Python递归函数未返回值

Question

This is my first question here so my sincerest apologies if I've written TOO much and thanks so much in advance for taking a look.

PROBLEM:

I've written a function which should, if necessary, call itself x times and if unsuccessful return nothing. However, when the function seems to be successful, it still returns None.

BACKGROUND: I have a number of directories which represent months in the year 2009. Not all of the months exist though, therefore if this is the case, I want to get the previous month, check if a directory for that month exists, and if not continue going back one month AT MOST for 6 months.

Below you see the month 09 in the date_tag, which in my test case does not exist. Neither does 08 or 07. Thus the function SHOULD return 06 but instead it returns None.

import pdb
import generate_months_module
import os

date_tag = '2000/2000/2009/09'
tree_location = '/Users/kelly/Documents/projects/komm/data/directory-tree/'

def iterateOverMonths(date_tag, x):
    if x <= 0:
        return_string = 'no dates found'
        return return_string
    else:
        new_date = generate_months_module.handleDateShifts(date_tag)[1]
        print '\tNEW DATE after calling handleDateShifts' + new_date
        full_path = tree_location + '/' + new_date
        if checkDirectoryExistance(full_path) == True:
            print '\t'+ full_path + ' is a real path'
            return full_path
        else:
            print 'dir does not exist'                                                                                                                       
            iterateOverMonths(new_date, x-1)

def checkDirectoryExistance(dir_path):
    "check if a directory exists, return true or false"
    if os.path.isdir(dir_path) == True:
        return True
    else:
        return False

print iterateOverMonths(date_tag, 6)

The generate_months_module.handleDateShifts is supposed to just get the previous month and return it. (This works in other test cases so I highly doubt the problem is here!)

My output from this then is:

6
    NEW DATE after calling handleDateShifts2000/2000/2009/08
    dir does not exist
5
    NEW DATE after calling handleDateShifts2000/2000/2009/07
    dir does not exist
4
    NEW DATE after calling handleDateShifts2000/2000/2009/06

    /Users/kelly/Documents/projects/komm/data/directory-tree/2000/2000/2009/06 is a real path

    returning full path
    None

When I use pdb.set_trace() prior to "return full_path" it seems like the function is being called again DESPITE the IF clause being True and thus writing over the "full_path" variable I want to return.

Why isn't the path "/Users/kelly/Documents/projects/komm/data/directory-tree/2000/2000/2009/06" being returned??

imported function:

If one should be interested and want to re-create this the handleDateShifts function is as follows (my apologies, it's a bit messy):

def handleDateShifts(corpus_date_string):
    "get background corpus date strings ALSO call this function if month does not exist and need to go back even further"
    century, decade, year, month = corpus_date_string.split('/')
    if month == '01' or month == '02':
        #handle date boundaries which can affect year, decade and cent                                                   
        background_mo_1 = '11'
        background_mo_2 = '12'
        millenium_shift = re.search('[1-9][0][0][0]$', year)
        century_shift = re.search('[1-9][0][0]$', year)
        decade_shift = re.search('[1-9][0]$',year)
        if century_shift or millenium_shift:
            century = int(year) - 100
            decade = int(year) - 10
            year = int(year) - 1
        elif decade_shift:
            decade = int(year) - 10
            year = int(year) - 1
        elif not decade_shift and not century_shift:
            year = int(year) - 1
        background_1_string = str(century) +'/'+ str(decade) +'/'+ str(year) +'/'+ str(background_mo_1)
        background_2_string = str(century) +'/'+ str(decade) +'/'+ str(year) +'/'+ str(background_mo_2)
    else: #the cent/dec/year can stay the same                                                                           
        background_mo_1 = int(month) - 2
    background_mo_2 = int(month) - 1
        if len(str(background_mo_1)) == 1:
            background_mo_1 = '0' + str(background_mo_1)
        if len(str(background_mo_2)) == 1:
            background_mo_2 = '0' + str(background_mo_2)
        background_1_string = str(century) +'/'+ str(decade) +'/'+ str(year) +'/'+ str(background_mo_1)
        background_2_string = str(century) +'/'+ str(decade) +'/'+ str(year)+'/'+ str(background_mo_2)
    return background_1_string, background_2_string

Answer 1

You don't return anything in this branch (your result is lost):

    else:
        print 'dir does not exist'                                                                                                                       
        iterateOverMonths(new_date, x-1)

If function runs it course without explicit return <smth> , None is returned.