python recursive function returning none instead of string [duplicate]

Question

The below python function prints value using print function but is returning none if made to return string value. can anyone help me on how to return a string value from this below function.

csv_full_list = ['mycsv_0', 'mycsv_1']


def create_csv_name(comm, index):
    global csv_full_list
    if comm + '_' + str(index) in csv_full_list:
        index += 1 # increment index by 1
        create_csv_name(comm, index=index)
    else:
        print '%s_%i' % (comm, index)
        return '%s_%i' % (comm, index)


print(create_csv_name('mycsv', 0))

out put expected :

mycsv_2

but returns:

None

You should return from the recursive call: return create_csv_name(comm, index=index) — cs95
– cs95, Commented Nov 20, 2017 at 6:13
can you elaborate. I tried but getting error: RuntimeError: maximum recursion depth exceeded while getting the str of an object — Romby Soft
– Romby Soft, Commented Nov 20, 2017 at 6:30

Akshay Agrawal · Accepted Answer · 2017-11-20 06:58:57Z

0

Try out the following code.

csv_full_list = ['mycsv_0', 'mycsv_1']


def create_csv_name(comm, index):
    global csv_full_list
    result = True
    print "ATTEMPT: COMM: {}, INDEX: {}".format(comm, index)

    while result:
        if comm + '_' + str(index) not in csv_full_list:
            result = False

        else:
            index += 1 # increment index by 1
            #print "--> Found COMM: {}, INDEX: {}".format(comm, index)
            create_csv_name(comm, index=index)

    if not result:
        return "\n\nStopping at >> COMM: {}, INDEX: {}".format(comm, index)

# Call the def
print(create_csv_name('mycsv', 0))

2nd variation:

csv_full_list = ['mycsv_0', 'mycsv_1']


def create_csv_name(comm, index):
    global csv_full_list
    result = True
    print "ATTEMPT: COMM: {}, INDEX: {}".format(comm, index)

    if comm + '_' + str(index) not in csv_full_list:
        return "\n\nStopping at >> COMM: {}, INDEX: {}".format(comm, index)
    else:
        index += 1 # increment index by 1
        return create_csv_name(comm, index=index)

print(create_csv_name('mycsv', 0))

output:

> python.exe .\sample.py
ATTEMPT: COMM: mycsv, INDEX: 0
ATTEMPT: COMM: mycsv, INDEX: 1
ATTEMPT: COMM: mycsv, INDEX: 2
ATTEMPT: COMM: mycsv, INDEX: 2


Stopping at >> COMM: mycsv, INDEX: 2

edited Nov 20, 2017 at 6:58

answered Nov 20, 2017 at 6:30

Akshay Agrawal

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2 Comments

user1741851 Over a year ago

Quite honestly, I don't see why you need the while loop. It's an overkill to achieve the result.

Akshay Agrawal Over a year ago

@nightgaunt agreed, but wanted to keep it simple, so it could be understood easily. As pointed out by your comment the best practice would be to go for return create_csv_name(comm, index=index), Updated the code block. Thanks !!

user1741851 · Accepted Answer · 2017-11-20 06:35:56Z

0

You need to use return create_csv_name(comm, index=index) inside the function. Otherwise, you are not returning anything from the first call to the function. That also explains your None return value.

Recursion is a topic that requires keen observation. And takes some time to get familiar and apply in real application.

answered Nov 20, 2017 at 6:35

user1741851

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