commands/envelope: keep editable headers order

Use the ordered set of header keys from the settings or from the current
envelope when generating an editable envelope.

Fixes https://github.com/pazz/alot/issues/898:
Message header ordering isn't preserved from the editor

Note: collections.OrderedDict has been used for backward compatibility with
Python <3.7.

1 files changed, 29 insertions, 0 deletions

diff --git a/alot/utils/collections.py b/alot/utils/collections.py
new file mode 100644
index 00000000..acde56ce
--- /dev/null
+++ b/alot/utils/collections.py
@@ -0,0 +1,29 @@
+# This file is released under the GNU GPL, version 3 or a later revision.
+# For further details see the COPYING file
+
+from collections.abc import Set
+
+# for backward compatibility with Python <3.7
+from collections import OrderedDict
+
+
+class OrderedSet(Set):
+    """
+    Ordered collection of distinct hashable objects.
+    Taken from https://stackoverflow.com/a/10006674
+    """
+
+    def __init__(self, iterable=()):
+        self.d = OrderedDict.fromkeys(iterable)
+
+    def __len__(self):
+        return len(self.d)
+
+    def __contains__(self, element):
+        return element in self.d
+
+    def __iter__(self):
+        return iter(self.d)
+
+    def __repr__(self):
+        return str(list(self))