gh-122239: Add actual count in unbalanced unpacking error message when possible (#122244)

1 files changed, 54 insertions, 3 deletions

diff --git a/Lib/test/test_unpack.py b/Lib/test/test_unpack.py
index 515ec12..adb30c0 100644
--- a/Lib/test/test_unpack.py
+++ b/Lib/test/test_unpack.py
@@ -18,6 +18,13 @@ Unpack list
     >>> a == 4 and b == 5 and c == 6
     True
 
+Unpack dict
+
+    >>> d = {4: 'four', 5: 'five', 6: 'six'}
+    >>> a, b, c = d
+    >>> a == 4 and b == 5 and c == 6
+    True
+
 Unpack implied tuple
 
     >>> a, b, c = 7, 8, 9
@@ -66,14 +73,14 @@ Unpacking tuple of wrong size
     >>> a, b = t
     Traceback (most recent call last):
       ...
-    ValueError: too many values to unpack (expected 2)
+    ValueError: too many values to unpack (expected 2, got 3)
 
 Unpacking tuple of wrong size
 
     >>> a, b = l
     Traceback (most recent call last):
       ...
-    ValueError: too many values to unpack (expected 2)
+    ValueError: too many values to unpack (expected 2, got 3)
 
 Unpacking sequence too short
 
@@ -140,8 +147,52 @@ Unpacking to an empty iterable should raise ValueError
     >>> () = [42]
     Traceback (most recent call last):
       ...
-    ValueError: too many values to unpack (expected 0)
+    ValueError: too many values to unpack (expected 0, got 1)
+
+Unpacking a larger iterable should raise ValuleError, but it
+should not entirely consume the iterable
 
+    >>> it = iter(range(100))
+    >>> x, y, z = it
+    Traceback (most recent call last):
+      ...
+    ValueError: too many values to unpack (expected 3)
+    >>> next(it)
+    4
+
+Unpacking unbalanced dict
+
+    >>> d = {4: 'four', 5: 'five', 6: 'six', 7: 'seven'}
+    >>> a, b, c = d
+    Traceback (most recent call last):
+      ...
+    ValueError: too many values to unpack (expected 3, got 4)
+
+Ensure that custom `__len__()` is NOT called when showing the error message
+
+    >>> class LengthTooLong:
+    ...     def __len__(self):
+    ...         return 5
+    ...     def __getitem__(self, i):
+    ...         return i*2
+    ...
+    >>> x, y, z = LengthTooLong()
+    Traceback (most recent call last):
+      ...
+    ValueError: too many values to unpack (expected 3)
+
+For evil cases like these as well, no actual count to be shown
+
+    >>> class BadLength:
+    ...     def __len__(self):
+    ...         return 1
+    ...     def __getitem__(self, i):
+    ...         return i*2
+    ...
+    >>> x, y, z = BadLength()
+    Traceback (most recent call last):
+      ...
+    ValueError: too many values to unpack (expected 3)
 """
 
 __test__ = {'doctests' : doctests}