使用 python 字典处理 keyerror

Question

How can I fix a KeyError in a python dictionary?

I was asked to create a dictionary and create a function number_dictionary(text, n) . I'm able to return True or False if the text matches the n , but there's a KeyError when I put something that's not in the dictionary when it is supposed to return False .

I've put the two conditions together (when it does not match and not in the dictionary), but then it returns everything as False .

def number_dictionary(text,n):
    numbers={'two': '2', 'three': '3', 'four': '4'}
    if n != numbers[text] or n or text not in numbers:
        return(False)
    else:
        return(True)

Answer 1

Three options:

• Catch the exception :

   try: foo = somedictionary[bar] except KeyError: return False

• Use the dict.get() method to return a default value instead, that default value defaults to None :

   foo = somedictionary.get(bar) # return None if bar is not a key

• Test for the key seperately:

   if bar not in somedictionary: return False

In your case you tested for the key after trying to access it. You could swap the test (dropping the or n ):

def number_dictionary(text, n):
    numbers={'two': '2', 'three': '3', 'four': '4'}
    return text not in numbers or n != numbers[text]

Or just use dict.get() , the default None will never match a string n :

def number_dictionary(text, n):
    numbers = {'two': '2', 'three': '3', 'four': '4'}
    return numbers.get(text) == n

Both the not in and == comparison tests already produce either True or False , so there is no need to use if and separate return statements.