Why I have to do a deepcopy to the argument?

[Dimitris Leventeas]

Hi!

I am trying to understand Python's method of passing arguments, the references 
to objects etc.  I fail to grasp why in populate_trie I have to make a deepcopy 
of trie locally instead of just referencing to it.

If it makes any difference, I use Python 3.


from copy import deepcopy

def access_trie(d, sequence, position=None):
    """ 
    Access the dictionary which is referred by applying consequently each
    term of the sequence.  In a more python terms, if sequence is 'key',
    access: d['k']['e']['y']  Assume that the dictionary is at the
    `position` of a list, if `position` is an argument.
    
    >>> a = {'k': [0, {'a': [0, {'l': [0, {'o': [1, {}]}]}]}]}
    >>> access_trie(a, 'kal', 1)
    {'o': [1, {}]}
    >>> access_trie(a, 'kalo', 1)
    {}
    >>> a = {'d': {'i': {'m': {'i': {'t': {'r': {'i': {'s': 1}}}}}}}}
    >>> access_trie(a, 'dimitr')
    {'i': {'s': 1}}
    >>> access_trie(a, '')
    {'d': {'i': {'m': {'i': {'t': {'r': {'i': {'s': 1}}}}}}}}
    >>> access_trie(a, 'dimitris')
    1
    >>> b = access_trie(a, 'dimitr')
    >>> b['o'] = {'s': 1}
    >>> a
    {'d': {'i': {'m': {'i': {'t': {'r': {'i': {'s': 1}, 'o': {'s': 1}}}}}}}}

    """
        
    for c in sequence:
        d = d[c]
        if position is not None:
            d = d[position]
        
    return d


def populate_trie(trie, sequence, position=None):
    """ 
    Populate a trie.

    Assume that the counter is always at `position` 0 while the `position`
    of the dictionary is the last one.
    
    >>> trie = {}
    >>> populate_trie(trie, 'taspython')
    {'t': {'a': {'s': {'p': {'y': {'t': {'h': {'o': {'n': {}}}}}}}}}}
    >>> trie = {}
    >>> populate_trie(trie, 'kalo', 1)
    {'k': [1, {'a': [1, {'l': [1, {'o': [1, {}]}]}]}]}
    >>> trie = {}
    >>> populate_trie(trie, 'heh', 2)
    {'h': [1, 0, {'e': [1, 0, {'h': [1, 0, {}]}]}]}
    >>> trie = {}
    >>> trie = populate_trie(trie, 'heh', 1)
    >>> populate_trie(trie, 'hah', 1)
    {'h': [2, {'a': [1, {'h': [1, {}]}], 'e': [1, {'h': [1, {}]}]}]}
    
        
    """
    
    if (position is not None) and (position >= 1):
        embedded_obj = [0] * position
        embedded_obj.append({})
    else:
        embedded_obj = {}
    
    d = deepcopy(trie)
    d2 = d
    for i, character in enumerate(sequence):
        d2 = access_trie(d, sequence[:i], position)
        if character not in d2:
            if position is None:
                d2[character] = deepcopy(embedded_obj)
            else:
                d2[character] = d2.get(character, deepcopy(embedded_obj))
                d2[character][0] += 1
        elif position is not None:
            d2[character][0] += 1

    return d
    


Best regargs,
Dimitris Leventeas
-- 
Dimitris Leventeas
http://students.ceid.upatras.gr/~lebenteas/