問題描述

我有一個對象 myobject，它可能返回 None.如果返回None，則不會返回屬性id:

I have an object myobject, which might return None. If it returns None, it won't return an attribute id:

a = myobject.id

所以當 myobject 為 None 時，上面的語句會導致 AttributeError:

So when myobject is None, the stament above results in a AttributeError:

AttributeError: 'NoneType' object has no attribute 'id'

如果 myobject 為 None，那么我希望 a 等于 None.如何在一行語句中避免此異常，例如:

If myobject is None, then I want a to be equal to None. How do I avoid this exception in one line statement, such as:

a = default(myobject.id, None)

推薦答案

你應該使用 getattr 包裝器，而不是直接檢索 id 的值.

a = getattr(myobject, 'id', None)

這就像說我想從對象myobject中檢索屬性id，但是如果里面沒有屬性id對象 myobject，然后返回 None."但它確實有效.

This is like saying "I would like to retrieve the attribute id from the object myobject, but if there is no attribute id inside the object myobject, then return None instead." But it does it efficiently.

有些對象還支持以下形式的getattr訪問:

Some objects also support the following form of getattr access:

a = myobject.getattr('id', None)

根據 OP 要求，'deep getattr':

def deepgetattr(obj, attr):
    """Recurses through an attribute chain to get the ultimate value."""
    return reduce(getattr, attr.split('.'), obj)
# usage: 
print deepgetattr(universe, 'galaxy.solarsystem.planet.name')

簡單解釋:

Reduce 就像一個就地遞歸函數.在這種情況下，它的作用是從 obj (universe)開始，然后為您嘗試使用 getattr 訪問的每個屬性遞歸地深入，所以在您的問題中它會是像這樣:

Reduce is like an in-place recursive function. What it does in this case is start with the obj (universe) and then recursively get deeper for each attribute you try to access using getattr, so in your question it would be like this:

a = getattr(getattr(myobject, 'id', None), 'number', None)

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