Python字典DataStructure方法d []还是d.get（）？ [英] Python Dictionary DataStructure which method d[] or d.get()?

问题描述

虽然使用Python字典DataStructure（包含键值对）如果我想从我的字典中检索一些值，我有两个选项 d ['']和g.get（'key'）

While Using Python Dictionary DataStructure (which contains key-value pair) if i want to retrieve some value from my Dictionary i have two options d[''] and g.get('key') so i am confused now which is better and Why ?? I understand both some way but when it comes to memory consumption and evaluation in memory which one is better ??

希望对一些积极的答复，

Hoping for some Positive reply,

请问。

推荐答案

从 Python Library Docs

d [key]

用键键返回 d 的项目。如果键不在地图中，则会引发 KeyError 。

d[key]
Return the item of d with key key. Raises a KeyError if key is not in the map.

如果dict的子类定义了一个方法 __缺少__（），如果密钥键不存在， d [key] 操作将键入键作为参数调用该方法。 d [key] 操作然后返回或引发由 __缺少__（键）调用返回或引发的任何值，如果钥匙不存在没有其他操作或方法调用 __缺少__（）。如果 __缺少__（）未定义，则会引发 KeyError 。 __缺少__（）必须是一种方法;它不能是一个实例变量。 [...]

If a subclass of dict defines a method __missing__(), if the key key is not present, the d[key] operation calls that method with the key key as argument. The d[key] operation then returns or raises whatever is returned or raised by the __missing__(key) call if the key is not present. No other operations or methods invoke __missing__(). If __missing__() is not defined, KeyError is raised. __missing__() must be a method; it cannot be an instance variable. [...]

和

get（key [，default]）

如果键返回键的值在字典中，否则默认。如果没有给出默认值，则默认为 None ，以便此方法不会引发 KeyError 。

get(key[, default])
Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.

区别在于返回值。当您要求与不存在的密钥对应的值时，您需要

The difference lies in the return value. When you ask for the value corresponding to a non-existing key, you either want

1. A KeyError raise


2. 回调调用


3. 返回的默认值

1. A KeyError raised
2. A callback invoked
3. A default value returned

Python通过多种方法提供不同的功能。

Python provides the different functionalities through multiple methods.

使用 [] 当找不到密钥时，无论是在调用 _missing _ 还是引发异常。当密钥IS存在时，哪个是更快的，我检查了源代码。 （我使用2.7.2进行此检查。）在 dictobject.c 中，我们看到：

There will be a performance hit using [] when the key is not found, either in calling _missing_ or raising the exception. As to which one is faster when the key IS present, I checked the source code. (I used 2.7.2 for this check.) In dictobject.c we see:

• get call dict_get


• ] 调用 dict_subscript

• get calls dict_get
• [] calls dict_subscript

现在如果值存在于 dict_get 我们有

Now if the values are present, in dict_get we have

if (!PyArg_UnpackTuple(args, "get", 1, 2, &key, &failobj))
    return NULL;

if (!PyString_CheckExact(key) ||
    (hash = ((PyStringObject *) key)->ob_shash) == -1) {
    hash = PyObject_Hash(key);
    if (hash == -1)
        return NULL;
}
ep = (mp->ma_lookup)(mp, key, hash);

和 dict_subscript 我们有

assert(mp->ma_table != NULL);
if (!PyString_CheckExact(key) ||
    (hash = ((PyStringObject *) key)->ob_shash) == -1) {
    hash = PyObject_Hash(key);
    if (hash == -1)
        return NULL;
ep = (mp->ma_lookup)(mp, key, hash);

唯一的区别是 get 额外的打包元组！

The only difference is that get does an extra unpack tuple!

重要？我不知道。 ： - ）

Significant? I have no idea. :-)

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