allow switch abbrevations as an option

docs/cli.rst

@@ -424,6 +424,27 @@ specify the validators. For example::
 
 Annotations are ignored if the positional decorator is present.
 
+Switch Abbreviations
+^^^^^^^^^^^^^^^^^^^^
+
+The cli supports switches which have been abbreviated by the user, for example, "--h", "--he", or
+"--hel" would all match an actual switch name of"--help", as long as no ambiguity arises from
+multiple switches that might match the same abbreviation. This behavior is disabled by default but
+can be enabled by defining the class-level attribute ``ALLOW_ABBREV`` to True. For example::
+
+    class MyApp(cli.Application):
+        ALLOW_ABBREV = True
+        cheese = cli.Flag(["cheese"], help = "cheese, please")
+        chives = cli.Flag(["chives"], help = "chives, instead")
+
+With the above definition, running the following will raise an error due to ambiguity::
+
+    $ python example.py --ch   # error! matches --cheese and --chives
+
+However, the following two lines are equivalent::
+
+    $ python example.py --che
+    $ python example.py --cheese
 
 
 .. _guide-subcommands:

plumbum/cli/application.py

@@ -124,6 +124,10 @@ def main(self, src, dst):
     * ``SUBCOMMAND_HELPMSG`` - Controls the printing of extra "see subcommand -h" help message.
       Default is a message, set to False to remove.
 
+    * ``ALLOW_ABBREV`` - Controls whether partial switch names are supported, for example '--ver' will match
+      '--verbose'. Default is False for backward consistency with previous plumbum releases. Note that ambiguous
+      abbreviations will not match, for example if --foothis and --foothat are defined, then --foo will not match.
+
     A note on sub-commands: when an application is the root, its ``parent`` attribute is set to
     ``None``. When it is used as a nested-command, ``parent`` will point to its direct ancestor.
     Likewise, when an application is invoked with a sub-command, its ``nested_command`` attribute
@@ -141,6 +145,7 @@ def main(self, src, dst):
     COLOR_GROUPS = None
     CALL_MAIN_IF_NESTED_COMMAND = True
     SUBCOMMAND_HELPMSG = T_("see '{parent} {sub} --help' for more info")
+    ALLOW_ABBREV = False
 
     parent = None
     nested_command = None
@@ -259,6 +264,13 @@ def wrapper(subapp):
 
         return wrapper(subapp) if subapp else wrapper
 
+    def _get_partial_matches(self, partialname):
+        matches = []
+        for switch in self._switches_by_name:
+            if switch.startswith(partialname):
+                matches += [switch, ]
+        return matches
+
     def _parse_args(self, argv):
         tailargs = []
         swfuncs = {}
@@ -289,6 +301,15 @@ def _parse_args(self, argv):
                     argv.insert(0, a[eqsign:])
                 else:
                     name = a[2:]
+
+                if self.ALLOW_ABBREV:
+                    partials = self._get_partial_matches(name)
+                    if len(partials) == 1:
+                        name = partials[0]
+                    elif len(partials) > 1:
+                        raise UnknownSwitch(
+                            T_("Ambiguous partial switch {0}").format("--" + name))
+
                 swname = "--" + name
                 if name not in self._switches_by_name:
                     raise UnknownSwitch(

tests/test_cli.py

@@ -15,6 +15,8 @@ def bacon(self, param):
         print ("!!b", param)
 
     eggs = cli.SwitchAttr(["e"], str, help = "sets the eggs attribute", envname="PLUMBUM_TEST_EGGS")
+    cheese = cli.Flag(["--cheese"], help = "cheese, please")
+    chives = cli.Flag(["--chives"], help = "chives, instead")
     verbose = cli.CountOf(["v"], help = "increases the verbosity level")
     benedict = cli.CountOf(["--benedict"], help = """a very long help message with lots of
         useless information that nobody would ever want to read, but heck, we need to test
@@ -291,5 +293,15 @@ def test_mandatory_env_var(self, capsys):
         stdout, stderr = capsys.readouterr()
         assert "bacon is mandatory" in stdout
 
+    def test_partial_switches(self, capsys):
+        app = SimpleApp
+        app.ALLOW_ABBREV = True
+        inst, rc = app.run(["foo", "--bacon=2", "--ch"], exit=False)
+        stdout, stderr = capsys.readouterr()
+        assert 'Ambiguous partial switch' in stdout
+        assert rc == 2
 
-
+        inst, rc = app.run(["foo", "--bacon=2", "--chee"], exit=False)
+        assert rc == 0
+        assert inst.cheese is True
+        assert inst.chives is False