python find all occurrences in list using built in code example

Example: get all occurrence indices in list python

a_list = [1, 2, 3, 1]

indices = []
for i in range(len(a_list)):
   if a_list[i] == 1:
      indices.append(i)

# more concise way
a_list = [1, 2, 3, 1]
indices = [index for index, element in enumerate(a_list) if element == 1]