在python中，为什么要从内置模块导入“对象"? [英] In python, why import 'object' from builtins module?

问题描述

为了过渡到 python 3，我试图理解编写 python 2 和 python 3 兼容代码.下面的代码来自 python-future.org，它说明了一种构造一个迭代器的方法，该迭代器兼容两个版本的 python.

from builtins import object类上(对象):def __init__(self, iterable):self._iter = iter(可迭代)def __next__(self): # Py3 风格的迭代器接口return next(self._iter).upper() # 内置 next() 函数调用def __iter__(self):回归自我itr = Upper('你好')assert next(itr) == 'H' # 兼容样式断言列表(itr)==列表('ELLO')

代码在 python 2 中运行良好，但令我惊讶的是，如果我删除导入语句，则会收到错误 TypeError: Upper object is not an iterator.我经常从 object 派生我的自定义类，但我从未从内置程序中导入它.为什么简单地导入 object 会改变代码的行为?

解决方案

您正在(间接)从 future.builtins 模块;它提供了一个 custom object 基类，它添加了向前指向的特殊名称.

在 Python 2 中，迭代器必须具有 next()方法(以及__iter__)；这个方法在 Python 3 中被重命名为 __next__.如果不使用 future.builtins.object 版本，您只是缺少 Python 2 中提供的 next -> __next__ 别名.

查看源代码对于 future.types.newobject.py:

<块引用>

def next(self):如果 hasattr(self, '__next__'):返回类型(self).__next__(self)raise TypeError('newobject 不是迭代器')

请注意，如果您运行的是 Python 3，builtins 将返回标准的内置对象，该模块只为 Python 2 返回这些垫片.

您可以简单地自己添加相同的别名:

class Upper(object):def __init__(self, iterable):self._iter = iter(可迭代)def __iter__(self):回归自我def __next__(self): # Py3 风格的迭代器接口return next(self._iter).upper() # 内置 next() 函数调用next = __next__ # Py2 别名

In order to transition to python 3, I am trying to understand writing python 2 and python 3 compatible codes. The following code is from python-future.org and illustrates a way to construct an iterator that would be compatible with both versions of python.

from builtins import object

class Upper(object):
    def __init__(self, iterable):
        self._iter = iter(iterable)
    def __next__(self):      # Py3-style iterator interface
        return next(self._iter).upper()  # builtin next() function calls
    def __iter__(self):
        return self

itr = Upper('hello')
assert next(itr) == 'H'      # compatible style
assert list(itr) == list('ELLO')

The code runs fine in python 2, but to my surprise, if i remove the import statement then i get an error TypeError: Upper object is not an iterator. I often derive my custom classes from object but i have never imported it from builtins. Why does simply importing object change the behaviour of the code?

解决方案

You are (indirectly) importing from the future.builtins module; it provides a custom object baseclass that adds forward-pointing special names.

In Python 2, iterators must have a next() method (as well as __iter__); this method was renamed to __next__ in Python 3. By not using the future.builtins.object version you are simply missing the next -> __next__ alias provided in Python 2.

See the source code for future.types.newobject.py:

def next(self):
    if hasattr(self, '__next__'):
        return type(self).__next__(self)
    raise TypeError('newobject is not an iterator')

Note that builtins will return standard built-in objects if you are running Python 3, the module only returns shims like these for Python 2.

You could simply add that same alias yourself:

class Upper(object):
    def __init__(self, iterable):
        self._iter = iter(iterable)

    def __iter__(self):
        return self

    def __next__(self):      # Py3-style iterator interface
        return next(self._iter).upper()  # builtin next() function calls
    next = __next__          # Py2 alias

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