astype attempts to convert datetime64[ms] as nanoseconds when missing value in data

[dschallis]

When creating a DataFrame with a millisecond timestamp, created with dtype='datetime64[ms]', this works as expected when there are no missing values in the data:

import pandas as pd
df = pd.DataFrame([1036713600000], dtype='float64')
print(df[0].astype('datetime64[ms]'))

Output:

0   2002-11-08
Name: 0, dtype: datetime64[ns]

Adding a missing value to the data causes the values to get parsed as nanoseconds rather than microseconds, which causes an exception:

df = pd.DataFrame([1036713600000, None], dtype='float64')
print(df[0].astype('datetime64[ms]'))

Output:

Traceback (most recent call last):                                          
  File "./f.py", line 6, in <module>                                        
    print(df[0].astype('datetime64[ms]'))                                   
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/generic.py", line 2411, in astype
    dtype=dtype, copy=copy, raise_on_error=raise_on_error, **kwargs)        
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 2504, in astype
    return self.apply('astype', dtype=dtype, **kwargs)                      
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 2459, in apply
    applied = getattr(b, f)(**kwargs)                                       
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 373, in astype
    values=values, **kwargs)                                                
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 407, in _astype
    fastpath=True, dtype=dtype, klass=klass)                                
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 2101, in make_block
    placement=placement)                                                    
  File "/Users/dsc/.virtualenvs/p3default/lib/python3.4/site-packages/pandas/core/internals.py", line 1795, in __init__
    values = tslib.cast_to_nanoseconds(values)                              
  File "pandas/tslib.pyx", line 2622, in pandas.tslib.cast_to_nanoseconds (pandas/tslib.c:43295)
  File "pandas/tslib.pyx", line 1333, in pandas.tslib._check_dts_bounds (pandas/tslib.c:23332)
pandas.tslib.OutOfBoundsDatetime: Out of bounds nanosecond timestamp: 292278994-08-16 16:47:04

Expected output was something like:

0   2002-11-08
1   NaT
Name: 0, dtype: datetime64[ns]

This was using python 3.4.2, Pandas 0.16.2.

[jreback]

I guess this is a bug, but you should in general simply use pd.to_datetime

In [7]: pd.to_datetime(df[0],unit='ms')
Out[7]: 
0   2002-11-08
1          NaT
Name: 0, dtype: datetime64[ns]

actually I think the astype should simply raise on anything that isn't quite simple.

[jreback]

pull-requests are welcome!

[dschallis]

Thanks @jreback, I'll use pd.to_datetime for now, and see if I can track down the cause of the error when I get a chance.

[jreback]

@dschallis gr8!. as an FYI, you almost always want to use int64 and not float with timestamps

[dschallis]

@jreback Yup, unfortunately I'm reading from a JSON dataset with some missing values for timestamps, so pd.read_json is coercing the timestamp to float64.

[jreback]

http://pandas.pydata.org/pandas-docs/stable/io.html#reading-json

are you passing date_unit ?

how does the JSON represent missing values?

[dschallis]

@jreback hmm, no, I wasn't using date_unit. My data and code looks something like:

# timestamp field omitted when missing:
data = '''
[{"id": 1, "timestamp": 1036713600000},
 {"id": 2}]
'''

df = pd.read_json(data, orient='records', dtype={'id': 'int32', 'timestamp': 'datetime64[ms]'})

print(df); print(df.info()) produces:

id     timestamp
0   1  1.036714e+12
1   2           NaN

<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 2 columns):
id           2 non-null int32
timestamp    1 non-null float64
dtypes: float64(1), int32(1)
memory usage: 40.0 bytes

[jreback]

hmm, I think that should work actually (if you provided date_unit). So sort of a two-fer here.