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]