Add DownloadAction argparse action for --download

Encapsulate the dbip.csv download process into an argparse action
subclass: OptionalBooleanAction.
(see https://docs.python.org/3/library/argparse.html#action)

This way the download of the required dbip.com file is earlier and the
blocks file parameter (args.blocks) can be set as soon as possible in
order to avoid trying to read a non existing file.

Enables the usage of --download/--no-download.

Bumps the required minimum Python version to 3.9.

Author: pvxe

nft_geoip.py

@@ -51,6 +51,32 @@
                           'country_alpha_2')
 
 
+class DownloadAction(argparse.BooleanOptionalAction):
+    """
+    Custom BooleanOptionalAction to download db-ip csv in case the user
+    specified so.
+    """
+    def __call__(self, parser, namespace, values, option_string=None):
+        super().__call__(parser, namespace, values, option_string=option_string)
+        if namespace.download:
+            filename = namespace.dir+'dbip.csv.gz'
+            url = 'https://download.db-ip.com/free/dbip-country-lite-{}.csv.gz'.format(time.strftime("%Y-%m"))
+            print('Downloading db-ip.com geoip csv file...')
+            r = requests.get(url, stream=True)
+            if r.status_code == 200:
+                with open(filename, 'wb') as f:
+                    r.raw.decode_content = True
+                    shutil.copyfileobj(r.raw, f)
+            else:
+                sys.exit('Error trying to download DB-IP lite geoip csv file. Bailing out...')
+
+            with gzip.open(filename, 'rb') as f_in:
+                with open(namespace.dir+'dbip.csv', 'wb') as f_out:
+                    shutil.copyfileobj(f_in, f_out)
+                    os.remove(filename)
+            # Update blocks arg with the downloaded file
+            setattr(namespace, 'blocks', open(namespace.dir+'dbip.csv', 'r', encoding='utf-8'))
+
 def strip_accent(text):
     """
     Remove accented characters. Convert to ASCII.
@@ -263,10 +289,10 @@ def create_parser():
                             f'(default: {DEFAULT_FILE_ADDRESS})',
                         required=False,
                         dest='blocks')
-    parser.add_argument('-d', '--download', action='store_true',
-                        help='fetch geoip data from db-ip.com. This option overrides --file-address',
-                        required=False,
-                        dest='download')
+    parser.add_argument('-d', '--download', action=DownloadAction,
+                        help='Fetch geoip data from db-ip.com. Overrides --file-address.',
+                        default=False,
+                        required=False)
     parser.add_argument('-o', '--output-dir',
                         help='Existing directory where downloads and output will be saved. '
                              '(default: working directory)',
@@ -295,26 +321,6 @@ def create_parser():
         # Add trailing / for folder path if there isn't
         args.dir += '/' if args.dir[-1:] != '/' else ''
 
-    if args.download:
-        filename = args.dir+'dbip.csv.gz'
-        url = 'https://download.db-ip.com/free/dbip-country-lite-{}.csv.gz'.format(time.strftime("%Y-%m"))
-        print('Downloading db-ip.com geoip csv file...')
-        r = requests.get(url, stream=True)
-        if r.status_code == 200:
-            with open(filename, 'wb') as f:
-                r.raw.decode_content = True
-                shutil.copyfileobj(r.raw, f)
-        else:
-            sys.exit('Error trying to download DB-IP lite geoip csv file. Bailing out...')
-
-        with gzip.open(filename, 'rb') as f_in:
-            with open(args.dir+'dbip.csv', 'wb') as f_out:
-                shutil.copyfileobj(f_in, f_out)
-                os.remove(filename)
-
-        # Update blocks arg with the downloaded file
-        args.blocks = open(args.dir+'dbip.csv', 'r', encoding='utf-8')
-
     if not (args.blocks or args.locations):
         parser.print_help()
         sys.exit('Missing required address and location csv files.')