Priority formula in excel or python pandas

Let's consider that valid addresses in A contain a @ character:

df['C'] = df['B'].where(df['B'].str.contains('@'), df['A'])

# or if you don't want to fill with potentially invalid addresses in B
# df['C'] = df['B'].where(df['B'].str.contains('@'),
#                         df['A'].where(df['A'].str.contains('@')))

Or testing if the value is not 'noemail':

df['C'] = df['B'].where(df['B'].ne('noemail'), df['A'])

output:

                A                B                C
0   tim@email.com   josh@email.com   josh@email.com
1  math@email.com  angel@email.com  angel@email.com
2   nil@email.com          noemail    nil@email.com
3                  jorme@email.com  jorme@email.com

I achieved desired result by using excel if conditions , See below

In column C, Paste below formula

=IF(B1="noemail",A1,B1)

Note : you should mark "noemail" if no email id is present in column B .