how to change a list to a specific string.

Question

class Bag:
    def __init__(self, i=None):
        self.bag = []
        if i == None:
            pass  # i is None, do nothing after create empty bag
        elif type(i)==list:
            self.bag.extend(i) # append whole list i into bag with extend method
        else:
            self.bag.append(i) # append single item i into bag 

    def __repr__(self):
        for s in self.bag :
            return s

    def __str__(self):
        for s in self.bag :
            return s

In the __str__method. It supposed to return a string.

The list is Bag(['a', 'c', 'b', 'b', 'd', 'd', 'd']). And

__str__ is supposed to return Bag(a[1], c[1], b[2], d[3])

Can anyone tell me how to make it work? Many thanks.

Answer 1

You can use a collections.Counter here to help:

def __str__(self):
    counts = Counter(self.bag)
    count_str = ','.join('{}[{}]'.format(k, v) for k, v in counts.items())
    return 'Bag({})'.format(count_str)

The great thing about this is that the counter does all the work of figuring out how many of each item are in your bag . All the rest of it is just formatting details. One downside is that the counter is not ordered. If you want an order, you could use the .most_common method (which will order the counts from most common to least-common), or you could use something like an ordered unique recipe to figure out how you want to iterate over the counter.