Fix Python – What is the best way to exit a function (which has no return value) in python before the function ends (e.g. a check fails)?

Let’s assume an iteration in which we call a function without a return value. The way I think my program should behave is explained in this pseudocode:
for element in some_list:
foo(element)

def foo(element):
do something
if check is true:
do more (because check was succesful)
else:
return None
do much much mor….