python
diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
@@ -2501,6 +2501,7 @@ fields, or any other data types containing pointer type fields.
       An optional small integer that allows overriding the alignment of
       structure fields in the instance.  :attr:`_pack_` must already be defined
       when :attr:`_fields_` is assigned, otherwise it will have no effect.
+      Setting this attribute to 0 is the same as not setting it at all.
 
 
    .. attribute:: _anonymous_

diff --git a/Lib/test/test_ctypes/test_pep3118.py b/Lib/test/test_ctypes/test_pep3118.py
@@ -86,6 +86,20 @@ class PackedPoint(Structure):
     _pack_ = 2
     _fields_ = [("x", c_long), ("y", c_long)]
 
+class PointMidPad(Structure):
+    _fields_ = [("x", c_byte), ("y", c_uint64)]
+
+class PackedPointMidPad(Structure):
+    _pack_ = 2
+    _fields_ = [("x", c_byte), ("y", c_uint64)]
+
+class PointEndPad(Structure):
+    _fields_ = [("x", c_uint64), ("y", c_byte)]
+
+class PackedPointEndPad(Structure):
+    _pack_ = 2
+    _fields_ = [("x", c_uint64), ("y", c_byte)]
+
 class Point2(Structure):
     pass
 Point2._fields_ = [("x", c_long), ("y", c_long)]
@@ -185,10 +199,13 @@ class Complete(Structure):
 
     ## structures and unions
 
-    (Point,                     "T{<l:x:<l:y:}".replace('l', s_long),  (),  Point),
-    # packed structures do not implement the pep
-    (PackedPoint,               "B",                                   (),  PackedPoint),
     (Point2,                    "T{<l:x:<l:y:}".replace('l', s_long),  (),  Point2),
+    (Point,                     "T{<l:x:<l:y:}".replace('l', s_long),  (),  Point),
+    (PackedPoint,               "T{<l:x:<l:y:}".replace('l', s_long),  (),  PackedPoint),
+    (PointMidPad,               "T{<b:x:7x<Q:y:}",                     (),  PointMidPad),
+    (PackedPointMidPad,         "T{<b:x:x<Q:y:}",                      (),  PackedPointMidPad),
+    (PointEndPad,               "T{<Q:x:<b:y:7x}",                     (),  PointEndPad),
+    (PackedPointEndPad,         "T{<Q:x:<b:y:x}",                      (),  PackedPointEndPad),
     (EmptyStruct,               "T{}",                                 (),  EmptyStruct),
     # the pep doesn't support unions
     (aUnion,                    "B",                                   (),  aUnion),

diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-02-05-21-54-46.bpo-32780.Dtiz8z.rst b/Misc/NEWS.d/next/Core and Builtins/2018-02-05-21-54-46.bpo-32780.Dtiz8z.rst
@@ -0,0 +1,3 @@
+Inter-field padding is now inserted into the PEP3118 format strings obtained
+from :class:`ctypes.Structure` objects, reflecting their true representation in
+memory.
diff --git a/Modules/_ctypes/stgdict.c b/Modules/_ctypes/stgdict.c
@@ -337,6 +337,29 @@ MakeAnonFields(PyObject *type)
     return 0;
 }
 
+/*
+  Allocate a memory block for a pep3118 format string, copy prefix (if
+  non-null) into it and append `{padding}x` to the end. 
+  Returns NULL on failure, with the error indicator set.
+*/
+char *
+_ctypes_alloc_format_padding(const char *prefix, Py_ssize_t padding)
+{
+    /* int64 decimal characters + x + null */
+    char buf[19 + 1 + 1];
+
+    assert(padding > 0);
+
+    if (padding == 1) {
+        /* Use x instead of 1x, for brevity */
+        return _ctypes_alloc_format_string(prefix, "x");
+    }
+
+    int ret = PyOS_snprintf(buf, sizeof(buf), "%zdx", padding);
+    assert(0 <= ret && ret < sizeof(buf));
+    return _ctypes_alloc_format_string(prefix, buf);
+}
+
 /*
   Retrieve the (optional) _pack_ attribute from a type, the _fields_ attribute,
   and create an StgDictObject.  Used for Structure and Union subclasses.
@@ -346,11 +369,10 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
 {
     StgDictObject *stgdict, *basedict;
     Py_ssize_t len, offset, size, align, i;
-    Py_ssize_t union_size, total_align;
+    Py_ssize_t union_size, total_align, aligned_size;
     Py_ssize_t field_size = 0;
     int bitofs;
     PyObject *tmp;
-    int isPacked;
     int pack;
     Py_ssize_t ffi_ofs;
     int big_endian;
@@ -374,7 +396,6 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
         return -1;
     }
     if (tmp) {
-        isPacked = 1;
         pack = _PyLong_AsInt(tmp);
         Py_DECREF(tmp);
         if (pack < 0) {
@@ -389,7 +410,7 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
         }
     }
     else {
-        isPacked = 0;
+        /* Setting `_pack_ = 0` amounts to using the default alignment */
         pack = 0;
     }
 
