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# bytes

# contents

* [related file](#related-file)

* [memory layout](#memory-layout)

* [example](#example)

* [empty bytes](#empty-bytes)

* [ascii characters](#ascii-characters)

* [nonascii characters](#nonascii-characters)

* [summary](#summary)

* [ob_shash](#ob_shash)

* [ob_size](#ob_size)

* [summary](#summary)

# related file

* cpython/Objects/bytesobject.c

* cpython/Include/bytesobject.h

* cpython/Objects/clinic/bytesobject.c.h

# memory layout

![memory layout](https://img-blog.csdnimg.cn/20190318160629447.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMxNzIwMzI5,size_16,color_FFFFFF,t_70)

The memory layout of **PyBytesObject** looks like [memory layout of tuple object](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/tuple/tuple.md#memory-layout) and [memory layout of int object](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/long/long.md#memory-layout), but simpler than any of them.

# example

## empty bytes

**bytes** object is an immutable object, whenever you need to modify a **bytes** object, you need to create a new one, which keeps the implementation simple.

s = b""

![empty](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/bytes/empty.png)

## ascii characters

let's initialize a byte object with ascii characters

s = b"abcdefg123"

![ascii](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/bytes/ascii.png)

## nonascii characters

s = "我是帅哥".encode("utf8")

![nonascii](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/bytes/nonascii.png)

# summary

## ob_shash

The field **ob_shash** should store the hash value of the byte object, value **-1** means not computed yet.

The first time the hash value computed, it will be cached in the **ob_shash** field

the cached hash value can save recalculation and speeds up dictionary lookups

## ob_size

field **ob_size** is inside every **PyVarObject**, the **PyBytesObject** uses this **field** to store size information to keep O(1) time complexity for **len()** operation and tracks the size of non-ascii string(may be null characters inside)

## summary

The **PyBytesObject** is a python wrapper of c style null terminate string, with **ob_shash** for caching hash value and **ob_size** for storing the size information of **PyBytesObject**

The implementation of **PyBytesObject** looks like the **embstr** encoding in redis

redis-cli

127.0.0.1:6379> set a "hello"

127.0.0.1:6379> object encoding a

"embstr"

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