#14897: Enhance error messages of struct.pack and struct.pack_into

Patch by Matti M?ki.

3 files changed, 18 insertions, 4 deletions

diff --git a/Misc/ACKS b/Misc/ACKS
index 3966ba67cc..1db819477a 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -820,6 +820,7 @@ Michael Muller
 Neil Muller
 Louis Munro
 R. David Murray
+Matti Mäki
 Dale Nagata
 John Nagle
 Takahiro Nakayama
diff --git a/Misc/NEWS b/Misc/NEWS
index 1db5843019..76d4ad9b13 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -49,6 +49,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #14897: Enhance error messages of struct.pack and
+  struct.pack_into. Patch by Matti Mäki.
+
 - Issue #12890: cgitb no longer prints spurious <p> tags in text
   mode when the logdir option is specified.
 
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 0b20e26e6b..0cd0512dcf 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1662,7 +1662,7 @@ s_pack(PyObject *self, PyObject *args)
     if (PyTuple_GET_SIZE(args) != soself->s_len)
     {
         PyErr_Format(StructError,
-            "pack requires exactly %zd arguments", soself->s_len);
+            "pack expected %zd items for packing (got %zd)", soself->s_len, PyTuple_GET_SIZE(args));
         return NULL;
     }
 
@@ -1701,9 +1701,19 @@ s_pack_into(PyObject *self, PyObject *args)
     assert(soself->s_codes != NULL);
     if (PyTuple_GET_SIZE(args) != (soself->s_len + 2))
     {
-        PyErr_Format(StructError,
-                     "pack_into requires exactly %zd arguments",
-                     (soself->s_len + 2));
+        if (PyTuple_GET_SIZE(args) == 0) {
+            PyErr_Format(StructError,
+                        "pack_into expected buffer argument");
+        }
+        else if (PyTuple_GET_SIZE(args) == 1) {
+            PyErr_Format(StructError,
+                        "pack_into expected offset argument");
+        }
+        else {
+            PyErr_Format(StructError,
+                        "pack_into expected %zd items for packing (got %zd)",
+                        soself->s_len, (PyTuple_GET_SIZE(args) - 2));
+        }
         return NULL;
     }