quick_replace

s0 = "foobarbaz" # --> friendbarbaz
s1 ='foofoofoo'  # --> friendfriendfriend
s2 = 'foodoof goof foog' # --> frienddoof goof friendg
s = 'f o o d' #--> f o o d


tobereplace = 'foo'
replacement= 'friend'


new_s = ''
i=0
while i < len(s):
    temp = ''
    j = 0
    while j< len(tobereplace) and s[i] == tobereplace[j]:
        temp += s[i]
        i+=1
        j+=1

    if temp == tobereplace:
        new_s += replacement
    else:
        new_s += temp+s[i] if temp != '' else s[i]
        i+=1

print(new_s)