对于类python的实例，getattr查找失败

Question

I'm doing an exercise in which you're supposed to calculate a score of how classy your items are. A tophat gives you 2 points, a bowtie gives you 4 points and a monocle gives you 5 points. I've initiated a dictionary with these items on each class instance, yet when I use getattr to check if the item has the attribute it always returns None .

class Classy(object):

    def __init__(self):
        self.items = []
        self.classyItems = {'tophat': 2, 'bowtie': 4, 'monocle': 5}

    def addItem(self, str):
      self.items.append(str)

    def getClassiness(self):
      classiness = 0
      for item in self.items:
        itemIsClassy = getattr(self.classyItems, item, None) # Why does this always return none?
        if itemIsClassy:
          classiness += itemIsClassy
        else:
          pass
      return classiness


# Test cases
me = Classy()

# Should be 0
print me.getClassiness()

me.addItem("tophat")
# Should be 2
print me.getClassiness() # I'm getting 0

Why is getattr returning None when self.classyItems clearly has the attribute of tophat?

Thanks for the help everyone! I was still confused after reading the answer and simply reading that dictionary keys are not attributes here helped me. Is Python dict an Object?

Answer 1

getattr accesses attributes and/or methods, but classyItems is a dict which doesn't have any (visible) attributes ¹ and stores its contents as key-value pairs ² .

In your case you should use dict.get instead of getattr , more specifically self.classyItems.get(item, None) , to access the values with default.

¹ Dictionaries have methods that could be accessed using getattr though.

² That's why you access them using d[key] instead of d.key .

Answer 2

getattr() takes an object and a string of the attribute to look for, and returns that object . You don't have an attribute named item in your object, but you do have classyItems : Using your code you can see that:

>>> getattr(me, 'classyItems')
{'bowtie': 4, 'tophat': 2, 'monocle': 5}

It is much easier (and readable) to use self.classyItems.get(item, None) which looks up a dict and returns default value. You can't lookup a dict values with getattr , but you can however check if it has certain methods:

>>> getattr(me.classyItems, 'update')
<built-in method update of dict object at 0x028ADE40>

Answer 3

将itemIsClassy = getattr(self.classyItems, item, None)替换为itemIsClassy = self.classyItems.get(item, None)

Answer 4

As mentioned by @Dharmesh and @MSeifert, replace itemIsClassy = getattr(self.classyItems, item, None) to itemIsClassy = self.classyItems.get(item, None)

The reason why your code didn't work is not because classyItems doesn't have many attributes, it is because the getattr function works on objects, not dictionary.