python recursive function returning none instead of string

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

Asked By: Romby Soft

||

Source

Answer 1

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 "nnStopping 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 "nnStopping 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

Answered By: Akshay Agrawal

Answer 2

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 By: user1741851

python recursive function returning none instead of string

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