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SF #1086675: restore "Extending optparse" section (which was dropped
in Python 2.4).

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 \code{"-c"}.  Fixing this is left as an exercise for the reader.
 % $Id: callbacks.txt 415 2004-09-30 02:26:17Z greg $ 
+\subsection{Extending \module{optparse}\label{optparse-extending-optparse}}
+Since the two major controlling factors in how \module{optparse} interprets
+command-line options are the action and type of each option, the most
+likely direction of extension is to add new actions and new types.
+Also, the \code{examples/} directory of the source distribution includes
+several demonstrations of extending \module{optparse} in different ways: e.g. a
+case-insensitive option parser, or two kinds of option parsers that
+implement ``required options''.
+\subsubsection{Adding new types\label{optparse-adding-new-types}}
+To add new types, you need to define your own subclass of \module{optparse}'s Option
+class.  This class has a couple of attributes that define \module{optparse}'s types:
+\member{TYPES} and \member{TYPE{\_}CHECKER}.
+\member{TYPES} is a tuple of type names; in your subclass, simply define a new
+tuple \member{TYPES} that builds on the standard one.
+\member{TYPE{\_}CHECKER} is a dictionary mapping type names to type-checking
+functions.  A type-checking function has the following signature:
+def check_mytype(option, opt, value)
+where \code{option} is an \class{Option} instance, \code{opt} is an option string
+(e.g., \code{"-f"}), and \code{value} is the string from the command line that
+must be checked and converted to your desired type.  \code{check{\_}mytype()}
+should return an object of the hypothetical type \code{mytype}.  The value
+returned by a type-checking function will wind up in the OptionValues
+instance returned by \method{OptionParser.parse{\_}args()}, or be passed to a
+callback as the \code{value} parameter.
+Your type-checking function should raise OptionValueError if it
+encounters any problems.  OptionValueError takes a single string
+argument, which is passed as-is to OptionParser's \method{error()} method,
+which in turn prepends the program name and the string \code{"error:"} and
+prints everything to stderr before terminating the process.
+Here's a silly example that demonstrates adding a \code{complex} option
+type to parse Python-style complex numbers on the command line.  (This
+is even sillier than it used to be, because \module{optparse} 1.3 added built-in
+support for complex numbers, but never mind.)
+First, the necessary imports:
+from copy import copy
+from optparse import Option, OptionValueError
+You need to define your type-checker first, since it's referred to later
+(in the \member{TYPE{\_}CHECKER} class attribute of your Option subclass):
+def check_complex(option, opt, value):
+    try:
+        return complex(value)
+    except ValueError:
+        raise OptionValueError(
+            "option %s: invalid complex value: %r" % (opt, value))
+Finally, the Option subclass:
+class MyOption (Option):
+    TYPES = Option.TYPES + ("complex",)
+    TYPE_CHECKER["complex"] = check_complex
+(If we didn't make a \function{copy()} of \member{Option.TYPE{\_}CHECKER}, we would end
+up modifying the \member{TYPE{\_}CHECKER} attribute of \module{optparse}'s Option class.
+This being Python, nothing stops you from doing that except good manners
+and common sense.)
+That's it!  Now you can write a script that uses the new option type
+just like any other \module{optparse}-based script, except you have to instruct your
+OptionParser to use MyOption instead of Option:
+parser = OptionParser(option_class=MyOption)
+parser.add_option("-c", type="complex")
+Alternately, you can build your own option list and pass it to
+OptionParser; if you don't use \method{add{\_}option()} in the above way, you
+don't need to tell OptionParser which option class to use:
+option_list = [MyOption("-c", action="store", type="complex", dest="c")]
+parser = OptionParser(option_list=option_list)
+\subsubsection{Adding new actions\label{optparse-adding-new-actions}}
+Adding new actions is a bit trickier, because you have to understand
+that \module{optparse} has a couple of classifications for actions:
+\item[``store'' actions]
+actions that result in \module{optparse} storing a value to an attribute of the
+current OptionValues instance; these options require a \member{dest}
+attribute to be supplied to the Option constructor
+\item[``typed'' actions]
+actions that take a value from the command line and expect it to be
+of a certain type; or rather, a string that can be converted to a
+certain type.  These options require a \member{type} attribute to the
+Option constructor.
+These are overlapping sets: some default ``store'' actions are \code{store},
+\code{store{\_}const}, \code{append}, and \code{count}, while the default ``typed''
+actions are \code{store}, \code{append}, and \code{callback}.
