{"id":1895,"date":"2019-05-24T10:31:28","date_gmt":"2019-05-24T09:31:28","guid":{"rendered":"https:\/\/rosetta.vn\/short\/?p=1895"},"modified":"2019-05-24T10:31:28","modified_gmt":"2019-05-24T09:31:28","slug":"python-understanding-dict-copy-shallow-or-deep-stack-overflow","status":"publish","type":"post","link":"https:\/\/rosetta.vn\/short\/2019\/05\/24\/python-understanding-dict-copy-shallow-or-deep-stack-overflow\/","title":{"rendered":"python – Understanding dict.copy() – shallow or deep? – Stack Overflow"},"content":{"rendered":"
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By “shallow copying” it means the\u00a0content<\/em>\u00a0of the dictionary is not copied by value, but just creating a new reference.<\/p>\n

>>><\/span> a <\/span>=<\/span> {<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span>2<\/span>,<\/span>3<\/span>]}<\/span>\r\n>>><\/span> b <\/span>=<\/span> a<\/span>.<\/span>copy<\/span>()<\/span>\r\n>>><\/span> a<\/span>,<\/span> b\r\n<\/span>({<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>]},<\/span> {<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>]})<\/span>\r\n>>><\/span> a<\/span>[<\/span>1<\/span>].<\/span>append<\/span>(<\/span>4<\/span>)<\/span>\r\n>>><\/span> a<\/span>,<\/span> b\r\n<\/span>({<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>,<\/span> 4<\/span>]},<\/span> {<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>,<\/span> 4<\/span>]})<\/span><\/code><\/pre>\n
In contrast, a deep copy will copy all contents by value.<\/p>\n
>>><\/span> import<\/span> copy\r\n<\/span>>>><\/span> c <\/span>=<\/span> copy<\/span>.<\/span>deepcopy<\/span>(<\/span>a<\/span>)<\/span>\r\n>>><\/span> a<\/span>,<\/span> c\r\n<\/span>({<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>,<\/span> 4<\/span>]},<\/span> {<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>,<\/span> 4<\/span>]})<\/span>\r\n>>><\/span> a<\/span>[<\/span>1<\/span>].<\/span>append<\/span>(<\/span>5<\/span>)<\/span>\r\n>>><\/span> a<\/span>,<\/span> c\r\n<\/span>({<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>,<\/span> 4<\/span>,<\/span> 5<\/span>]},<\/span> {<\/span>1<\/span>:<\/span> [<\/span>1<\/span>,<\/span> 2<\/span>,<\/span> 3<\/span>,<\/span> 4<\/span>]})<\/span><\/code><\/pre>\n
So:<\/p>\n
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  1. b = a<\/code>: Reference assignment, Make\u00a0a<\/code>\u00a0and\u00a0b<\/code>\u00a0points to the same object.\n
    \"Illustration<\/li>\n
  2. b = a.copy()<\/code>: Shallow copying,\u00a0a<\/code>\u00a0and\u00a0b<\/code>\u00a0will become two isolated objects, but their contents still share the same reference\n
    \"Illustration<\/li>\n
  3. b = copy.deepcopy(a)<\/code>: Deep copying,\u00a0a<\/code>\u00a0and\u00a0b<\/code>‘s structure and content become completely isolated.\n
    \"Illustration<\/li>\n<\/ol>\n<\/div>\n