使用python中的条件语句检查字典是否为空会引发键错误

Question

I am sort of new to Python so I was reading through Pro Python and it had this section about passing variable keyword arguments to function. After reading that section I wrote the following code which doesn't seem to work.

def fun(**a):
    return a['height'] if a is not {} else 0

I got a key error when I called fun with no arguments. How should I change the code so that it returns a['height'] if a is not a null dictionary and returns 0 if it is a null dictionary?

Answer 1

You are testing if a the same object as a random empty dictionary, not if that dictionary has the key height . is not and is test for identity , not equality.

You can create any number of empty dictionaries, but they would not be the same object :

>>> a = {}
>>> b = {}
>>> a == b
True
>>> a is b
False

If you want to return a default value if a key is missing, use the dict.get() method here:

return a.get('height', 0)

or use not a to test for an empty dictionary:

return a['height'] if a else 0

but note you'll still get a KeyError if the dictionary has keys other than 'height' . All empty containers test as false in a boolean context, see Truth Value Testing .

Answer 2

To check if a dictionary has a specific key, do this:

if 'height' not in a: return 0

Or you can use get() with a default:

return a.get('height', 0)