Always specify encodings to open()

Fixes some neat windows behavior where the default code page can be cp1252
Add BOM unit test

core/dbt/compat.py

@@ -82,19 +82,20 @@ def to_native_string(s):
 
 def write_file(path, s):
     if WHICH_PYTHON == 2:
-        with codecs.open(path, 'w', encoding='utf-8') as f:
-            return f.write(to_string(s))
+        open = codecs.open
     else:
-        with open(path, 'w') as f:
-            return f.write(to_string(s))
+        open = builtins.open
+    with open(path, 'w', encoding='utf-8') as f:
+        return f.write(to_string(s))
 
 
 def open_file(path):
-    """Open the path for reading"""
+    """Open the path for reading. It must be utf-8 encoded."""
     if WHICH_PYTHON == 2:
-        return codecs.open(path, encoding='utf-8')
+        open = codecs.open
     else:
-        return open(path)
+        open = builtins.open
+    return open(path, encoding='utf-8')
 
 
 if WHICH_PYTHON == 2:

test/unit/test_agate_helper.py

@@ -13,6 +13,8 @@
 1,n,test,3.2,20180806T11:33:29.320Z,True,NULL
 2,y,asdf,900,20180806T11:35:29.320Z,False,a string"""
 
+SAMPLE_CSV_BOM_DATA = u'\ufeff' + SAMPLE_CSV_DATA
+
 
 EXPECTED = [
     [1, 'n', 'test', Decimal('3.2'),
@@ -41,6 +43,15 @@ def test_from_csv(self):
         for idx, row in enumerate(tbl):
             self.assertEqual(list(row), EXPECTED[idx])
 
+    def test_bom_from_csv(self):
+        path = os.path.join(self.tempdir, 'input.csv')
+        with open(path, 'wb') as fp:
+            fp.write(SAMPLE_CSV_BOM_DATA.encode('utf-8'))
+        tbl = agate_helper.from_csv(path)
+        self.assertEqual(len(tbl), len(EXPECTED))
+        for idx, row in enumerate(tbl):
+            self.assertEqual(list(row), EXPECTED[idx])
+
     def test_from_data(self):
         column_names = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
         data = [