python pandas: how to find rows in one dataframe but not in another?

Question

Let's say that I have two tables: people_all and people_usa, both with the same structure and therefore the same primary key.

How can I get a table of the people not in the USA? In SQL I'd do something like:

select a.*
from people_all a

left outer join people_usa u
on a.id = u.id

where u.id is null

What would be the Python equivalent? I cannot think of a way to translate this where statement into pandas syntax.

The only way I can think of is to add an arbitrary field to people_usa (e.g. people_usa['dummy']=1), do a left join, then take only the records where 'dummy' is nan, then delete the dummy field - which seems a bit convoluted.

Thanks!

Answer 1

use isin and negate the boolean mask:

people_usa[~people_usa['ID'].isin(people_all ['ID'])]

Example:

In [364]:
people_all = pd.DataFrame({ 'ID' : np.arange(5)})
people_usa = pd.DataFrame({ 'ID' : [3,4,6,7,100]})
people_usa[~people_usa['ID'].isin(people_all['ID'])]

Out[364]:
    ID
2    6
3    7
4  100

so 3 and 4 are removed from the result, the boolean mask looks like this:

In [366]:
people_usa['ID'].isin(people_all['ID'])

Out[366]:
0     True
1     True
2    False
3    False
4    False
Name: ID, dtype: bool

using ~ inverts the mask