how to change the unit of time variable in NetCdf files using python

Question

I want the unit of time to be "hours since" like other which have unit is "day since"

Answer 1

I usually have a code like this:

import numpy as np
import os, subprocess
from netCDF4 import num2date,date2num,Dataset
# --------------------
filein = 'bdy.3d.climatology.nc'
fileout= 'bdy.3d.climatology.from1970.nc'
newunit = 'seconds since 1970-01-01 00:00:00'
# ------------------------------
#cstr = 'cp '+filein+' '+fileout
cstr = 'ncap2 -s "time=double(time)"'+' '+filein+' '+fileout
subprocess.call(cstr,shell=True)
# ------------------------------

ncout = Dataset(fileout,'a');
timevar = ncout.variables['time'];
timein = timevar[:];
# --------------------------------------------
datesin = num2date(timein,timevar.units);
timevar.setncattr('units',newunit)
timevar[:] = date2num(datesin,newunit)
ncout.close()
# --------------------------------------------

In this example I also wanted to change the variable type from "float" to "double" and therefore made a new file using ncap2 instead of ordinary copy (cp oldfile newfile).