python – Get year, month or day from numpy datetime64-ThrowExceptions

Exception or error:

I have an array of datetime64 type:

dates = np.datetime64(['2010-10-17', '2011-05-13', "2012-01-15"])

Is there a better way than looping through each element just to get np.array of years:

years = f(dates)
#output:
array([2010, 2011, 2012], dtype=int8) #or dtype = string

I’m using stable numpy version 1.6.2.

How to solve:

As datetime is not stable in numpy I would use pandas for this:

In [52]: import pandas as pd

In [53]: dates = pd.DatetimeIndex(['2010-10-17', '2011-05-13', "2012-01-15"])

In [54]: dates.year
Out[54]: array([2010, 2011, 2012], dtype=int32)

Pandas uses numpy datetime internally, but seems to avoid the shortages, that numpy has up to now.

Answer:

I find the following tricks give between 2x and 4x speed increase versus the pandas method described above (i.e. pd.DatetimeIndex(dates).year etc.). The speed of [dt.year for dt in dates.astype(object)] I find to be similar to the pandas method. Also these tricks can be applied directly to ndarrays of any shape (2D, 3D etc.)

dates = np.arange(np.datetime64('2000-01-01'), np.datetime64('2010-01-01'))
years = dates.astype('datetime64[Y]').astype(int) + 1970
months = dates.astype('datetime64[M]').astype(int) % 12 + 1
days = dates - dates.astype('datetime64[M]') + 1

Answer:

There should be an easier way to do this, but, depending on what you’re trying to do, the best route might be to convert to a regular Python datetime object:

datetime64Obj = np.datetime64('2002-07-04T02:55:41-0700')
print datetime64Obj.astype(object).year
# 2002
print datetime64Obj.astype(object).day
# 4

Based on comments below, this seems to only work in Python 2.7.x and Python 3.6+

Answer:

Using numpy version 1.10.4 and pandas version 0.17.1,

dates = np.array(['2010-10-17', '2011-05-13', '2012-01-15'], dtype=np.datetime64)
pd.to_datetime(dates).year

I get what you’re looking for:

array([2010, 2011, 2012], dtype=int32)

Answer:

This is how I do it.

import numpy as np

def dt2cal(dt):
    """
    Convert array of datetime64 to a calendar array of year, month, day, hour,
    minute, seconds, microsecond with these quantites indexed on the last axis.

    Parameters
    ----------
    dt : datetime64 array (...)
        numpy.ndarray of datetimes of arbitrary shape

    Returns
    -------
    cal : uint32 array (..., 7)
        calendar array with last axis representing year, month, day, hour,
        minute, second, microsecond
    """

    # allocate output 
    out = np.empty(dt.shape + (7,), dtype="u4")
    # decompose calendar floors
    Y, M, D, h, m, s = [dt.astype(f"M8[{x}]") for x in "YMDhms"]
    out[..., 0] = Y + 1970 # Gregorian Year
    out[..., 1] = (M - Y) + 1 # month
    out[..., 2] = (D - M) + 1 # dat
    out[..., 3] = (dt - D).astype("m8[h]") # hour
    out[..., 4] = (dt - h).astype("m8[m]") # minute
    out[..., 5] = (dt - m).astype("m8[s]") # second
    out[..., 6] = (dt - s).astype("m8[us]") # microsecond
    return out

It’s vectorized across arbitrary input dimensions, it’s fast, its intuitive, it works on numpy v1.15.4, it doesn’t use pandas.

I really wish numpy supported this functionality, it’s required all the time in application development. I always get super nervous when I have to roll my own stuff like this, I always feel like I’m missing an edge case.

Answer:

If you upgrade to numpy 1.7 (where datetime is still labled as experimental) the following should work.

dates/np.timedelta64(1,'Y')

Answer:

Another possibility is:

np.datetime64(dates,'Y') - returns - numpy.datetime64('2010')

or

np.datetime64(dates,'Y').astype(int)+1970 - returns - 2010

but works only on scalar values, won’t take array

Answer:

Use dates.tolist() to convert to native datetime objects, then simply access year. Example:

>>> dates = np.array(['2010-10-17', '2011-05-13', '2012-01-15'], dtype='datetime64')
>>> [x.year for x in dates.tolist()]
[2010, 2011, 2012]

This is basically the same idea exposed in https://stackoverflow.com/a/35281829/2192272, but using simpler syntax.

Tested with python 3.6 / numpy 1.18.

Answer:

There’s no direct way to do it yet, unfortunately, but there are a couple indirect ways:

[dt.year for dt in dates.astype(object)]

or

[datetime.datetime.strptime(repr(d), "%Y-%m-%d %H:%M:%S").year for d in dates]

both inspired by the examples here.

Both of these work for me on Numpy 1.6.1. You may need to be a bit more careful with the second one, since the repr() for the datetime64 might have a fraction part after a decimal point.

Answer:

Anon’s answer works great for me, but I just need to modify the statement for days

from:

days = dates - dates.astype('datetime64[M]') + 1

to:

days = dates.astype('datetime64[D]') - dates.astype('datetime64[M]') + 1

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