import string

class SqlString:
    _str = ''
    def __init__(self, param):
        if type(param).__name__ == 'str':
            self._str = param
        else:
            return false
    
    def getSql(self):
        return self._str


class SqlComb:
    """A SQL parser"""
    _select = []
    _from = []
    def __init__(self,  tableName):
        self._from.append(tableName)
    
    def select(self,  fields):
        if type(fields).__name__ == 'str':
            fields = SqlString(fields)
        
        if type(fields).__name__ == 'instance':
            self._select.append(fields)
        
        if type(fields).__name__ == 'list':
            for v in fields:
                self.select(v)
    
    def getSql(self):
        r = 'SELECT ' + implode(self._select)  
        return r

def implode(values):
    print values[0],  values[1:] # возвращает < 'instance' of SqlString>, [<instance ... >, <instance >] 
    r = v = values[0]
    if type(v).__name__ == 'instance':
        r = v.getSql()
    
    if len(values) == 1:
        return r
    
    return r + ', ' + implode(values[1:])

a = SqlComb('aaa')
a.select(['b', 'c, d, e'])

print a.getSql() # тут всё нормально, выводит то, что ожидаю

b = SqlComb('bbb')
b.select(a)
print b.getSql()

class SqlString (object) :
    def __init__ (self, param) :
        self._str = ''
        if isinstance(param, str) :
            self._str = param

    @property
    def Sql (self) :
        return self._str

class SqlComb (object) :
    """A SQL parser"""
    def __init__ (self,  tableName) :
        self._select = []
        self._from   = [tableName,]

    def select (self, fields) :
        if isinstance(fields, str) :
            fields = SqlString(fields)

        if isinstance(fields, (SqlString, SqlComb)) :
            self._select.append(fields)

        if isinstance(fields, list) :
            for v in fields :
                self.select(v)

    @property
    def Sql (self) :
        r = 'SELECT ' + implode(self._select)
        return r
##___________________________________________________________________________________

def implode (values) :
    if isinstance(values, (list, tuple)) :
        return ", ".join([v.Sql for v in values if isinstance(v, (SqlString, SqlComb))])
    else : return ""
#####################################################################################

if "__main__" == __name__ :
    a = SqlComb('aaa')
    a.select(['b', 'c, d, e', "f", "h"])

    print(a.Sql) # тут всё нормально, выводит то, что ожидаю

    b = SqlComb('bbb')
    b.select(a)
    print(b.Sql)
#################################### End Of File ####################################

def implode(values):
    if isinstance(values, (list, tuple)) :
        return ', '.join([v.Sql() for v in values if isinstance(v, (SqlComb, SqlString))])
    else :
        return ''