前言

在默認情況下,Python的新類和舊類的實例都有一個字典來存儲屬性值。這對于那些沒有實例屬性的對象來說太浪費空間了，當需要創建大量實例的時候，這個問題變得尤為突出。

因此這種默認的做法可以通過在新式類中定義了一個__slots__屬性從而得到了解決。__slots__聲明中包含若干實例變量，并為每個實例預留恰好足夠的空間來保存每個變量，因此沒有為每個實例都創建一個字典，從而節省空間。

本文主要介紹了關于python使用__slots__讓你的代碼更加節省內存的相關內容，分享出來供大家參考學習，下面話不多說了，來一起看看詳細的介紹吧

現在來說說python中dict為什么比list浪費內存？

和list相比，dict 查找和插入的速度極快，不會隨著key的增加而增加；dict需要占用大量的內存，內存浪費多。

而list查找和插入的時間隨著元素的增加而增加；占用空間小，浪費的內存很少。

python解釋器是Cpython，這兩個數據結構應該對應C的哈希表和數組。因為哈希表需要額外內存記錄映射關系，而數組只需要通過索引就能計算出下一個節點的位置，所以哈希表占用的內存比數組大，也就是dict比list占用的內存更大。

如果想更加詳細了解，可以查看C的源代碼。python官方鏈接：https://www.python.org/downloads/source/

如下代碼是我從python官方截取的代碼片段：

List 源碼：

typedef struct { PyObject_VAR_HEAD /* Vector of pointers to list elements. list[0] is ob_item[0], etc. */ PyObject **ob_item;  /* ob_item contains space for 'allocated' elements. The number * currently in use is ob_size. * Invariants: * 0 <= ob_size <= allocated * len(list) == ob_size * ob_item == NULL implies ob_size == allocated == 0 * list.sort() temporarily sets allocated to -1 to detect mutations. * * Items must normally not be NULL, except during construction when * the list is not yet visible outside the function that builds it. */ Py_ssize_t allocated;} PyListObject;

Dict源碼：

/* PyDict_MINSIZE is the minimum size of a dictionary. This many slots are * allocated directly in the dict object (in the ma_smalltable member). * It must be a power of 2, and at least 4. 8 allows dicts with no more * than 5 active entries to live in ma_smalltable (and so avoid an * additional malloc); instrumentation suggested this suffices for the * majority of dicts (consisting mostly of usually-small instance dicts and * usually-small dicts created to pass keyword arguments). */#define PyDict_MINSIZE 8 typedef struct { /* Cached hash code of me_key. Note that hash codes are C longs. * We have to use Py_ssize_t instead because dict_popitem() abuses * me_hash to hold a search finger. */ Py_ssize_t me_hash; PyObject *me_key; PyObject *me_value;} PyDictEntry; /*To ensure the lookup algorithm terminates, there must be at least one Unusedslot (NULL key) in the table.The value ma_fill is the number of non-NULL keys (sum of Active and Dummy);ma_used is the number of non-NULL, non-dummy keys (== the number of non-NULLvalues == the number of Active items).To avoid slowing down lookups on a near-full table, we resize the table whenit's two-thirds full.*/typedef struct _dictobject PyDictObject;struct _dictobject { PyObject_HEAD Py_ssize_t ma_fill; /* # Active + # Dummy */ Py_ssize_t ma_used; /* # Active */  /* The table contains ma_mask + 1 slots, and that's a power of 2. * We store the mask instead of the size because the mask is more * frequently needed. */ Py_ssize_t ma_mask;  /* ma_table points to ma_smalltable for small tables, else to * additional malloc'ed memory. ma_table is never NULL! This rule * saves repeated runtime null-tests in the workhorse getitem and * setitem calls. */ PyDictEntry *ma_table; PyDictEntry *(*ma_lookup)(PyDictObject *mp, PyObject *key, long hash); PyDictEntry ma_smalltable[PyDict_MINSIZE];};