为什么此代码返回None值？

Question

I have written this very simple code that should respond to specific user inputs. I know I'm doing something wrong, but I don't know what is it. When I type a word or leave blank I get the response "None". Thank you in advance :) here is my code:

import random
# Sentences we'll respond with if the user greeted us

GREETING_KEYWORDS = ("hello", "hi", "greetings", "sup", "whats up", "You")
GREETING_RESPONSES = ["sup buddy", "Hola", "Halo", "Hi back?"]


def check_for_greeting(sentence):
  """If any of the words in the user's input was a greeting, return a greeting response"""
   if sentence is not None:
       for word in sentence:
           if word.lower() in GREETING_KEYWORDS:
               return random.choice(GREETING_RESPONSES)
           else:
               pass
   elif sentence is None:
       return "You got to say something if you want me to do the same."
   else:
       return "Something is wrong..."


def chat_input():
    x = input("")
    print(check_for_greeting(x))

# Run
print("chat with me...")
chat_input()

Answer 1

for word in sentence:
    if word.lower() in GREETING_KEYWORDS:
        return random.choice(GREETING_RESPONSES)
    else:
        pass

What if none of the words is in GREETING_KEYWORDS ? What is returned then? There is no explicit return statement for when that happens, so the answer is: None is returned.

You can fix that by returning a default response if the for loop completes without finding a greeting.

for word in sentence:
    if word.lower() in GREETING_KEYWORDS:
        return random.choice(GREETING_RESPONSES)
return "default response"

The next problem is that for word in sentence iterates over each letter of the sentence, not each word . To iterate over words, split the sentence apart:

for word in sentence.split():
    if word.lower() in GREETING_KEYWORDS:
        return random.choice(GREETING_RESPONSES)
return "default response"

Next, I see that you want to detect a two-word greeting: whats up . That's not going to work since splitting the sentence into words means you can only check for one-word greetings. I'd suggest inverting the logic: loop over all the possible greetings and see if they're in the sentence. (Notice that now you don't want to split the sentence into words.)

for greeting in GREETING_KEYWORDS:
    if greeting in sentence:
        return random.choice(GREETING_RESPONSES)
return "default response"

Finally, this is a little more advanced, but you could use a generator expression plus any() to simplify the check down to a simple if/else.

if any(greeting in sentence for greeting in GREETING_KEYWORDS):
    return random.choice(GREETING_RESPONSES)
else:
    return "default response"

I like this because it reads more like plain English.