Python Recursive function to fetch data from dict

Question

I have created a recursive function to fetch the data from a dictionary. The dictionary consists of keys and each key has a list of keys and it goes on. So I need to fetch the flatten list of keys when I give a key input.

My Dict :

data = {"p": ["s1", "s2", "s3", "s4"],
        "s1": ["s1s1", "s1s2"],
        "s2": [],
        "s3": [],
        "s4": [],
        "s1s1": [],
        "s1s2": ["s1s2s1"],
        "s1s2s1": []
        }

My function :

def get_data(key):
    items = data[key]
    if items:
        for key in items:
            items += get_data(key)
    return items

when i call get_data("p") it returns

['s1', 's2', 's3', 's4', 's1s1', 's1s2', 's1s2s1', 's1s2s1']

But the expected output is :

['s1', 's2', 's3', 's4', 's1s1', 's1s2', 's1s2s1']

Thanks in advance for any help ...

@MuthuKumar does the order of the list in which they are present matter? — Devanshu Misra
– Devanshu Misra, Commented Feb 27, 2019 at 7:00

Shashank Agarwal · Accepted Answer · 2019-02-27 07:13:37Z

3

The problem is in these lines -

for key in items:
    items += get_data(key)

Here you are modifying items as you're iterating over it. So in the last iteration, your items ends up getting with the same key multiple times; you can add a logging statement to see which key is being used to call get_data.

You want to obtain all the new items separately, and then update items after the iteration is done -

new_items = []
for key in items:
    new_items += get_data(key)
items += new_items

answered Feb 27, 2019 at 7:13

Shashank Agarwal

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Comments

Devanshu Misra · Accepted Answer · 2019-02-27 07:16:12Z

Here's a code which will help you achieve what you want. The common approach to get a unique collection of items is to use a set. Sets are unordered collections of distinct objects. To create a set from any iterable, you can simply pass it to the built-in set() function. If you later need a real list again, you can similarly pass the set to the list() function.

data = {"p": ["s1", "s2", "s3", "s4"],
        "s1": ["s1s1", "s1s2"],
        "s2": [],
        "s3": [],
        "s4": [],
        "s1s1": [],
        "s1s2": ["s1s2s1"],
        "s1s2s1": []
        }

def get_data(key):
    items = data[key]
    if items:
        for keys in items:
            items += get_data(keys)
    return list(set(items))

print(get_data("p"))

OUTPUT:

['s3', 's1s1', 's1', 's1s2', 's2', 's1s2s1', 's4']

Update: an order-preserving approach is two lines:

from collections import OrderedDict
return OrderedDict((x, True) for x in items).keys()

replace the return line of the given solution with the return line of the Update: Solution and you will get an ordered list in return.

OUTPUT:

odict_keys(['s1', 's2', 's3', 's4', 's1s1', 's1s2', 's1s2s1'])

Use list() function with the achieved output to get the following list:

['s1', 's2', 's3', 's4', 's1s1', 's1s2', 's1s2s1']

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