Fix Python – Set value for particular cell in pandas DataFrame using index

I have created a Pandas DataFrame
df = DataFrame(index=[‘A’,’B’,’C’], columns=[‘x’,’y’])

and have got this

x y
A NaN NaN
B NaN NaN
C NaN NaN

Now, I would like to assign a value to particular cell, for example to row C and column x.
I would expect to get this result:

x y
A NaN NaN
B NaN NaN
C 10 NaN

with this code:
df….