title: How to create a netCDF file with python netCDF4?
body: <p>I am learning how to use netCDF4 using Python module in Anaconda. I am trying to append values to two variables I have created <code>time</code> and <code>field</code>:</p>
  <pre><code>from netCDF4 import Dataset
  import numpy as np
  
  root_grp = Dataset('py_netcdf4.nc', 'w', format='NETCDF4')
  root_grp.description = 'Example simulation data'
  
  ndim = 128 # Size of the matrix ndim*ndim
  xdimension = 0.75
  ydimension = 0.75
  # dimensions
  root_grp.createDimension('time', None)
  root_grp.createDimension('x', ndim)
  root_grp.createDimension('y', ndim)
  
  # variables
  time = root_grp.createVariable('time', 'f8', ('time',))
  x = root_grp.createVariable('x', 'f4', ('x',))
  y = root_grp.createVariable('y', 'f4', ('y',))
  field = root_grp.createVariable('field', 'f8', ('time', 'x', 'y',))
  
  # data
  x_range =  np.linspace(0, xdimension, ndim)
  y_range =  np.linspace(0, ydimension, ndim)
  x[:] = x_range
  y[:] = y_range
  for i in range(5):
      time[i] = i*50.0
      field[i,:,:] = np.random.uniform(size=(len(x_range), len(y_range)))
  
  
  root_grp.close
  </code></pre>
  <p>But now when I print one of the variables I get that it's empty(!!):</p>
  <pre><code>Python 2.7.10 |Anaconda 2.4.1 (64-bit)| (default, Sep 15 2015, 14:50:01) 
  [GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] on linux2
  Type &quot;help&quot;, &quot;copyright&quot;, &quot;credits&quot; or &quot;license&quot; for more information.
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  &gt;&gt;&gt; from netCDF4 import Dataset
  &gt;&gt;&gt; root_grp = Dataset('py_netcdf4.nc', 'r', format='NETCDF4')
  &gt;&gt;&gt; print root_grp.variables[&quot;field&quot;][:,:,:]
  []
  &gt;&gt;&gt; 
  </code></pre>
  <p>What am I doing wrong here?</p>