Add db.schema framework

jw.db.schema is a set of classes meant as an interface to describe a database schema.

Signed-off-by: Jan Lindemann <jan@janware.com>

tools/python/jwutils/db/schema/ColumnSet.py

@@ -0,0 +1,43 @@
+# -*- coding: utf-8 -*-
+
+from typing import Optional, List, Iterable, Any
+
+class ColumnSet: # export
+
+    def __init__(self, *args: List[Any], columns: List[Any]=[], table: Optional[Any]=None, names: Optional[List[str]]=None):
+        self.__columns: List[Any] = [*args]
+        self.__columns.extend(columns)
+        self.__table = table
+        if names is not None:
+            assert(table is not None)
+            for name in names:
+                self.__columns.append(table.column(name))
+        if self.__table is not None:
+            for col in columns:
+                assert(col.table == self.__table)
+
+    def __len__(self):
+        return len(self.__columns)
+
+    def __iter__(self):
+        yield from self.__columns
+
+    def __repr__(self):
+        return '|'.join([col.name for col in self.__columns])
+
+    def __getitem__(self, index):
+        return self.__columns[index]
+
+    def __eq__(self, rhs):
+        if self.__table != rhs.__table:
+            return False
+        if len(self.__columns) != len(rhs.__columns):
+            return False
+        for i in range(0, len(self.__columns)):
+            if self.__columns[i].name != rhs.__columns[i].name:
+                return False
+        return True
+
+    @property
+    def columns(self) -> List[Any]:
+        return self.__columns