[ Python 常見問題 ] How to find all occurrences of an element in a list?

Source From Here 
Question 
index() will just give the first occurrence of an item in a list. Is there a neat trick which returns all indices in a list? 

How-To 
You can use a list comprehension: 

>>> alist = [1, 2, 3, 3, 2, 4, 1, 5, 6, 5, 5, 7] 
>>> indices_of_2 = [i for i, v in enumerate(alist) if v == 2] # Collecting index of value=2 
>>> indices_of_2 
[1, 4]

A functional programming way for reference: 

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1. def foldLeft(alist, identity, fun):  
2.     rst = identity  
3.     for e in alist:  
4.         rst = fun(rst, e)  
5.     return rst  
6.   
7. def indexAll(rst, t, n):  
8.     def ia(rst, t):  
9.         if t[1] == n:  
10.             rst.append(t[0])  
11.         return rst  
12.     return ia  
13.   
14. foldLeft(list(enumerate(alist)), [], indexAll(2))  # Return index of '2' = [1, 4]  
15. foldLeft(list(enumerate(alist)), [], indexAll(5))  # Return index of '5' = [7, 9, 10]