UPD(index) preserve db by default ,

-  Uncouple --source from -overwrite.
- drop PROMNESIA_INDEX_POLICY env-var.

doc/GUIDE.org

@@ -110,7 +110,9 @@ Also see [[https://github.com/karlicoss/promnesia/issues/172][issues/172]].
 
 ** partial update
 
-(experimental) Set env variable =PROMNESIA_INDEX_POLICY=update=.
+Only index sources given in =promnesia index --sources SOURCE [SOURCE] ...=
+(or all sources, if no =--sources= given), unless =--overwrite= is given,
+in which case all existing visits are removed from db prior to indexing.
 
 ** exclude files from =auto= indexer
 

src/promnesia/__main__.py

@@ -1,8 +1,6 @@
 import argparse
-import os
 import logging
 import inspect
-import os
 import sys
 from typing import List, Tuple, Optional, Dict, Sequence, Iterable, Iterator, Union
 from pathlib import Path
@@ -303,24 +301,15 @@ def main() -> None:
         nargs="+",
         type=_ordinal_or_name,
         metavar="SOURCE",
-        help="Source names (or their 0-indexed position) to index."
-        "  If missing, db is recreated empty and all sources are indexed.",
+        help="Source names (or their 0-indexed position) to index.",
     )
-    overwrite = ep.add_mutually_exclusive_group()
-    overwrite.add_argument(
-        '--update',
+    ep.add_argument(
+        '--overwrite',
         required=False,
-        action="store_const",
-        const=True,
-        dest="overwrite_db",
-        help="Keep existing visits in db and merge new ones collected."
-        "  If neither is given, --update assumed when --sources given (the default)"
-        ", unless PROMNESIA_INDEX_POLICY=(update|overwrite) env-var defined"
-        ", which takes precendance."
-        "  Conflicts with --update.%(default)0.0s"
+        action="store_true",
+        help="Empty db before populating it with newly indexed visits."
+        "  If interrupted, db is left untouched."
     )
-    overwrite.add_argument('--overwrite', required=False, action="store_const", const=False, dest="overwrite_db",
-                    help="The opposite of --update:  recreate db with newly indexed visits%(default)0.0s")
 
     sp = subp.add_parser('serve', help='Serve a link database', formatter_class=F) # type: ignore
     server.setup_parser(sp)
@@ -342,7 +331,7 @@ def main() -> None:
         help='Promnesia source to index as (see https://github.com/karlicoss/promnesia/tree/master/src/promnesia/sources for the full list)',
     )
     ap.add_argument('--sources', required=False, action="extend", nargs="+", type=_ordinal_or_name,
-                    help="Subset of source(s) to run (name or 0-indexed position);  use `promnisia --dry` to view sources")
+                    help="Source names (or their 0-indexed position) to index.")
     ap.add_argument('params', nargs='*', help='Optional extra params for the indexer')
 
     isp = subp.add_parser('install-server', help='Install server as a systemd service (for autostart)', formatter_class=F)
@@ -376,19 +365,6 @@ def main() -> None:
         p.print_help(sys.stderr)
         sys.exit(1)
 
-    overwrite_policy_var = os.environ.get("PROMNESIA_INDEX_POLICY")
-    if overwrite_policy_var:
-        overwrite_policy_var = overwrite_policy_var.lower()
-        if overwrite_policy_var not in ("update", "overwrite"):
-            print(
-                f"Invalid value for PROMNESIA_INDEX_POLICY env-var: {overwrite_policy_var}"
-                "\n  Must be one of (update | overwrite).",
-                file=sys.stderr)
-            sys.exit(2)
-        args.overwrite_db = overwrite_policy_var == "overwrite"
-    if args.overwrite_db is None:
-        args.overwrite_db = not bool(args.sources)
-
     logger.info("CLI args: %s", args)
 
     # TODO maybe, it's better for server to compute intermediate represetnation?
@@ -401,7 +377,7 @@ def main() -> None:
                 config_file=args.config,
                 dry=args.dry,
                 sources_subset=args.sources,
-                overwrite_db=args.overwrite_db,
+                overwrite_db=args.overwrite,
             )
         elif args.mode == 'serve':
             server.run(args)
@@ -415,7 +391,7 @@ def main() -> None:
                 config_file=args.config,
                 name=args.name,
                 sources_subset=args.sources,
-                overwrite_db=args.overwrite_db,
+                overwrite_db=args.overwrite,
                 )
         elif args.mode == 'install-server': # todo rename to 'autostart' or something?
             install_server.install(args)