Finding Subsequences of a large String

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I am trying to get all the Subsequences of a String. Example:-

firstString = "ABCD"

O/P should be; 'ABCD', 'BCD', 'ACD', 'ABD', 'ABC', 'CD', 'BD', 'BC', 'AD', 'AC', 'AB', 'D', 'C', 'B', 'A'

For that I am using following part of code:-

---

#!usr/bin/python

from __future__ import print_function

from operator import itemgetter

from subprocess import call

import math

import itertools

import operator

call(["date"])

firstArray = []

firstString = "ABCD"

firstList = list(firstString)

for L in range(0, len(firstList)+1):

    for subset in itertools.combinations(firstList, L):

            firstArray.append(''.join(subset))

firstArray.reverse()

print (firstArray)

call(["date"])

---

But this code is not scalable.

If I provide :- firstString = "ABCDABCDABCDABCDABCDABCDABCD"

The program takes almost 6 mins time to complete.

---------------- Capture while running the script --------------------

python sample-0012.py

Wed Feb 8 21:30:30 PST 2017

Wed Feb 8 21:30:30 PST 2017

Can someone please help?

asked 38 secs ago

Manas Pal

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Finding Subsequences of a large String