Recursive function with same arguments

Question

I'm creating a text-based game in python and have hit a road block. I have a function that checks if user input contains certain words, and if it does it returns the user-input, otherwise it will re-ask for the input. If you write something that doesn't contains one of the words, it will re-call the function.

def contains_words(prompt, words):

    user_input = raw_input(prompt).strip().lower()


    if user_input == "instructions":
        print
        instructions()
        print
        contains_words(prompt, words)

    elif user_input == "i" or user_input == "inventory":
        if len(inventory) == 0:
            print 
            print "There is nothing in your inventory."
            print
            contains_words(prompt, words)
        else:
            print "Your inventory contains: " + inventory
            contains_words(prompt, words)

    else:
        if user_input in words:
            return user_input
        else:
            print
            print "I did not understand your answer, consider rephrasing."
            contains_words(prompt , words)

Here is me calling it:

pizza = contains_words("Do you like pizza?", ["yes", "no"])

Within this function you can bring up the instructions or your inventory and then it will recall the function. Everything works like normal if you put one of the words in your answer the first time you are asked. The problem happens when you enter something incorrect, bring up the inventory or bring up the instructions. It causes the function to return nothing, instead of user-input. Why is this happening? Is it because the function resets so the parameter equal none?

(a) Can you correct the indentation? Use ctrl+k with your code block for that (b) why are you calling the method `contains_words`with and without `self` at different places? — Anshul Goyal, May 17 '15 at 17:36
All your recursive calls need to have their results returned. `return contains_words(prompt, words)` — Ry-, May 17 '15 at 17:37

score 1 · Answer 1 · answered May 17 '15 at 17:53

1

You only need to replace the statements in your code contains_words(prompt, words) by return contains_words(prompt, words)

answered May 17 '15 at 17:53

Pierre

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What is the reasoning behind this? – Conyare May 17 '15 at 18:05

score 1 · Answer 2 · answered May 17 '15 at 17:53

1

When you recurse, you need to return the result.

return contains_words(prompt , words)

answered May 17 '15 at 17:53

recursive

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What is the reasoning behind this? – Conyare May 17 '15 at 17:57
If you don't return anything, the function returns `None` implicitly, but you said it should return the user input. – recursive May 17 '15 at 18:54

score 1 · Accepted Answer · answered May 17 '15 at 18:15

Let's walk through a sample call of this function.

pizza = contains_words("Do you like pizza?", ["yes", "no"])

Say the user inputs instructions. Your first if statement is True, so we enter that block, instructions() is called (presumably printing instructions to the console), and contains_words is called again. Let's say the user inputs yes this time. We'll get down to the last if statement, it'll be True, and this call of contains_words will return yes -- to where it was called.

So, now we're back up the stack to the original call of contains_words. The return value is ignored, because the function is called on a line by itself and not as an argument to another function or statement. Now we're done with this if block, and the next thing in the function is ... nothing. The rest of the ifs. elifs and elses mean nothing (as the original if evaluated to True), and we drop out of the bottom of the function. It returns nothing (actually None). (Check the type of pizza to see.)

The solution is to either change your recursive calls to return contains_words(prompt, words) so when the function drops out of each recursive call it passes the return value up the stack, or, since this is just tail-recursion anyway, replace it with a loop:

def contains_words(prompt, words):

    while True:
        user_input = raw_input(prompt).strip().lower()


        if user_input == "instructions":
            print
            instructions()
            print


        elif user_input == "i" or user_input == "inventory":
            if len(inventory) == 0:
                print 
                print "There is nothing in your inventory."
                print
            else:
                print "Your inventory contains: " + inventory

        else:
            if user_input in words:
                return user_input
            else:
                print
                print "I did not understand your answer, consider rephrasing."

Which will avoid memory issues involved with potentially many recursions.

Recursive function with same arguments

3 Answers3