Counting occurrences in data-frame without aggregating

Question

I would like to count the number of occurrences in a data frame without aggregating or deleting the duplicated value.

this is what I have tried thus far: df['counts']=df.no_.value_counts() df['counts']=df.groupby(no_).count()

key1 key2  
a  one         
a  two         
b  one         
b  two         
a  one         
c  two         

I would like the result to look like this, the count column need only reflect the column in question (in this case key1)

key1 key2 count(key1)
a  one  3
a  two  3      
b  one  2       
b  two  2      
a  one  3       
c  two  1