Declare function at end of file in Python

One way that is sort of idiomatic in Python is writing:

def main():    print Kerma()def Kerma():    return "energy / mass"    if __name__ == '__main__':    main()

This allows you to write you code in the order you like as long as you keep calling the function main at the end.

When a Python module (.py file) is run, the top level statements in it are executed in the order they appear, from top to bottom (beginning to end). This means you can't reference something until you've defined it. For example the following will generate the error shown:

c = a + b  # -> NameError: name 'a' is not defineda = 13b = 17

Unlike with many other languages, def and class statements are executable in Python—not just declarative—so you can't reference either a or b until that happens and they're defined. This is why your first example has trouble—you're referencing the Kerma() function before its def statement has executed and body have been processed and the resulting function object bound to the function's name, so it's not defined at that point in the script.

Programs in languages like C++ are usually preprocessed before being run and during this compilation stage the entire program and any #include files it refers to are read and processed all at once. Unlike Python, this language features declarative statements which allow the name and calling sequence of functions (or static type of variables) to be declared (but not defined), before use so that when the compiler encounters their name it has enough information to check their usage, which primarily entails type checking and type conversions, none of which requires their actual contents or code bodies to have been defined yet.

This isn't possible in Python, but quite frankly you will soon find you don't need it at all. The Pythonic way to write code is to divide your program into modules that define classes and functions, and a single "main module" that imports all the others and runs.

For simple throw-away scripts get used to placing the "executable portion" at the end, or better yet, learn to use an interactive Python shell.