Dictionary Assignment Not Functioning As Intended

CONTEXT

I’m creating a calculator. I’m at the point where I want my calculator to do BODMAS (start calculations with the inner most bracket).

The user enters an expression, in this example it is: expression = (1+(1+1))

I have a function that splits up this function into ‘terms’ and saves the result in expression_list.

expression_list = ['(', '1', '+', '(', '1', '+', '1', ')', ')']

I have a function that locates the indexes of all brackets in expression_list and saves it in b_pairs.

For the above expression the intended output for b_pairs should be of the form:

b_pairs = [{'ob_1', 0}, {'ob_2', 3}, {'cb_1', 7}, {'cb_2', 8}]

Here is the function:

for term in expression_list:
    if term == '(':
        b_pairs.append({"ob_"+str(ob),index})
        ob+=1
    elif term == ')':
        b_pairs.append({"cb_"+str(cb),index})
        cb+=1
    index+=1

Where 'ob' represents an open bracket and 'oc' represents a close bracket.

PROBLEM

The function works because it identifies the location of the open/close brackets and their respective locations correctly, but it mixes up the format. Here is the actual result:

b_pairs = [{0, 'ob_1'}, {'ob_2', 3}, {'cb_1', 7}, {8, 'cb_2'}]

Notice the order in (a) b_pairs[0] and (b) b_pairs[1]. I want the key to be the name of the bracket and the value to be the location of the bracket in expression_array, e.g. {‘ob_1,0}, not the other way around.

I’ve coded such that the format should be as in b_pairs[1], but for some strange reason it’s not always so.

I’ve debugged the code but I can’t see any logical reason for this happening. I know that a dictionary with multiple key/value pairs will not always be printed in order, but in this example the dictionary variables in expression_list are single key/value and I code such that the key should be the name of the bracket, not the index of the bracket in expression_list. Please help.