检查 Python 变量类型的最佳(惯用)方法是什么? [英] What is the best (idiomatic) way to check the type of a Python variable?

问题描述

我需要知道 Python 中的变量是字符串还是字典.下面的代码有什么问题吗?

I need to know if a variable in Python is a string or a dict. Is there anything wrong with the following code?

if type(x) == type(str()):
    do_something_with_a_string(x)
elif type(x) == type(dict()):
    do_somethting_with_a_dict(x)
else:
    raise ValueError

更新:我接受了 avisser 的回答(尽管如果有人解释为什么 isinstance 比 type(x) is 更受欢迎，我会改变主意)).

Update: I accepted avisser's answer (though I will change my mind if someone explains why isinstance is preferred over type(x) is).

但是感谢nakedfanatic提醒我使用dict(作为case语句)比if/elif/else系列更干净.

But thanks to nakedfanatic for reminding me that it's often cleaner to use a dict (as a case statement) than an if/elif/else series.

让我详细说明我的用例.如果一个变量是一个字符串，我需要把它放在一个列表中.如果它是一个字典，我需要一个唯一值的列表.这是我想出的:

Let me elaborate on my use case. If a variable is a string, I need to put it in a list. If it's a dict, I need a list of the unique values. Here's what I came up with:

def value_list(x):
    cases = {str: lambda t: [t],
             dict: lambda t: list(set(t.values()))}
    try:
        return cases[type(x)](x)
    except KeyError:
        return None

如果首选 isinstance，你会如何编写这个 value_list() 函数?

If isinstance is preferred, how would you write this value_list() function?

推荐答案

如果有人将 unicode 字符串传递给您的函数会发生什么?还是从 dict 派生的类?或者一个类实现了一个类似 dict 的接口?以下代码涵盖了前两种情况.如果您使用的是 Python 2.6，您可能需要使用 collections.Mapping 而不是 dict 根据 ABC PEP.

What happens if somebody passes a unicode string to your function? Or a class derived from dict? Or a class implementing a dict-like interface? Following code covers first two cases. If you are using Python 2.6 you might want to use collections.Mapping instead of dict as per the ABC PEP.

def value_list(x):
    if isinstance(x, dict):
        return list(set(x.values()))
    elif isinstance(x, basestring):
        return [x]
    else:
        return None

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