colon in python function

#It's a function annotation; function arguments and the return value can be tagged with arbitrary Python expressions. Python itself ignores the annotation (other than saving it), but third-party tools can make use of them.

#In this case, it is intended as type hint: programs like mypy can analyze your code statically (that is, without running it, but only looking at the source code itself) to ensure that only str values are passed as arguments to splitComma.

#A fuller annotation to also specify the return type of the function:
def splitComma(line: str) -> str:
    ...