Python, not able to append to a list from a recursive function

I am in the mid-way of writing a code to find all possible solutions of a input similar like "a&b|c!d|a", where a,b,c,d all are booleans and &-and, |-or !-not are the operators. By solution I mean the set of values of these variables which makes the input expression give True.

I am able to print all possible combinations of the variables, but I am not able to retain them (in this case, in a list) for later use.

What's wrong in the way I am doing it? Are there better ways to store them? generate_combination is the method in which I am trying to do this.

Code:

import operator

# global all_combinations
all_combinations=[]
answers=[]

def solve(combination, input, rank):
    try:
        substituted_str=""
        for i in input:
            if i in combination:
                substituted_str+=combination[i]
            else:
                substituted_str+=i
        print substituted_str
        # for item in rank:
    except:
        pass

def generate_combination(variables,comb_dict, length, current_index):
    if len(comb_dict)==length:
        print comb_dict #Each combination , coming out right
        all_combinations.append(comb_dict)
        print all_combinations,"\n" #This is not working as expected
    else:
        for i in [1,0]:
            comb_dict[variables[current_index]]=i
            generate_combination(variables,comb_dict, length,current_index+1)
            comb_dict.pop(variables[current_index], None)


def main(input,variables,order):
    rank=sorted(order.items(), key=operator.itemgetter(1))

    generate_combination(variables, {}, len(variables), 0)

    for combination in all_combinations:
        print combination
        ans=solve(combination, input, rank)
        ans=[]
        answers.extend(ans)

    # for answer in answers:
    #     print answer

def nothing():
    pass

if __name__ == '__main__':
    # print "Enter your symbols for :\n"
    # And=raw_input("And = ")
    # Or=raw_input("Or = ")
    # Not=raw_input("Not = ")
    # input_str=raw_input("Enter the expression :")

    And,Or,Not,input_str='&','|','!','a&b|c!d|a'
    input_str=input_str.replace(" ","")


    mapping={And:"&", Or:"|", Not:"!"}
    order={"&":3, "|":2, "!":1}


    variables=[]
    processed_str=""

    for i in input_str:
        if i in mapping:
            processed_str+=mapping[i]
        else:
            processed_str+=i
            variables.append(i)
    variables=list(set(variables))

    print "Reconstituted string : ",processed_str
    print "Variables : ",variables,"\n"

    main(processed_str,variables,order)

Current Output:

Reconstituted string :  a&b|c!d|a
Variables :  ['a', 'c', 'b', 'd'] 

{'a': 1, 'c': 1, 'b': 1, 'd': 1}
[{'a': 1, 'c': 1, 'b': 1, 'd': 1}] 

{'a': 1, 'c': 1, 'b': 1, 'd': 0}
[{'a': 1, 'c': 1, 'b': 1, 'd': 0}, {'a': 1, 'c': 1, 'b': 1, 'd': 0}] 

{'a': 1, 'c': 1, 'b': 0, 'd': 1}
[{'a': 1, 'c': 1, 'b': 0, 'd': 1}, {'a': 1, 'c': 1, 'b': 0, 'd': 1}, {'a': 1, 'c': 1, 'b': 0, 'd': 1}] 

{'a': 1, 'c': 1, 'b': 0, 'd': 0}
[{'a': 1, 'c': 1, 'b': 0, 'd': 0}, {'a': 1, 'c': 1, 'b': 0, 'd': 0}, {'a': 1, 'c': 1, 'b': 0, 'd': 0}, {'a': 1, 'c': 1, 'b': 0, 'd': 0}] 

{'a': 1, 'c': 0, 'b': 1, 'd': 1}
[{'a': 1, 'c': 0, 'b': 1, 'd': 1}, {'a': 1, 'c': 0, 'b': 1, 'd': 1}, {'a': 1, 'c': 0, 'b': 1, 'd': 1}, {'a': 1, 'c': 0, 'b': 1, 'd': 1}, {'a': 1, 'c': 0, 'b': 1, 'd': 1}] 

{'a': 1, 'c': 0, 'b': 1, 'd': 0}
[{'a': 1, 'c': 0, 'b': 1, 'd': 0}, {'a': 1, 'c': 0, 'b': 1, 'd': 0}, {'a': 1, 'c': 0, 'b': 1, 'd': 0}, {'a': 1, 'c': 0, 'b': 1, 'd': 0}, {'a': 1, 'c': 0, 'b': 1, 'd': 0}, {'a': 1, 'c': 0, 'b': 1, 'd': 0}] 

{'a': 1, 'c': 0, 'b': 0, 'd': 1}
[{'a': 1, 'c': 0, 'b': 0, 'd': 1}, {'a': 1, 'c': 0, 'b': 0, 'd': 1}, {'a': 1, 'c': 0, 'b': 0, 'd': 1}, {'a': 1, 'c': 0, 'b': 0, 'd': 1}, {'a': 1, 'c': 0, 'b': 0, 'd': 1}, {'a': 1, 'c': 0, 'b': 0, 'd': 1}, {'a': 1, 'c': 0, 'b': 0, 'd': 1}] 

{'a': 1, 'c': 0, 'b': 0, 'd': 0}
[{'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}, {'a': 1, 'c': 0, 'b': 0, 'd': 0}] 

{'a': 0, 'c': 1, 'b': 1, 'd': 1}
[{'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}, {'a': 0, 'c': 1, 'b': 1, 'd': 1}] 

{'a': 0, 'c': 1, 'b': 1, 'd': 0}
[{'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}, {'a': 0, 'c': 1, 'b': 1, 'd': 0}] 

{'a': 0, 'c': 1, 'b': 0, 'd': 1}
[{'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}, {'a': 0, 'c': 1, 'b': 0, 'd': 1}] 

{'a': 0, 'c': 1, 'b': 0, 'd': 0}
[{'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}, {'a': 0, 'c': 1, 'b': 0, 'd': 0}] 

{'a': 0, 'c': 0, 'b': 1, 'd': 1}
[{'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}, {'a': 0, 'c': 0, 'b': 1, 'd': 1}] 

{'a': 0, 'c': 0, 'b': 1, 'd': 0}
[{'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}, {'a': 0, 'c': 0, 'b': 1, 'd': 0}] 

{'a': 0, 'c': 0, 'b': 0, 'd': 1}
[{'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}, {'a': 0, 'c': 0, 'b': 0, 'd': 1}] 

{'a': 0, 'c': 0, 'b': 0, 'd': 0}
[{'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}, {'a': 0, 'c': 0, 'b': 0, 'd': 0}]