You're missing a key part of the picture: the actual datetime struct definitions, which lie in Include/datetime.h. There are also important comments in there. Here are some key excerpts:
/* Fields are packed into successive bytes, each viewed as unsigned and
* big-endian, unless otherwise noted:
*
* byte offset
* 0 year 2 bytes, 1-9999
* 2 month 1 byte, 1-12
* 3 day 1 byte, 1-31
* 4 hour 1 byte, 0-23
* 5 minute 1 byte, 0-59
* 6 second 1 byte, 0-59
* 7 usecond 3 bytes, 0-999999
* 10
*/
...
/* # of bytes for year, month, day, hour, minute, second, and usecond. */
#define _PyDateTime_DATETIME_DATASIZE 10
...
/* The datetime and time types have hashcodes, and an optional tzinfo member,
* present if and only if hastzinfo is true.
*/
#define _PyTZINFO_HEAD \
PyObject_HEAD \
Py_hash_t hashcode; \
char hastzinfo; /* boolean flag */
...
/* All datetime objects are of PyDateTime_DateTimeType, but that can be
* allocated in two ways too, just like for time objects above. In addition,
* the plain date type is a base class for datetime, so it must also have
* a hastzinfo member (although it's unused there).
*/
...
#define _PyDateTime_DATETIMEHEAD \
_PyTZINFO_HEAD \
unsigned char data[_PyDateTime_DATETIME_DATASIZE];
typedef struct
{
_PyDateTime_DATETIMEHEAD
} _PyDateTime_BaseDateTime; /* hastzinfo false */
typedef struct
{
_PyDateTime_DATETIMEHEAD
unsigned char fold;
PyObject *tzinfo;
} PyDateTime_DateTime; /* hastzinfo true */
Additionally, note the following lines in Modules/_datetimemodule.c:
static PyTypeObject PyDateTime_DateTimeType = {
PyVarObject_HEAD_INIT(NULL, 0)
"datetime.datetime", /* tp_name */
sizeof(PyDateTime_DateTime), /* tp_basicsize */
That tp_basicsize line says sizeof(PyDateTime_DateTime), not sizeof(_PyDateTime_BaseDateTime), and the type doesn't implement any special __sizeof__ handling. That means the datetime.datetime type reports its instance size as the size of a time-zone aware datetime, even for unaware instances.
The 48-byte count you're seeing breaks down as follows:
- 8-byte refcount
- 8-byte type pointer
- 8-byte cached hash
- 1-byte "hastzinfo" flag
- 10-byte manually packed
unsigned char[10] containing datetime data
- 1-byte "fold" flag (DST-related)
- 4-byte padding, to align the tzinfo pointer
- 8-byte tzinfo pointer
This is true even though the actual memory layout of your unaware instance doesn't have a fold flag or tzinfo pointer.
This is, of course, all implementation details. It may be different on a different Python implementation, or a different CPython version, or a 32-bit CPython build, or a CPython debug build (there's extra stuff in the PyObject_HEAD when CPython is compiled with Py_TRACE_REFS defined).