@@ -470,12 +491,10 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
     }
 
     assert(stgdict->format == NULL);
-    if (isStruct && !isPacked) {
+    if (isStruct) {
         stgdict->format = _ctypes_alloc_format_string(NULL, "T{");
     } else {
-        /* PEP3118 doesn't support union, or packed structures (well,
-           only standard packing, but we don't support the pep for
-           that). Use 'B' for bytes. */
+        /* PEP3118 doesn't support union. Use 'B' for bytes. */
         stgdict->format = _ctypes_alloc_format_string(NULL, "B");
     }
     if (stgdict->format == NULL)
@@ -543,24 +562,53 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
         } else
             bitsize = 0;
 
-        if (isStruct && !isPacked) {
+        if (isStruct) {
             const char *fieldfmt = dict->format ? dict->format : "B";
             const char *fieldname = PyUnicode_AsUTF8(name);
             char *ptr;
             Py_ssize_t len;
             char *buf;
+            Py_ssize_t last_size = size;
+            Py_ssize_t padding;
 
             if (fieldname == NULL)
             {
                 Py_DECREF(pair);
                 return -1;
             }
 
+            /* construct the field now, as `prop->offset` is `offset` with
+               corrected alignment */
+            prop = PyCField_FromDesc(desc, i,
+                                   &field_size, bitsize, &bitofs,
+                                   &size, &offset, &align,
+                                   pack, big_endian);
+            if (prop == NULL) {
+                Py_DECREF(pair);
+                return -1;
+            }
+
+            /* number of bytes between the end of the last field and the start
+               of this one */
+            padding = ((CFieldObject *)prop)->offset - last_size;
+
+            if (padding > 0) {
+                ptr = stgdict->format;
+                stgdict->format = _ctypes_alloc_format_padding(ptr, padding);
+                PyMem_Free(ptr);
+                if (stgdict->format == NULL) {
+                    Py_DECREF(pair);
+                    Py_DECREF(prop);
+                    return -1;
+                }
+            }
+
             len = strlen(fieldname) + strlen(fieldfmt);
 
             buf = PyMem_Malloc(len + 2 + 1);
             if (buf == NULL) {
                 Py_DECREF(pair);
+                Py_DECREF(prop);
                 PyErr_NoMemory();
                 return -1;
             }
@@ -578,15 +626,9 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
 
             if (stgdict->format == NULL) {
                 Py_DECREF(pair);
+                Py_DECREF(prop);
                 return -1;
             }
-        }
-
-        if (isStruct) {
-            prop = PyCField_FromDesc(desc, i,
-                                   &field_size, bitsize, &bitofs,
-                                   &size, &offset, &align,
-                                   pack, big_endian);
         } else /* union */ {
             size = 0;
             offset = 0;
@@ -595,14 +637,14 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
                                    &field_size, bitsize, &bitofs,
                                    &size, &offset, &align,
                                    pack, big_endian);
+            if (prop == NULL) {
+                Py_DECREF(pair);
+                return -1;
+            }
             union_size = max(size, union_size);
         }
         total_align = max(align, total_align);
 
-        if (!prop) {
-            Py_DECREF(pair);
-            return -1;
-        }
         if (-1 == PyObject_SetAttr(type, name, prop)) {
             Py_DECREF(prop);
             Py_DECREF(pair);
@@ -612,26 +654,41 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
         Py_DECREF(prop);
     }
 
-    if (isStruct && !isPacked) {
-        char *ptr = stgdict->format;
+    if (!isStruct) {
+        size = union_size;
+    }
+
+    /* Adjust the size according to the alignment requirements */
+    aligned_size = ((size + total_align - 1) / total_align) * total_align;
+
+    if (isStruct) {
+        char *ptr;
+        Py_ssize_t padding;
+
+        /* Pad up to the full size of the struct */
+        padding = aligned_size - size;
+        if (padding > 0) {
+            ptr = stgdict->format;
+            stgdict->format = _ctypes_alloc_format_padding(ptr, padding);
+            PyMem_Free(ptr);
+            if (stgdict->format == NULL) {
+                return -1;
+            }
+        }
+
+        ptr = stgdict->format;
         stgdict->format = _ctypes_alloc_format_string(stgdict->format, "}");
         PyMem_Free(ptr);
         if (stgdict->format == NULL)
             return -1;
     }
 
-    if (!isStruct)
-        size = union_size;
-
-    /* Adjust the size according to the alignment requirements */
-    size = ((size + total_align - 1) / total_align) * total_align;
-
     stgdict->ffi_type_pointer.alignment = Py_SAFE_DOWNCAST(total_align,
                                                            Py_ssize_t,
                                                            unsigned short);
-    stgdict->ffi_type_pointer.size = size;
+    stgdict->ffi_type_pointer.size = aligned_size;
 
-    stgdict->size = size;
+    stgdict->size = aligned_size;
     stgdict->align = total_align;
     stgdict->length = len;      /* ADD ffi_ofs? */