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enum done
and readme change category to content readme move learning material down
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‎BasicObject/bytearray/bytearray.md‎

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# bytearray
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### category
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### contents
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* [related file](#related-file)
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* [memory layout](#memory-layout)

‎BasicObject/bytes/bytes.md‎

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# bytes
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* [related file](#related-file)
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* [memory layout](#memory-layout)

‎BasicObject/class/class.md‎

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# class
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### category
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* [related file](#related-file)
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* [memory layout](#memory-layout)

‎BasicObject/complex/complex.md‎

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# complex
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* [related file](#related-file)
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* [memory layout](#memory-layout)

‎BasicObject/dict/dict.md‎

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# dict
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### category
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Because the **PyDictObject** is a little bit more complicated than other basic object, I will not show _\_setitem_\_/_\_getitem_\_ step by step, instead, I will illustrate in the middle of some concept
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‎BasicObject/enum/enum.md‎

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# enum
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### contents
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* [related file](#related-file)
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* [memory layout](#memory-layout)
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>>> type(e)
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<class 'enumerate'>
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before iter through the object **e**, the **en_index** field is 0, **en_sit** stores the actual generator object being iterated, **en_result** stores the previous result
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before iter through the object **e**, the **en_index** field is 0, **en_sit** stores the actual generator object being iterated, **en_result** points a tuple object with two empty value
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we will see the meaning of **en_longindex** later
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>>> id(t1)
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4469348888
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now, the **en_index** becomes 1, the tuple in **en_result** is the last tuple object returned, the elements in the tuple are changed, but the address **en_result** doesn't change, not because of the [free-list mechanism in tuple](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/tuple/tuple.md#free-list)
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now, the **en_index** becomes 1, the tuple in **en_result** is the last tuple object returned, the elements in the tuple are changed, but the address in **en_result** doesn't change, not because of the [free-list mechanism in tuple](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/tuple/tuple.md#free-list)
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it's a trick in the **enumerate** iterating function
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return result;
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}
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it's clear, because only the current enumerate object keep a reference to the old `tuple object -> (None, None)` **enum_next** reset the 0th element in the tuple to 0, and 1th element in the tuple to 'I', so the address **en_result** points to is the same
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it's clear, because only the current enumerate object keep a reference to the old `tuple object -> (None, None)` object
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**enum_next** reset the 0th element in the tuple to 0, and 1th element in the tuple to 'I', so the address **en_result** points to is the same
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![example1](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/example1.png)
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>>> e = enumerate(gen(), (1 << 63) + 100)
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![longindex1](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/longindex1.png)
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![longindex1](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/longindex1.png)
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‎BasicObject/enum/enum_cn.md‎

