning np.nan vs. pd.NA for missing values), independent of the dtype of the Series s:

s.astype(str)
s.astype('string')

[TomAugspurger]

Right, that's definitely desirable. But I don't think NDFrame.astype is the place for the fix.

[TomAugspurger]

For example, consider SparseArray. It implements astype such that the result is also a SparseArray and preserves the sparsity. So Series[SparseArray].astype("string") would return a Series[SparseArray[string]]. But NDFrame.astype has no awareness of that.

[Dr-Irv]

For example, consider SparseArray. It implements astype such that the result is also a SparseArray and preserves the sparsity. So Series[SparseArray].astype("string") would return a Series[SparseArray[string]]. But NDFrame.astype has no awareness of that.

I guess this depends on the semantics of astype() in the following sense.

If I have an EA of type "current_dtype" and I write s.astype("target_dtype"), is it either:

1. the responsibility of the EA with type "current_dtype" to know how to convert to every possible "target_dtype", or
2. the responsibility of EA's of type "target_dtype" to know how to convert whatever types it can to "target_dtype"?

I think you are saying that the design we have supports (1), and I'm suggesting a design corresponding to (2).

Now, the reason that I prefer (2) is that when I construct a Series, and I provide the dtype as an argument, then pandas figures out how to convert the data passed to the Series to the corresponding dtype if it can. That is behavior corresponding to (2). So s=pd.Series([1,2,3], dtype="category") and s=pd.Series([1,0,pd.NA], "boolean") both work, but s=pd.Series([1,2,3], dtype="string") does not.

Another possible design would be to have a property of EA's called something like can_convert_anydtype being True or False, and if True, then astype knows it can ask the EA to convert any dtype, and if False, it then asks the target dtype to do the conversion. So, for StringDtype, we set it to be can_convert_anydtype to True, and for other dtypes set it to False

[TomAugspurger]

We have another issue for an astype dispatch mechanism.

[tritemio]

I want to chime in just to give another use-case from the duplicated issue #31839 .

In addition to conversion to "string", converting "string" to "Int8/16/64" when the initial series contains pd.NA is currently quite tricky:

s = pd.Series(['0', pd.NA], dtype='string')
s.astype('object').replace(pd.NA, np.nan).astype('float64').astype('Int8')

It should be possible to do simply s.astype('Int8')

[vadella]

I want to chime in just to give another use-case from the duplicated issue #31839 .

In addition to conversion to "string", converting "string" to "Int8/16/64" when the initial series contains pd.NA is currently quite tricky:

s = pd.Series(['0', pd.NA], dtype='string')
s.astype('object').replace(pd.NA, np.nan).astype('float64').astype('Int8')

It should be possible to do simply s.astype('Int8')

Your solution can give rounding errors when dealing with large integers. (I've been bitten by this when importing production data. The batch numbers were too large to fit exactly in a float64)

s = pd.Series(["0", pd.NA, str(2 ** 60 + 2)], dtype="string")
s.to_frame().assign(
    a=s.astype("object")
    .replace(pd.NA, np.nan)
    .astype("float64")
    .astype("Int64"),
    b=s.apply(lambda x: int(x) if pd.notnull(x) else x).astype("Int64"),
)

	0	a	b
0	0	0	0
1
2	1152921504606846978	1152921504606846976	1152921504606846978

This explicitly loops over the column, so is not ideal performance wise