dynamically import dateformatter subclasses (#50)

* Use pkgutils to dynamically import dateformat formatter subclasses

* Adjust formatter import and test to confirm we only import once

* Add pytest-ordering dependency to tox

* Use python3.8 compatible caching

* Add pytest-ordering to tox coverage deps

pytest.ini

@@ -1,3 +1,4 @@
 [pytest]
 markers =
-    last: run marked tests after all others
+    last: run marked tests after all others
+    first: run marked tests before all others

src/undate/dateformat/base.py

@@ -1,18 +1,26 @@
-# base class for date format parsers
-from typing import Dict
-
-"""Base class for date format parsing and serializing
+"""
+Base class for date format parsing and serializing
 
 To add support for a new date format:
 
 - create a new file under undate/dateformat
 - extend BaseDateFormat and implement parse and to_string methods
   as desired/appropriate
-- Add your new formatter to [... details TBD ...]
-  so that it will be included in the available formatters
+
+It should be loaded automatically and included in the formatters
+returned by :meth:`BaseDateFormat.available_formatters`
 
 """
 
+import importlib
+import logging
+import pkgutil
+from typing import Dict
+from functools import lru_cache  # functools.cache not available until 3.9
+
+
+logger = logging.getLogger(__name__)
+
 
 class BaseDateFormat:
     """Base class for parsing and formatting dates for specific formats."""
@@ -30,10 +38,34 @@ def to_string(self, undate) -> str:
         # convert an undate or interval to string representation for this format
         raise NotImplementedError
 
+    # cache import class method to ensure we only import once
     @classmethod
-    def available_formatters(cls) -> Dict[str, "BaseDateFormat"]:
-        # FIXME: workaround for circular import problem"
+    @lru_cache
+    def import_formatters(cls):
+        """Import all undate.dateformat formatters
+        so that they will be included in available formatters
+        even if not explicitly imported. Only import once.
+        returns the count of modules imported."""
+        logger.debug("Loading formatters under undate.dateformat")
+        import undate.dateformat
+
+        # load packages under this path with curent package prefix
+        formatter_path = undate.dateformat.__path__
+        formatter_prefix = f"{undate.dateformat.__name__}."
 
-        from undate.dateformat.iso8601 import ISO8601DateFormat
+        import_count = 0
+        for importer, modname, ispkg in pkgutil.iter_modules(
+            formatter_path, formatter_prefix
+        ):
+            # import everything except the current file
+            if not modname.endswith(".base"):
+                importlib.import_module(modname)
+                import_count += 1
 
+        return import_count
+
+    @classmethod
+    def available_formatters(cls) -> Dict[str, "BaseDateFormat"]:
+        # ensure undate formatters are imported
+        cls.import_formatters()
         return {c.name: c for c in cls.__subclasses__()}  # type: ignore

tests/test_dateformat/test_base.py

@@ -1,3 +1,5 @@
+import logging
+
 import pytest
 
 from undate.dateformat.base import BaseDateFormat
@@ -29,6 +31,23 @@ def test_parse_to_string(self):
             BaseDateFormat().to_string(1991)
 
 
+@pytest.mark.first
+def test_import_formatters_import_only_once(caplog):
+    # run first so we can confirm it runs once
+    with caplog.at_level(logging.DEBUG):
+        import_count = BaseDateFormat.import_formatters()
+    # should import at least one thing (iso8601)
+    assert import_count >= 1
+    # should have log entry
+    assert "Loading formatters" in caplog.text
+
+    # if we clear the log and run again, should not do anything
+    caplog.clear()
+    with caplog.at_level(logging.DEBUG):
+        BaseDateFormat.import_formatters()
+    assert "Loading formatters" not in caplog.text
+
+
 @pytest.mark.last
 def test_formatters_unique_error():
     # confirm that our uniqe formatters check fails when it should