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# enum
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### 目录
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* [相关位置文件](#相关位置文件)
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* [内存构造](#内存构造)
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* [示例](#示例)
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* [normal](#normal)
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* [en_longindex](#en_longindex)
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#### 相关位置文件
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* cpython/Objects/enumobject.c
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* cpython/Include/enumobject.h
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* cpython/Objects/clinic/enumobject.c.h
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#### 内存构造
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**enumerate** 是一个类型, **enumerate** 的实例是一个可迭代对象, 你可以在迭代的过程中同时获得这个迭代的对象和一个计数器
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![layout](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/layout.png)
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#### 示例
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##### normal
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def gen():
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yield "I"
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yield "am"
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yield "handsome"
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e = enumerate(gen())
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>>> type(e)
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<class 'enumerate'>
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在迭代对象 **e** 之前, 它的 **en_index** 字段为 0, **en_sit** 指向了真正的 **generator** 对象, **en_result** 指向了一个有两个空值的 **tuple**
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我们后面会提到 **en_longindex** 的作用
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![example0](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/example0.png)
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>>> t1 = next(e)
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>>> t1
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(0, 'I')
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>>> id(t1)
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4469348888
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现在 **en_index** 字段变成了 1, **en_result** 里面指向的元组对象为最近一次返回的元组对象, 元组里面的两个元素都改变了, 但是 **en_result** 里的地址没有变化, 没有变化的原因不是 [tuple 缓冲池](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/tuple/tuple_cn.md#free-list) 机制的原因
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而是和 **enumerate** 的迭代函数使用的技巧有关
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static PyObject *
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enum_next(enumobject *en)
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{
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/* omit */
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PyObject *result = en->en_result;
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/* omit */
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if (result->ob_refcnt == 1) {
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/*
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tuple 对象的引用计数器为 is 1
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说明当前唯一引用这个 tuple 对象的就是这个 enumerate 实例本身
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既然这个 tuple 对象的两个旧元素已经不需要了
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我们可以把这两个元素设置为新的元素, 并返回这个 tuple 对象
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*/
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Py_INCREF(result);
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old_index = PyTuple_GET_ITEM(result, 0);
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old_item = PyTuple_GET_ITEM(result, 1);
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PyTuple_SET_ITEM(result, 0, next_index);
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PyTuple_SET_ITEM(result, 1, next_item);
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Py_DECREF(old_index);
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Py_DECREF(old_item);
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return result;
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}
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/*
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到达这里, 这个 tuple 对象的引用计数器不为 1, 处理自己本身还有其他的变量在使用它
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我们不能重置这个 tuple 里面的元素
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只能创建新的返回给调用者
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*/
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result = PyTuple_New(2);
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if (result == NULL) {
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Py_DECREF(next_index);
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Py_DECREF(next_item);
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return NULL;
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}
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PyTuple_SET_ITEM(result, 0, next_index);
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PyTuple_SET_ITEM(result, 1, next_item);
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return result;
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}
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很明显了, 因为旧的 **tuple** `tuple object -> (None, None)` 对象唯一的引用来自当前的 **enumerate** 对象, **enum_next** 会把这个 **tuple** 对象的第 0 个元素变为 0, 第 1 个元素变为 'I'， 之后把这个旧 tuple 返回
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所以 **en_result** 指向的地址未发生改变, 并且 **en_result** 指向的对象为这次迭代返回的对象
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![example1](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/example1.png)
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这个 **tuple** 对象 `(0, 'I') # id(4469348888)` 现在的引用计数器变为 2 了, 一个来自 **enumerate** 实例的引用, 还有一个来自变量名称 t1, **enum_next** 这次会进入下面的分支, 创建一个新的 **tuple** 对象并返回而不是重置那个旧的 **tuple** 对象
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**en_index** 中的数值增加了, **en_result** 仍然指向这个旧的 **tuple** 对象`(0, 'I') # id(4469348888)`
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>>> next(e)
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(1, 'am')
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![example2](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/example2.png)
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`del t1` 语句执行之后, 这个 tuple 对象 `(0, 'I') # id(4469348888)` 的引用计数器变回了 1, 此时 **enum_next** 会像上面一样重置这个 **tuple** 对象, **en_result** 仍然指向这个对象, 并且这次返回的为 **en_result** 指向的对象
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>>> del t1 # decrement the reference count of the object referenced by t1
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>>> next(e)
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(2, 'handsome')
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![example3](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/example3.png)
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结束标记是被 **en_sit** 里面存储的对象所存储的, **enumerate** 本身不存储结束标记等信息
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>>> next(e)
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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StopIteration
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![example3](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/example3.png)
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##### en_longindex
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通常情况下, 计数器的值是存储在 **en_index** 里面的, **en_index** 的类型是 **Py_ssize_t**, 我们来看下 **Py_ssize_t** 的定义
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#ifdef HAVE_SSIZE_T
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typedef ssize_t Py_ssize_t;
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#elif SIZEOF_VOID_P == SIZEOF_SIZE_T
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typedef Py_intptr_t Py_ssize_t;
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#else
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# error "Python needs a typedef for Py_ssize_t in pyport.h."
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#endif
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大部分情况下他是一个 **ssize_t**, 32位操作系统下为 int, 64位下为 **long int**
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在我的机器上他是 **long int**
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如果这个计数器的值大到这 64个 bit 都装不下呢?
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e = enumerate(gen(), 1 << 62)
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这个时候 **en_index** 是可以放得下这个值的
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![longindex0](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/longindex0.png)
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**en_index** 能表示的最大的值为 ((1 << 63) - 1) (PY_SSIZE_T_MAX)
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现在实际上的计数器的值已经比 PY_SSIZE_T_MAX 还要大了, 此时 **en_longindex** 会被用来存储真正的计数器
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**en_longindex** 指向的是一个 [PyLongObject(python 类型 int)](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/long/long_cn.md), 对象, 可以表示任意长度的整数大小
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>>> e = enumerate(gen(), (1 << 63) + 100)
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![longindex1](https://github.com/zpoint/CPython-Internals/blob/master/BasicObject/enum/longindex1.png)
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‎BasicObject/enum/example3.png‎

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‎BasicObject/float/float.md‎

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# float
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* [memory layout](#memory-layout)

‎BasicObject/func/func.md‎

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# func
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* [related file](#related-file)
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* [memory layout](#memory-layout)

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