Trying to concatenate strings in a function with parameter self in python

Question

I am working on an application to save and analyze my cryptocurrency data. I use 2 different apps, hotbit and binance. Since I have bought, for example, BTC on both hotbit and binance, I want to add HB to the end of BTC to specify that it needs to go into the hotbit table I've created. It should look like 'BTCHB'.

def which_application(self):

    print("Which application was this transaction made on? Type either 'hotbit' or 'binance'")
    which_application_prompt = input("").strip().lower()

    if which_application_prompt == 'hotbit':
        self = self + 'HB'
        return(self)

    elif which_application_prompt == 'binance':
        pass

    else:
        print("Invalid input.")
        which_application(self)

I've created this function and everything works smoothly inside the function. However, when I call the function and add a value in place of self, it doesn't add HB to it. Any ideas?

No, it is not. I can add the code I want to add HB to in the post if it helps. — Taimur Adam
– Taimur Adam, Commented Oct 20, 2021 at 17:11
self is not a reserved word in python, so you can use it in this context, but by convention we used it also to refeer to a instance of a class. If this don't make sense to you yet, just know that it's better to use a name other than self for that parameter. How are you calling the function? — Miguel Pinheiro
– Miguel Pinheiro, Commented Oct 20, 2021 at 17:14

Mandraenke · Accepted Answer · 2021-10-20 17:49:32Z

1

You are missing reassigning the value to self in your recursive call, a fixed version could be:

def which_application(self):
    print("Which application was this transaction made on? Type either 'hotbit' or 'binance'")
    which_application_prompt = input("").strip().lower()
    if which_application_prompt == 'hotbit':
        self = self + 'HB'
        return(self)
    elif which_application_prompt == 'binance':
        return self
    else:
        print("Invalid input.")
        self = which_application(self)
        return self

Works:

$ python3 demo.py
Which application was this transaction made on? Type either 'hotbit' or 'binance'
hotbit
testHB

If you call which_application again in the else statement a possibly returned value is lost as you do not assign it to self.

Another flaw is when you type binance nothing is returned - None to be exact.

$ python3 demo.py
Which application was this transaction made on? Type either 'hotbit' or 'binance'
binance
None

Last, a while loop is better than a recursion. Loop until a return.

edited Oct 20, 2021 at 17:49

answered Oct 20, 2021 at 17:26

Mandraenke

3,2761 gold badge17 silver badges28 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Miguel Pinheiro Over a year ago

if you enter a wrong input in the first time, and a correct one in the second time, it still returns None, why is this happening?

Mandraenke Over a year ago

The else did not have a return statement, returning None. I will fix it above.

Mathias R. Jessen · Accepted Answer · 2021-10-20 17:16:42Z

0

When you recurse (call the function from within itself), you're also "recreating" its locals, so self is no longer referring to the same object as self in the parent call.

Use a loop instead of recursion (and consider using a less confusing parameter name than self):

def which_application(transaction):
    while True:
        print("Which application was this transaction made on? Type either 'hotbit' or 'binance'")
        which_application_prompt = input("").strip().lower()

        if which_application_prompt == 'hotbit':
            transaction = transaction + 'HB'
            return(transaction)

        elif which_application_prompt == 'binance':
            break

        else:
            print("Invalid input.")
            # since we're neither `break`'ing nor `return`'ing from here, 
            # the loop will start over

edited Oct 20, 2021 at 17:16

answered Oct 20, 2021 at 17:12

Mathias R. Jessen

178k13 gold badges175 silver badges234 bronze badges

2 Comments

Taimur Adam Over a year ago

Thanks for the help, but that didn't fix it. The main problem is with this part I believe. if which_application_prompt == 'hotbit': self = self + 'HB' return(self) Say I added print(self) at the end of this if statement, it will work as intended. The result will be 'BTCHB'. But when I want to return 'BTCHB' to another function, it comes back as 'BTC'

Mathias R. Jessen Over a year ago

@TaimurAdam Odd. If you define this function as listed in my answer, which_application("trans1") returns "trans1HB" as expected when I input "hotbit"

Collectives™ on Stack Overflow

Trying to concatenate strings in a function with parameter self in python

2 Answers 2

2 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related