Python如何在内部存储日期时间？ [英] How does Python store datetime internally?

问题描述

我找到了 _datetimemodule.c 这似乎是正确的文件，但是我需要一点帮助，因为C不是我的强项。

I found _datetimemodule.c which seems to be the right file, but I need a bit of help as C is not my strength.

>>> import datetime
>>> import sys
>>> d = datetime.datetime.now()
>>> sys.getsizeof(d)
48
>>> d = datetime.datetime(2018, 12, 31, 23, 59, 59, 123)
>>> sys.getsizeof(d)
48

因此，时区未知的datetime对象需要48个字节。查看 PyDateTime_DateTimeType ，它似乎是 PyDateTime_DateType 和 PyDateTime_TimeType 。也许还 _PyDateTime_BaseTime ？

So a timezone-unaware datetime object nees 48 Bytes. Looking at the PyDateTime_DateTimeType, it seems to be a PyDateTime_DateType and a PyDateTime_TimeType. Maybe also _PyDateTime_BaseTime?

通过查看代码，我的印象是每个字段存储一个组件在 YYYY-mm-dd HH：MM：ss 中，意思是：

From looking at the code, I have the impression that one component is stored for each field in YYYY-mm-dd HH:MM:ss, meaning:

• 年份：例如int（例如 int16_t 为16位）


• 月：例如 int8_t


• 天：例如 int8_t


• 小时：例如 int8_t


• 分钟：例如 int8_t


• 第二个：例如 int8_t


• 微秒：例如 uint16_t

• Year: e.g. int (e.g int16_t would be 16 bit)
• Month: e.g int8_t
• day: e.g. int8_t
• Hour: e.g. int8_t
• Minute: e.g. int8_t
• Second: e.g. int8_t
• Microsecond: e.g. uint16_t

但这将是2 * 16 + 5 * 8 = 72位= 9字节，而不是Python告诉我的48字节。

But that would be 2*16 + 5 * 8 = 72 Bit = 9 Byte and not 48 Byte as Python tells me.

我对日期时间内部结构的假设在哪里呢？

（我想这在Python实现之间可能会有所不同-如果是这样，请关注cPython）

(I guess this might differ between Python implementations - if so, please focus on cPython)

推荐答案

您缺少图片的关键部分：实际的日期时间结构定义，该定义位于 Include / datetime.h 。那里也有重要的评论。以下是一些关键摘录：

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 */

您看到的48字节计数分解如下：

The 48-byte count you're seeing breaks down as follows:

• 8字节引用计数


• 8字节类型指针


• 8字节缓存的哈希


• 1字节 hastzinfo标志


• 7字节填充


• 10字节手动打包的 char [10] 包含日期时间数据


• 6字节填充

• 8-byte refcount
• 8-byte type pointer
• 8-byte cached hash
• 1-byte "hastzinfo" flag
• 7-byte padding
• 10-byte manually packed char[10] containing datetime data
• 6-byte padding

当然，这是所有实施细节。在不同的Python实现，不同的CPython版本，32位CPython构建或CPython调试构建（在定义了Py_TRACE_REFS的情况下编译CPython时，PyObject_HEAD中会有多余的东西）可能会有所不同。

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).

这篇关于Python如何在内部存储日期时间？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！