Allow verbose/quite level to be specified via config file or env var

src/pip/_internal/cli/parser.py

@@ -197,15 +197,20 @@ def _update_defaults(self, defaults):
             if option is None:
                 continue
 
-            if option.action in ('store_true', 'store_false', 'count'):
+            if option.action in ('store_true', 'store_false'):
                 try:
                     val = strtobool(val)
                 except ValueError:
-                    error_msg = invalid_config_error_message(
-                        option.action, key, val
-                    )
-                    self.error(error_msg)
-
+                    self.error(invalid_store_bool_message(key, val))
+            elif option.action == 'count':
+                try:
+                    value = int(val)
+                except ValueError:
+                    self.error(invalid_count_message(key, val))
+                if value < 0:
+                    self.error(invalid_count_message(key, val))
+                else:
+                    val = value
             elif option.action == 'append':
                 val = val.split()
                 val = [self.check_default(option, key, v) for v in val]
@@ -253,14 +258,20 @@ def error(self, msg):
         self.exit(UNKNOWN_ERROR, "{}\n".format(msg))
 
 
-def invalid_config_error_message(action, key, val):
+def invalid_store_bool_message(key, value):
+    # type: (str, str) -> str
+    """Returns a better error message when invalid configuration option
+    of action store_false or store_true is provided.
+    """
+    return ("{} is not a valid value for {} option, "
+            "please specify a boolean value like yes/no, "
+            "true/false or 1/0 instead.").format(value, key)
+
+
+def invalid_count_message(key, value):
+    # type: (str, str) -> str
     """Returns a better error message when invalid configuration option
-    is provided."""
-    if action in ('store_true', 'store_false'):
-        return ("{0} is not a valid value for {1} option, "
-                "please specify a boolean value like yes/no, "
-                "true/false or 1/0 instead.").format(val, key)
-
-    return ("{0} is not a valid value for {1} option, "
-            "please specify a numerical value like 1/0 "
-            "instead.").format(val, key)
+    of action count is provided.
+    """
+    return ("{} is not a valid value for {} option, "
+            "please specify a natural number instead.").format(value, key)