core: fix utils.format_amount for negative amounts

core/src/trezor/utils.py

@@ -74,8 +74,13 @@ def chunks(items: List[Chunked], size: int) -> Iterator[List[Chunked]]:
 
 
 def format_amount(amount: int, decimals: int) -> str:
+    if amount < 0:
+        amount = -amount
+        sign = "-"
+    else:
+        sign = ""
     d = pow(10, decimals)
-    s = ("%d.%0*d" % (amount // d, decimals, amount % d)).rstrip("0").rstrip(".")
+    s = ("%s%d.%0*d" % (sign, amount // d, decimals, amount % d)).rstrip("0").rstrip(".")
     return s
 
 

core/tests/test_trezor.utils.py

@@ -13,6 +13,16 @@ class TestUtils(unittest.TestCase):
             self.assertEqual(c[i].stop, 100 if (i == 14) else (i + 1) * 7)
             self.assertEqual(c[i].step, 1)
 
+    def test_format_amount(self):
+        VECTORS = [
+            (123456, 3, "123.456"),
+            (4242, 7, "0.0004242"),
+            (-123456, 3, "-123.456"),
+            (-4242, 7, "-0.0004242"),
+        ]
+        for v in VECTORS:
+            self.assertEqual(utils.format_amount(v[0], v[1]), v[2])
+
 
 if __name__ == '__main__':
     unittest.main()