Question or problem about Python programming:
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 the problem:
Solution 1:
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.
Solution 2:
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
Solution 3:
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+
Solution 4:
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.
Solution 5:
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)
Solution 6:
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.
Solution 7:
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
Solution 8:
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
Solution 9:
If you upgrade to numpy 1.7 (where datetime is still labeled as experimental) the following should work.
dates/np.timedelta64(1,'Y')
Solution 10:
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.