Designing functions
Often functions will act upon global data, or data passed in by the caller. It is important that when the function completes, it leaves this data in a consistent state. Equally so, it is important that, the function can make assumptions about the data before it accesses it.
Pre- and post-conditions
A function will typically alter some data: values passed into the function, data returned by the function, or some global data. It is important when designing a function that you determine what data will be accessed and changed and that these rules are documented.
A function will have pre-conditions, assumptions about the data that it will use. For example, if a function is passed a filename, with the intention that the function will extract some data from the file, whose responsibility is it to check that the file exists? You can make it the responsibility of the function, and so the first few lines will check that the name is a valid path to a file and call operating system...
