Processing (possibly) optional arguments in Python

Question

I am working on a series of command line tools which connect to the same server and do related but different things. I'd like users to be able to have a single configuration file where they can place common arguments such as connection information that can be shared across all the tools. Ideally, I'd like something that does the following for me:

1. If the server address is specified at the command line use this and ignore any other values
2. If the server address is not specified at the command line but is in a config file that is specified at the command line use this address. Ignore any other values.
3. If the server address is not specified at the command line or a config file specified at the command, but is available in a in a config file in the user's home directory (say .myapprc), use this value.
4. If the server address is not specified in any of the above mechinisms exit with an error message.

The closest I've seen to this is the configparse module, which from what I can tell offers an option parser that will also look at config files, but does not seem to have the notion of "Must be specified somewhere" which I need.

Does anyone know of an existing module that can cover my use case above? If not, a simple extension to optparse, configparse, or some other module I have not reviewed would also be greatly appreciated.

Answer 1

This-party module configparse is written to extend optparse from the standard Python library. As the optparse docs I pointed to mention, "optparse doesn’t prevent you from implementing required options, but doesn’t give you much help at it either" (though it follows with a couple of URLs that show you ways to do it). Simplest is to use the default value functionality: specify a default value that's not actually a legal value (for something like a server's address, that's pretty easy) -- then, once options are processed, verify that the specified value is legal (which is a good idea anyway!-) and raise the appropriate exception otherwise.

Answer 2

I've used opster's middleware feature together with SafeConfigParser to achieve a similar (but slightly simpler) effect as you ask. You have to implement the specific logic you described yourself, but it assists you enough to make it relatively painless. An example of opster's middleware use is in its test/test.py example.

Answer 3

use a dict to store options to your program.

first parse the option file in the user's directory and store every options in a dict (configparse or any other module is welcome). then parse the command line (using any module you want, optparse might fit well), if an arguments specifies a config file, parse the specified file in a dict and update your options from what you read (dict.update is really handy to merge 2 dict). then store all other arguments into another dict, and merge them again (dict.update again...).

this way, you are sure that the dict in which you stored the options contains the value you want, which was either read from the user's file, from the specified config file or directly from the command line. if it does not contain a required value, exit with an error.