+When you add an action, you need to decide if it's a ``store'' action, a
+``typed'' action, neither, or both.  Three class attributes of
+Option (or your Option subclass) control this:
+all actions must be listed in ACTIONS
+``store'' actions are additionally listed here
+``typed'' actions are additionally listed here
+In order to actually implement your new action, you must override
+Option's \method{take{\_}action()} method and add a case that recognizes your
+For example, let's add an \code{extend} action.  This is similar to the
+standard \code{append} action, but instead of taking a single value from
+the command-line and appending it to an existing list, \code{extend} will
+take multiple values in a single comma-delimited string, and extend an
+existing list with them.  That is, if \code{"-{}-names"} is an \code{extend}
+option of type \code{string}, the command line
+--names=foo,bar --names blah --names ding,dong
+would result in a list
+["foo", "bar", "blah", "ding", "dong"]
+Again we define a subclass of Option:
+class MyOption (Option):
+    ACTIONS = Option.ACTIONS + ("extend",)
+    STORE_ACTIONS = Option.STORE_ACTIONS + ("extend",)
+    TYPED_ACTIONS = Option.TYPED_ACTIONS + ("extend",)
+    def take_action(self, action, dest, opt, value, values, parser):
+        if action == "extend":
+            lvalue = value.split(",")
+            values.ensure_value(dest, []).extend(lvalue)
+        else:
+            Option.take_action(
+                self, action, dest, opt, value, values, parser)
+Features of note:
+\item {} 
+\code{extend} both expects a value on the command-line and stores that
+value somewhere, so it goes in both \member{STORE{\_}ACTIONS} and
+\item {} 
+\method{MyOption.take{\_}action()} implements just this one new action, and
+passes control back to \method{Option.take{\_}action()} for the standard
+\module{optparse} actions
+\item {} 
+\code{values} is an instance of the optparse{\_}parser.Values class,
+which provides the very useful \method{ensure{\_}value()} method.
+\method{ensure{\_}value()} is essentially \function{getattr()} with a safety valve;
+it is called as
+values.ensure_value(attr, value)
+If the \code{attr} attribute of \code{values} doesn't exist or is None, then
+ensure{\_}value() first sets it to \code{value}, and then returns 'value.
+This is very handy for actions like \code{extend}, \code{append}, and
+\code{count}, all of which accumulate data in a variable and expect that
+variable to be of a certain type (a list for the first two, an integer
+for the latter).  Using \method{ensure{\_}value()} means that scripts using
+your action don't have to worry about setting a default value for the
+option destinations in question; they can just leave the default as
+None and \method{ensure{\_}value()} will take care of getting it right when
+it's needed.
+\subsubsection{Other reasons to extend \module{optparse}\label{optparse-other-reasons-to-extend-optparse}}
+Adding new types and new actions are the big, obvious reasons why you
+might want to extend \module{optparse}.  I can think of at least two other areas to
+play with.
+First, the simple one: OptionParser tries to be helpful by calling
+\function{sys.exit()} when appropriate, i.e. when there's an error on the
+command line or when the user requests help.  In the former case, the
+traditional course of letting the script crash with a traceback is
+unacceptable; it will make users think there's a bug in your script when
+they make a command-line error.  In the latter case, there's generally
+not much point in carrying on after printing a help message.
+If this behaviour bothers you, it shouldn't be too hard to ``fix'' it.
+You'll have to
+\item {} 
+subclass OptionParser and override \method{error()}
+\item {} 
+subclass Option and override \method{take{\_}action()}{---}you'll
+need to provide your own handling of the \member{help} action that
+doesn't call \function{sys.exit()}
+The second, much more complex, possibility is to override the
+command-line syntax implemented by \module{optparse}.  In this case, you'd leave the
+whole machinery of option actions and types alone, but rewrite the code
+that processes \var{sys.argv}.  You'll need to subclass OptionParser in any
+case; depending on how radical a rewrite you want, you'll probably need
+to override one or all of \method{parse{\_}args()}, \method{{\_}process{\_}long{\_}opt()}, and
+Both of these are left as an exercise for the reader.  I have not tried
+to implement either myself, since I'm quite happy with \module{optparse}'s default
+behaviour (naturally).
+Happy hacking, and don't forget: Use the Source, Luke.
+% $Id: extending.txt 413 2004-09-28 00:59:13Z greg $