How to find all positions of the maximum value in a list?

a.index(max(a))

will tell you the index of the first instance of the largest valued element of list a.

>>> m = max(a)>>> [i for i, j in enumerate(a) if j == m][9, 12]

The chosen answer (and most others) require at least two passes through the list.
Here's a one pass solution which might be a better choice for longer lists.

Edited: To address the two deficiencies pointed out by @John Machin. For (2) I attempted to optimize the tests based on guesstimated probability of occurrence of each condition and inferences allowed from predecessors. It was a little tricky figuring out the proper initialization values for max_val and max_indices which worked for all possible cases, especially if the max happened to be the first value in the list — but I believe it now does.

def maxelements(seq):    ''' Return list of position(s) of largest element '''    max_indices = []    if seq:        max_val = seq[0]        for i,val in ((i,val) for i,val in enumerate(seq) if val >= max_val):            if val == max_val:                max_indices.append(i)            else:                max_val = val                max_indices = [i]    return max_indices