pmb.helpers.cli: add tab completion option for ask() helper (!1875)

Add a helper class that provides readline-based tab completion and an
extra optional argument "completion_choices" with possible completion
targets.

While at this, also improve paramters documentation for ask() func.

pmb/helpers/cli.py

@@ -3,14 +3,46 @@
 import datetime
 import logging
 import re
+import readline
+
+
+class ReadlineTabCompleter:
+    """ Stores intermediate state for completer function """
+    def __init__(self, options):
+        """
+        :param options: list of possible completions
+        """
+        self.options = sorted(options)
+        self.matches = []
+
+    def completer_func(self, input_text, iteration):
+        """
+        :param input_text: text that shall be autocompleted
+        :param iteration: how many times "tab" was hit
+        """
+        # First time: build match list
+        if iteration == 0:
+            if input_text:
+                self.matches = [s for s in self.options if s and s.startswith(input_text)]
+            else:
+                self.matches = self.options[:]
+
+        # Return the N'th item from the match list, if we have that many.
+        if iteration < len(self.matches):
+            return self.matches[iteration]
+        return None
 
 
 def ask(args, question="Continue?", choices=["y", "n"], default="n",
-        lowercase_answer=True, validation_regex=None):
+        lowercase_answer=True, validation_regex=None, complete=None):
     """
-    Ask a question on the terminal. When validation_regex is set, the user gets
-    asked until the answer matches the regex.
-    :returns: the user's answer
+    Ask a question on the terminal.
+    :param question: display prompt
+    :param choices: short list of possible answers, displayed after prompt if set
+    :param default: default value to return if user doesn't input anything
+    :param lowercase_answer: if True, convert return value to lower case
+    :param validation_regex: if set, keep asking until regex matches
+    :param complete: set to a list to enable tab completion
     """
     while True:
         date = datetime.datetime.now().strftime("%H:%M:%S")
@@ -20,7 +52,16 @@ def ask(args, question="Continue?", choices=["y", "n"], default="n",
         if default:
             question_full += " [" + str(default) + "]"
 
+        if complete:
+            readline.parse_and_bind('tab: complete')
+            readline.set_completer(ReadlineTabCompleter(complete).completer_func)
+
         ret = input(question_full + ": ")
+
+        # Stop completing (question is answered)
+        if complete:
+            readline.set_completer(None)
+
         if lowercase_answer:
             ret = ret.lower()
         if ret == "":

test/test_questions.py

@@ -34,7 +34,7 @@ def fake_answers(monkeypatch, answers):
                     the second question with "n" and so on.
     """
     def fake_ask(args, question="Continue?", choices=["y", "n"], default="n",
-                 lowercase_answer=True, validation_regex=None):
+                 lowercase_answer=True, validation_regex=None, complete=None):
         answer = answers.pop(0)
         logging.info("pmb.helpers.cli.ask() fake answer: " + answer)
         return answer