[Numpy-discussion] loop through values in a array and find maximum as looping

I would like to produce an array with the maximum values out of many (10000s) of arrays. I need to loop through many multidimentional arrays and if a value is larger (in the same place as the previous array) then I would like that value to replace it. e.g. a=[1,1,2,2 11,2,2 1,1,2,2] b=[1,1,3,2 2,1,0,0 1,1,2,0] where b>a replace with value in b, so the new a should be : a=[1,1,3,2] 2,1,2,2 1,1,2,2] and then keep looping through many arrays and replace whenever value is larger. I have tried numpy.putmask but that results in TypeError: putmask() argument 1 must be numpy.ndarray, not list Any other ideas? Thanks