Python modules:

• Python code is placed in *.py files called modules.

• Modules can be executed directly by passing them as the first argument to the Python interpreter.

• Modules can also be imported into the REPL, at which point all top-level statements in the module are executed in order.

Python functions:

• Named functions are defined using the def keyword followed by the function name and the argument list in parentheses.

• We can return objects from functions using the return statement.

• Return statements without a parameter return None, as does the implicit return at the end of every function body.

Module execution:

• We can detect whether a module has been imported or executed by examining
  the value of the special __name__ variable. If it is equal to the string
  "__main__" our module has been executed directly as a program. By executing a function if this condition is met using the top-level if __name__ == '__main__' idiom at the end of our module, we can make our module both usefully importable and executable, an important testing technique even for short scripts.

• Module code is only executed once, on first import.

• The def keyword is a statement which binds executable code to a function name.

• Command line arguments can be accessed as a list of strings accessible through the argv attribute of the sys module. The zero-th command line argument is the script filename, so the item at index one is the first true argument.

• Python's dynamic typing means our functions can be very generic with respect to the type of their arguments.

Docstrings:

• A literal string as the first line of a function's definition forms the function's docstring. They are typically triple-quoted multiline strings containing usage information.

• Function documentation provided in docstrings can be retrieved using help() in the REPL.

• Module docstrings should be placed near the beginning of the module prior to any Python statements such as import statements.

Comments:

• Comments in Python commence with a hash character and continue to the end of the line.

• The first line of the module can contain a special comment called a shebang, allowing the program loader to launch the correct Python interpreter on all major platforms.