for循环中while循环在Python中不起作用

Question

I am facing very unusual problem, below is code inside a class where pitnamebasename is 2d list.
For example:=
pitnamebasename= [['a','b'],['n','m'],['b','c'],['m','f'],['c','d'],['d',''],['h','f']]
Above list is not necessary to be in any order like ['d',''] can be at 0th order.

Here is my function (inside a class):-

   def getRole(self,pitname):
            basename=pitname
            print '\n\n\nbasename=',basename
            ind=True
            while pitname and ind:
                    ind=False
                    basename=pitname
                    print 'basename=',basename
                    for i in self.pitnamebasename:
                            print 'comparing-',i[0],'and',pitname
                            if i[0] == pitname:
                                    pitname=i[1]
                                    print 'newpitname=',pitname
                                    ind=True
                                    break
            print 'returning-',basename
            return basename

pitname is the string for example here it can be 'a'. I want return value to be 'd' mean the traversing must be like a to b, b to c and d to None, hence return value must be d. Please don't suggest me any other methods to solve.
Now my problem is that in the for loop its not looping till last but getting out in middle. Like return value is either b or c or even d depends on what I am searching. Actually list is very very long. Strange thing I noted that for loop loops only to that index where it loops till its first time. Like here first time for loop gets end when it find 'a' and pitname becomes 'b' but when it search for 'b' it loops till it find 'a' only. Does anyone knows how it is happening?

Answer 1

pitnamebasename= [['a','b'],['n','m'],['b','c'],['m','f'],['c','d'],['d',''],['h','f']]

First, change your '2d' array into a dict :

pitnamebasename = dict(pitnamebasename)

Now, it should be a simple matter of walking from element to element, using the value associated with the current key as the next key, until the value is the empty string; then return the current key. If pitname ever fails to exist as a key, it's treated as if it does exist and maps to the empty string.

def getRole(self, pitname):
    while pitnamebasename.get('pitname', '') != '':
        pitname = pitnamebasename[pitname]
    return pitname

A defaultdict could also be used in this case:

import collections.defaultdict
pitnamebasename = defaultdict(str, pitnamebasename)
def getRole(self, pitname):
    while pitnamebasename[pitname] != '':
        pitname = pitnamebasename[pitname]
    return pitname

Answer 2

You asked for the solution to your problem but I am having trouble replicating the problem. This code does the same thing without requiring you to change your entire class storage system.

By converting your list of lists into a dictionary lookup (looks like the following)

as_list_of_lists = [['a','b'],['n','m'],['b','c'],['m','f'],['c','d'],['d',''],['h','f']]
as_dict = dict(as_list_of_lists)
# as_dict = {'a': 'b', 'c': 'd', 'b': 'c', 'd': '', 'h': 'f', 'm': 'f', 'n': 'm'}

we can do a default dictionary lookup using the dictionary method .get. This will look for an item (say we pass it 'a') and return the associated value ('b'). If we look for something that isn't in the dictionary, .get will return the second value (a default value) which we can supply as ''. Hence as_dict.get('z','') will return ''

class this_class
    def __init__(self):
        self.pitnamebasename= [['a','b'],['n','m'],['b','c'],['m','f'],['c','d'],['d',''],['h','f']]

    def getRole(self,pitname):
        lookup = dict(self.pitnamebasename)
        new_pitname = pitname
        while new_pitname != '':
            pitname = new_pitname
            new_pitname = lookup.get(pitname, '')
        return pitname