for combination in product():# 

 expression = ...  

 import ast
from itertools import product
number = 100
for combination in product(*[['+', '-', '']] * 8):
    expression = '1'+''.join(sum([[x, y] for x, y in zip(combination, '23456789')], []))
    print('Trying the expression:', expression)
    answer = ast.literal_eval(expression) # tested in Python 3.5.x, import ast first!
    if answer == number: # compare the result and the target value
        print('Target expression is: ', expression)
        break
else:  #see for-else docs for details... 
    print('Sorry... Target expression wasnt found.')

 .....
Trying the expression: 1+2+3+4567-89
Trying the expression: 1+2+3+45678+9
Trying the expression: 1+2+3+45678-9
Trying the expression: 1+2+3+456789
Trying the expression: 1+2+3-4+5+6+7+8+9
Trying the expression: 1+2+3-4+5+6+7+8-9
Trying the expression: 1+2+3-4+5+6+7+89
Trying the expression: 1+2+3-4+5+6+7-8+9
Trying the expression: 1+2+3-4+5+6+7-8-9
Trying the expression: 1+2+3-4+5+6+7-89
Trying the expression: 1+2+3-4+5+6+78+9
Target expression is:  1+2+3-4+5+6+78+9