Improve specifying crawlers via CLI

Instead of removing the sections of unselected crawlers from the config file,
crawler selection now happens in the Pferd after loading the crawlers and is
more sophisticated. It also has better error messages.

PFERD/__main__.py

@@ -6,7 +6,7 @@ from pathlib import Path
 from .cli import PARSER, load_default_section
 from .config import Config, ConfigDumpError, ConfigLoadError, ConfigOptionError
 from .logging import log
-from .pferd import Pferd
+from .pferd import Pferd, PferdLoadError
 from .transformer import RuleParseError
 from .version import NAME, VERSION
 
@@ -24,28 +24,10 @@ def load_config_parser(args: argparse.Namespace) -> configparser.ConfigParser:
             args.command(args, parser)
 
     load_default_section(args, parser)
-    prune_crawlers(args, parser)
 
     return parser
 
 
-def prune_crawlers(
-        args: argparse.Namespace,
-        parser: configparser.ConfigParser,
-) -> None:
-    if not args.crawler:
-        return
-
-    for section in parser.sections():
-        if section.startswith("crawl:"):
-            # TODO Use removeprefix() when switching to 3.9
-            name = section[len("crawl:"):]
-            if name not in args.crawler:
-                parser.remove_section(section)
-
-    # TODO Check if crawlers actually exist
-
-
 def load_config(args: argparse.Namespace) -> Config:
     try:
         return Config(load_config_parser(args))
@@ -119,9 +101,9 @@ def main() -> None:
         exit()
 
     try:
-        pferd = Pferd(config)
+        pferd = Pferd(config, args.crawler)
         asyncio.run(pferd.run())
-    except ConfigOptionError as e:
+    except (PferdLoadError, ConfigOptionError) as e:
         log.unlock()
         log.error(str(e))
         exit(1)

PFERD/pferd.py

@@ -1,4 +1,4 @@
-from typing import Dict
+from typing import Dict, List, Optional
 
 from rich.markup import escape
 
@@ -10,13 +10,22 @@ from .crawlers import CRAWLERS
 from .logging import log
 
 
+class PferdLoadError(Exception):
+    pass
+
+
 class Pferd:
-    def __init__(self, config: Config):
+    def __init__(self, config: Config, crawlers_to_run: Optional[List[str]]):
         """
-        May throw ConfigOptionError.
+        May throw PferdLoadError.
         """
 
+        if crawlers_to_run is not None and len(crawlers_to_run) != len(set(crawlers_to_run)):
+            raise PferdLoadError("Some crawlers were selected multiple times")
+
         self._config = config
+        self._crawlers_to_run = crawlers_to_run
+
         self._authenticators: Dict[str, Authenticator] = {}
         self._crawlers: Dict[str, Crawler] = {}
 
@@ -31,9 +40,13 @@ class Pferd:
             authenticator = authenticator_constructor(name, section, self._config)
             self._authenticators[name] = authenticator
 
-    def _load_crawlers(self) -> None:
+    def _load_crawlers(self) -> List[str]:
+        names = []
+
         for name, section in self._config.crawler_sections():
             log.print(f"[bold bright_cyan]Loading[/] {escape(name)}")
+            names.append(name)
+
             crawl_type = section.get("type")
             crawler_constructor = CRAWLERS.get(crawl_type)
             if crawler_constructor is None:
@@ -42,15 +55,56 @@ class Pferd:
             crawler = crawler_constructor(name, section, self._config, self._authenticators)
             self._crawlers[name] = crawler
 
+        return names
+
+    def _find_crawlers_to_run(self, loaded_crawlers: List[str]) -> List[str]:
+        log.explain_topic("Deciding which crawlers to run")
+
+        if self._crawlers_to_run is None:
+            log.explain("No crawlers specified on CLI")
+            log.explain("Running all loaded crawlers")
+            return loaded_crawlers
+
+        log.explain("Crawlers specified on CLI")
+
+        names: List[str] = []  # With 'crawl:' prefix
+        unknown_names = []  # Without 'crawl:' prefix
+
+        for name in self._crawlers_to_run:
+            section_name = f"crawl:{name}"
+            if section_name in self._crawlers:
+                log.explain(f"Found crawler section named {section_name!r}")
+                names.append(section_name)
+            else:
+                log.explain(f"There's no crawler section named {section_name!r}")
+                unknown_names.append(name)
+
+        if unknown_names:
+            if len(unknown_names) == 1:
+                [name] = unknown_names
+                raise PferdLoadError(f"There is no crawler named {name!r}")
+            else:
+                names_str = ", ".join(repr(name) for name in unknown_names)
+                raise PferdLoadError(f"There are no crawlers named {names_str}")
+
+        return names
+
     async def run(self) -> None:
+        """
+        May throw PferdLoadError or ConfigOptionError.
+        """
+
         # These two functions must run inside the same event loop as the
         # crawlers, so that any new objects (like Conditions or Futures) can
         # obtain the correct event loop.
         self._load_authenticators()
-        self._load_crawlers()
+        loaded_crawlers = self._load_crawlers()
 
-        for name, crawler in self._crawlers.items():
         log.print("")
+
+        for name in self._find_crawlers_to_run(loaded_crawlers):
+            crawler = self._crawlers[name]
+
             log.print(f"[bold bright_cyan]Running[/] {escape(name)}")
 
             try: