Python Error "TypeError: 'str' object does not support item assignment"

Dear OS community,

I downloaded a template for my experiment and I tried to run it. Unfortunately it did not run, instead I got this error message immediately:

This error occurred on line 7 in the prepare phase of item Imports_and_Defaults.

Traceback (most recent call last):
  File "<Imports_and_Defaults.prepare>", line 7, in <module>
TypeError: 'str' object does not support item assignment

The complete corresponding code of 'Imports_and_Defaults' is the following:

import random

var.balloon_sizes = {}

for i in range(0,20):

y_val = 300 - (15*i)

j = "%.2f" % ((20 - i) / 20.0)

var.balloon_sizes[j] = y_val

# Probability variable for random explosion time

var.Probability = 0.0

# Set up sound for successful inflation

src1 = exp.pool['casino960msMax_22k16bMono.wav']

Casino_sound = sampler(src1, volume=1)

# Set up sound for cashing out

src2 = exp.pool['finale_22k16bMono.wav']

Cash_out_sound = sampler(src2, volume=1)

total_pennies = 0.0

var.total_cash = "$%.2f" % total_pennies

prac_pennies = 0.0

var.prac_cash = "$%.2f" % prac_pennies

var.cash_out_button = "return"

var.inflate_button = "space"

var.response = 0

I could not manage to find out what the problem is. Would be great if anybody could help me with this. Thanks! 😊