How to find row with specific column value only - python-3.x

Question

I am trying to figure out the names who only have specific column value and nothing else.
I have tried filtering the rows according to the column value but that isn't what I want, I want the names who only went to eat pizza.
I want names who only had pizza, so my code should return John only and not peter as john only had pizza
Click to view data frame

Answer 1

Your description is not clear. At first, it looks like a simple .loc will be enough. However, after viewing your picture of sample data, I realized it is not that simple. To get what you want, you need to identify duplicated or non-duplicated names having one Restaurant value only, and pick it. To do this, you need to use nunique and check it eq(1), and assign it a mask m. Finally, using m with slicing to get your desire output:
Your sample data:
In [512]: df
Out[512]:
Name Restaurant
0 john pizza
1 peter kfc
2 john pizza
3 peter pizza
4 peter kfc
5 peter pizza
6 john pizza
m = df.groupby('Name').Restaurant.transform('nunique').eq(1)
df[m]
Out[513]:
Name Res
0 john pizza
2 john pizza
6 john pizza
If you want to show only one row, just chain additional .drop_duplicates
df[m].drop_duplicates()
Out[515]:
Name Restaurant
0 john pizza