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14.3.1.1 Terminology

argument
  a string entered on the command-line, and passed by the shell to `execl()` or `execv()`. In Python, arguments are elements of `sys.argv[1:]` (`sys.argv[0]` is the name of the program being executed). Unix shells also use the term ``word''.

  It is occasionally desirable to substitute an argument list other than `sys.argv[1:]`, so you should read ``argument'' as ``an element of `sys.argv[1:]`, or of some other list provided as a substitute for `sys.argv[1:]`''.

option
  an argument used to supply extra information to guide or customize the execution of a program. There are many different syntaxes for options; the traditional Unix syntax is a hyphen (``-'') followed by a single letter, e.g. `"-x"` or `"-F"`. Also, traditional Unix syntax allows multiple options to be merged into a single argument, e.g. `"-x -F"` is equivalent to `"-xF"`. The GNU project introduced a double hyphen followed by a series of hyphen-separated words, e.g. --file or --dry-run. These are the only two option syntaxes provided by `optparse`.

  Some other option syntaxes that the world has seen include:

  - a hyphen followed by a few letters, e.g. `"-pf"` (this is not the same as multiple options merged into a single argument)

  - a hyphen followed by a whole word, e.g. `"-file"` (this is technically equivalent to the previous syntax, but they aren't usually seen in the same program)

  - a plus sign followed by a single letter, or a few letters, or a word, e.g. `"+f"`, `"+rgb"`

  - a slash followed by a letter, or a few letters, or a word, e.g. `"/f"`, `"/file"`

  These option syntaxes are not supported by `optparse`, and they never will be. This is deliberate: the first three are non-standard on any environment, and the last only makes sense if you're exclusively targeting VMS, MS-DOS, and/or Windows.

option argument
  an argument that follows an option, is closely associated with that option, and is consumed from the argument list when that option is. With `optparse`, option arguments may either be in a separate argument from their option:

      -f foo
      --file foo

  or included in the same argument:

      -ffoo
      --file=foo

  Typically, a given option either takes an argument or it doesn't. Lots of people want an ``optional option arguments'' feature, meaning that some options will take an argument if they see it, and won't if they don't. This is somewhat controversial, because it makes parsing ambiguous: if `"-a"` takes an optional argument and `"-b"` is another option entirely, how do we interpret `"-ab"`? Because of this ambiguity, `optparse` does not support this feature.

positional argument
  something leftover in the argument list after options have been parsed, i.e. after options and their arguments have been parsed and removed from the argument list.
required option
  an option that must be supplied on the command-line; note that the phrase ``required option'' is self-contradictory in English. `optparse` doesn't prevent you from implementing required options, but doesn't give you much help at it either. See `examples/required_1.py` and `examples/required_2.py` in the `optparse` source distribution for two ways to implement required options with `optparse`.

For example, consider this hypothetical command-line:

    prog -v --report /tmp/report.txt foo bar

-v and --report are both options. Assuming that --report takes one argument, `"/tmp/report.txt"` is an option argument. `"foo"` and `"bar"` are positional arguments.