Finding indexes of substrings

I am trying to code a program to find the index of multiple substrings of a string but I am stuck! See the examples bellow:

Find : 'yes'
Input = 'adnyesdapodyesndudndnyesae'
Output = [3,11,21]

Find : 'b'
Input = 'bbbbbbb'
Output = [0,1,2,3,4,5,6]

>Solution :

str1 = "adnyesdapodyesndudndnyesae" #The String
substr = "yes" #The Substring
res = [i for i in range(len(str1)) if str1.startswith(substr, i)]
print(str(res))

The third line basically runs a list comprehension method wherein using a for loop you check for occurences of a substring in the string.