API: to_datetime with ordinals and no unit

[chris-b1]

xref #15828

Currently, if ordinals are passed to to_datetime without a unit, a ns resolution is assumed. Given relative rarity of ns ordinals in the wild, I've been tripped up by this, e.g., below. I wonder if it would be better to raise in the case of no unit, and force ordinal parsing to always be explicit?

ordinals = pd.Series([1483228800000, 1483315200000, 1483401600000])

pd.to_datetime(ordinals)  # wrong
Out[49]: 
0   1970-01-01 00:24:43.228800
1   1970-01-01 00:24:43.315200
2   1970-01-01 00:24:43.401600
dtype: datetime64[ns]

pd.to_datetime(ordinals, unit='ms')
Out[51]: 
0   2017-01-01
1   2017-01-02
2   2017-01-03
dtype: datetime64[ns]

[jreback]

I do almost exactly that (but only if origin is specfied) in #15828

unit=None is actually the default (though the docs say ns). would need something like this: f5361ec#L464

so +1 from me to require a unit if the arg is numeric

[atbd]

Hi, i'm working on it and have few questions.
Do I have to take long numbers into account (and so, python2/3 compatibility) ? I think there are too big to be supported.
Plus, with my added tests I found a bug

In [3]: pd.to_datetime(True, unit='ms')
Out[3]: Timestamp('1970-01-01 00:00:00.001000')

In [4]: pd.to_datetime(False, unit='ms')
Out[4]: Timestamp('1970-01-01 00:00:00')

Does somebody confirms it ?

[chris-b1]

Numbers than don't fit into int64 space should already raise, I wouldn't expect this change to hit that, though maybe you've found a corner case.

pd.to_datetime([np.iinfo('i8').max + 1])
OverflowError: int too big to convert

And I agree that's a bug - the no unit case seem to be caught ok.

pd.to_datetime(True)
TypeError: <class 'bool'> is not convertible to datetime

[atbd]

Okay, thanks.

The bug comes from array_with_unit_to_datetime which calls array_to_datetime with unit=None | ns but not in the other unit cases.
For now, I check if the arg type is either bool (must be done because bool inherits from int) or numerical and raise the right error. But this makes conflicts with errors='coerce' for example.

My actual check:

if arg is None:                                                                       
    return None                                                                       
elif isinstance(arg, (bool,)):                                                        
    raise TypeError("{0} is not convertible to datetime"
                    .format(type(arg)))                                               
elif isinstance(arg, (int, float)) and unit is None:                                  
    raise ValueError("a unit is required in case of numerical arg")

Do I keep this behaviour or skip the bool case and raise the same numerical error ?

[chris-b1]

I'd suggest opening a PR with your changes, even if work-in-progress, that will be easier to review.

I'm not sure I completely understand your question, but with errors='coerce', I suppose the right answer is to return an array full of NaTs , although it admittedly looks strange.

In [98]: pd.to_datetime([1, 2], errors='coerce') 
Out[98]: DatetimeIndex(['NaT', 'NaT'], dtype='datetime64[ns]', freq=None)

[jreback]

pd.to_datetime([1, 2], errors='coerce')

this would raise, because you haven't specified a unit and you have a numeric entry (which requires a unit).

The check for unit needs to done before any conversions. Its kind of like the origin= check (which also requires a unit to be specified and a numeric type (int/float)).