pandas check if column is null with query function

Question

I have pandas dataframe that I want to execute on it query function with isnull() or not isnull() condition like that:

In [67]: df_data = pd.DataFrame({'a':[1,20,None,40,50]})
In [68]: df_data
Out[68]:       a
         0   1.0
         1  20.0
         2   NaN
         3  40.0
         4  50.0

if I use this command:

df_data.query('a isnull', engine='python')

or this command:

df_data.query('a isnull()', engine='python')

I get an error:

In [75]: df_data.query('a isnull', engine='python')  
File "<unknown>", line 1    a isnull           
SyntaxError: invalid syntax

In [76]: df_data.query('a isnull()', engine='python')  
File "<unknown>", line 1    a isnull ()           
SyntaxError: invalid syntax

What is the right way to do that?

Thank you.

jezrael · Accepted Answer · 2021-10-04 12:35:08Z

60

Use .:

a = df_data.query('a.isnull()', engine='python')
print (a)
    a
2 NaN

b = df_data.query('a.notnull()', engine='python')
print (b)
      a
0   1.0
1  20.0
3  40.0
4  50.0

You can use also logic NaN != NaN:

a = df_data.query('a != a')
print (a)
    a
 2 NaN

b = df_data.query('a == a')
print (b)
      a
0   1.0
1  20.0
3  40.0
4  50.0

edited Oct 4, 2021 at 12:35

answered Sep 3, 2017 at 7:29

jezrael

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4 Comments

Wassadamo Over a year ago

df_data.query('a.isnull()') works without engine='python'

Priscilla Over a year ago

I can't get this to work without the engine='python'

NikoNyrh Over a year ago

I don't think df_data.query('a.isnull()') works in Pandas 1.x, not sure what broke it.

Daniel Ryan Over a year ago

I really like the a == a trick

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