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I believe I did a minor misconcept which I cannot find. Basically, I'm designing a simple data container to hold {key1:[values1], key2:[values2]}.

class Cells(object):
    def __init__(self, **kwargs):
        self.data = {}

    def __data_len__(self):
        """ Length of data[first key] list. """
        _ = 0
        for key in self.data.keys():
            if self.data[key]:
                _ = len(self.data[key])
                break
        return _

    def subtract_lists(self, x, y):
        return [item for item in x if item not in y]

    def add(self, to_add):
        """ Add columns if not exist """
        if not set(to_add.keys()).issubset(self.data.keys()):       # New key means adding
            new_keys = self.subtract_lists(to_add.keys(), \
                                            self.data.keys())       # it to our dict
            newdict = dict.fromkeys(new_keys, \
                                    [] * self.__data_len__())
            self.data.update(newdict)

        [self.data[key].append(to_add.get(key, '')) for key in self.data.keys()]
        print('* Updated data is: %s' % self.data)

##############################
# Now, tests...              #
##############################
if __name__ == '__main__':
    cells = Cells()
    cells.add({'one':1, 'two':2, 'three': 3})

desired output is to be like:

Updated data is: {'one': [1], 'two': [2], 'three': [3]}

but it outputs:

Updated data is: {'one': [1, 2, 3], 'two': [1, 2, 3], 'three': [1, 2, 3]}

i.e. adding every value to every key, which is frustrating. A typo somewhere?

Pavel M.
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1 Answers1

2

The problem is that you're assigning all the values in newdict to be the same, empty list. See here for an explanation of related behavior: Python initializing a list of lists . The value only gets evaluated once, and all the values in the dictionary store the same list object.

By the way, [] * n (for any n) is still just [].

Change this line: 

    newdict = dict.fromkeys(new_keys, \
                                [] * self.__data_len__())

to this: 

    newdict = {key: [] for key in new_keys}
perigon
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