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

Patch by Matti Mäki.

3 files changed, 19 insertions, 5 deletions

diff --git a/Misc/ACKS b/Misc/ACKS
index 2b97f51..a070be8 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -710,6 +710,7 @@ Brian Merrell
 Luke Mewburn
 Carl Meyer
 Mike Meyer
+Piotr Meyer
 Steven Miale
 Trent Mick
 Tom Middleton
@@ -742,7 +743,7 @@ Sape Mullender
 Michael Muller
 Neil Muller
 R. David Murray
-Piotr Meyer
+Matti Mäki
 Dale Nagata
 John Nagle
 Takahiro Nakayama
diff --git a/Misc/NEWS b/Misc/NEWS
index f2ba982..4484813 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -135,6 +135,9 @@ 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 558d24b..edbe9b9 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1575,7 +1575,7 @@ 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;
     }
 
@@ -1614,9 +1614,19 @@ 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;
     }