Saving dtype = datetime[ns] data array to text using numpy gives unexpected results

Question

I've extracted some data from a data frame and want to save it as a text file

# df is my dataframe with col1, col2...., as column headers
X = df.col1[df['col3']<35].unique()
X[0:3]
>>array(['1998-01-07T11:00:00.000000000', '1998-01-06T23:00:00.000000000',
   '1998-01-28T11:00:00.000000000'], dtype='datetime64[ns]')
np.savetxt('filename', X, delimiter=' ', header='Some Header')

I'm expected it to save the dates but instead, the file contains the first few lines as:

# Some Header 
8.841708000000000000e+17
8.841276000000000000e+17
8.859852000000000000e+1

What am I missing here?

Warren Weckesser · Accepted Answer · 2017-12-07 13:27:22Z

1

savetxt's default format for each field is '%.18e', which is a floating point format. Add the fmt argument to change the format, e.g.

np.savetxt('filename', X, fmt='%s', delimiter=' ', header='Some Header')

answered Dec 7, 2017 at 13:27

Warren Weckesser

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