In python, when argument data type are NaN, how to return an NaN, not an TypeError

In python, when argument data type are NaN, how to return an NaN, not an
TypeError

e.g. I defined an function which needs several input arguments, if some
keyword arguments not being assigned, typically there be an TypeError
message, but I want to change it, to output an NaN as the result, could it
be done?
def myfunc( S0, K ,r....):
if S0 = NaN or .....:
How to do it? Much appreciated.
Edited:
def myfunc(a):
return a / 2.5 + 5
print myfunc('whatever')
>python -u "bisectnewton.py"
Traceback (most recent call last):
File "bisectnewton.py", line 6, in <module>
print myfunc('whatever')
File "bisectnewton.py", line 4, in myfunc
return a / 2.5 + 5
TypeError: unsupported operand type(s) for /: 'str' and 'float'
>Exit code: 1
What I want is, the myfunc(a) only accpets an number as the input, if some
other data type like a string = 'whatever' inputed, I don't want to just
output an default error message, I want it to output something like return
'NaN' to tell others that the input should be an number.
Now I changed it to this, but still not working, btw, is none the same as
NaN? I think they're different.
def myfunc(S0):
if math.isnan(S0):
return 'NaN'
return a / 2.5 + 5
print myfunc('whatever')
>python -u "bisectnewton.py"
Traceback (most recent call last):
File "bisectnewton.py", line 8, in <module>
print myfunc('whatever')
File "bisectnewton.py", line 4, in myfunc
if math.isnan(S0):
TypeError: a float is required
>Exit code: 1
Thanks!