python recursive function returning none instead of string

问题

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

回答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 "\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

回答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.

来源：https://stackoverflow.com/questions/47385933/python-recursive-function-returning-none-instead-of-string

标签

python-2.7

recursion