string = "{part1}_{part2}_{part3}"

result = string.format(part1="A", part2="", part3="B")
actual_result = "A__B"
desired_result = "A_B"

result = string.format(part1="A__A", part2="", part3="B__B")
result = result.replace("__", "_")
actual_result = "A_A_B_B"
desired_result = "A__A_B__B"

ignore_str = "$IGNORE_STRING$"
string = "{part1}_{part2}_{part3}"

result = string.format(part1="A__A", part2=ignore_str, part3="B__B")
result = result.replace("_" + ignore_str, "")
actual_result = "A__A_B__B"