Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 26 2016, 10:47:25)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
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import math
math.fmod(-1,20)
-1.0

I do not see the value in always only having one return from a function. When reading the code, you still need to look through the function for every place retValue is set, so it is not easier to follow or understand. If there is some clean-up to be done, then it does make sense. But if not, why keep to this rule if it provides no benefit? In the validation example someone else posted in this thread, the early returns provide easier to read code, rather than a long block of if/then/else.