Remove PropertyList hack

Now that PropertySet supports __getitem__ and update there
is no need to convert them to OrderedDict before using the
headers.

For merging this now means that if you request to merge some PropertyList
headers you get back a PropertyList (which is critical if
you then want to pass the result to afw)

Still have to copy the input list since it is modified
in place.

python/astro_metadata_translator/headers.py

@@ -89,9 +89,8 @@ def merge_headers(headers, mode="overwrite", sort=False, first=None, last=None):
     if not headers:
         raise ValueError("No headers supplied.")
 
-    # Force PropertyList to OrderedDict
-    # In python 3.7 dicts are guaranteed to retain order
-    headers = [h.toOrderedDict() if hasattr(h, "toOrderedDict") else h for h in headers]
+    # Copy the input list because we will be reorganizing it
+    headers = list(headers)
 
     # With a single header provided return a copy immediately
     if len(headers) == 1:
@@ -246,16 +245,9 @@ class or else support automatic translation class determination.
     The first file located in the search path is used for the correction.
     """
 
-    # PropertyList is not dict-like so force to a dict here to allow the
-    # translation classes to work. We update the original header though.
-    if hasattr(header, "toOrderedDict"):
-        header_to_translate = header.toOrderedDict()
-    else:
-        header_to_translate = header
-
     if translator_class is None:
         try:
-            translator_class = MetadataTranslator.determine_translator(header_to_translate,
+            translator_class = MetadataTranslator.determine_translator(header,
                                                                        filename=filename)
         except ValueError:
             # if the header is not recognized, we should not complain
@@ -265,7 +257,7 @@ class or else support automatic translation class determination.
         raise TypeError(f"Translator class must be a MetadataTranslator, not {translator_class}")
 
     # Create an instance for this header
-    translator = translator_class(header_to_translate, filename=filename)
+    translator = translator_class(header, filename=filename)
 
     # To determine the file look up we need the observation_id and instrument
     try:

python/astro_metadata_translator/observationInfo.py

@@ -95,11 +95,6 @@ def __init__(self, header, filename=None, translator_class=None, pedantic=False,
         # Store the supplied header for later stripping
         self._header = header
 
-        # PropertyList is not dict-like so force to a dict here to simplify
-        # the translation code.
-        if hasattr(header, "toOrderedDict"):
-            header = header.toOrderedDict()
-
         if translator_class is None:
             translator_class = MetadataTranslator.determine_translator(header, filename=filename)
         elif not issubclass(translator_class, MetadataTranslator):

tests/test_headers.py

@@ -68,6 +68,8 @@ def test_one(self):
 
     def test_merging_overwrite(self):
         merged = merge_headers([self.h1, self.h2], mode="overwrite")
+        # The merged header should be the same type as the first header
+        self.assertIsInstance(merged, type(self.h1))
 
         expected = {
             "MJD-OBS": self.h2["MJD-OBS"],