[BUG-REPORT]sort string column failed

[quicksina]

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Description
I loaded the csv file. if the column of string type included none value, i sort accoding this column that it would show wrong below.

df

Out[72]:

   Class                     Address       Number
0    --                      'NY'            1
1   'Sporting Goods'         --              2
2   'Hand Tools'           'Tokyo'           3
3        --                'Shanghai'       nan
4    'Wood Products'       'London'          4
5    'Hardware'              --              6
6    'Paint Supplies'        --              5

df.sort(df.Class)

Traceback (most recent call last):

File "/Users/heyuqi/opt/anaconda3/envs/vaex4/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3418, in run_code exec(code_obj, self.user_global_ns, self.user_ns)
File "", line 1, in
df.sort(df.Class)
File "/Users/heyuqi/development/vaex/install/vaex/packages/vaex-core/vaex/dataframe.py", line 4123, in sort indices = np.argsort(values, kind=kind)
File "<array_function internals>", line 5, in argsort
File "/Users/heyuqi/opt/anaconda3/envs/vaex4/lib/python3.8/site-packages/numpy/core/fromnumeric.py", line 1107, in argsort return _wrapfunc(a, 'argsort', axis=axis, kind=kind, order=order)
File "/Users/heyuqi/opt/anaconda3/envs/vaex4/lib/python3.8/site-packages/numpy/core/fromnumeric.py", line 55, in _wrapfunc return _wrapit(obj, method, *args, **kwds)
File "/Users/heyuqi/opt/anaconda3/envs/vaex4/lib/python3.8/site-packages/numpy/core/fromnumeric.py", line 44, in _wrapit result = getattr(asarray(obj), method)(*args, **kwds)
TypeError: '<' not supported between instances of 'str' and 'NoneType'

Software information

• Vaex version (import vaex; vaex.__version__):
• {'vaex': '4.0.0a13',
  'vaex-core': '4.0.0a19',
  'vaex-viz': '0.5.0.dev1',
  'vaex-hdf5': '0.7.0a6',
  'vaex-server': '0.4.0a3',
  'vaex-astro': '0.8.0a1',
  'vaex-ui': '0.3.0',
  'vaex-jupyter': '0.6.0.dev1',
  'vaex-ml': '0.11.0a5',
  'vaex-graphql': '0.2.0.dev1'}
• Vaex was installed via: from source
• OS: MacOS Big Sur

[JovanVeljanoski]

Hi,

Thanks for the report. This is indeed a missing feature. I will make a test now let's see how easy it is to get this in.

Thanks!