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Yup. It was faster to: Use lists for the append, then transform into an array, then transform into a binary string Rather than Create empty arrays and use its append method, then transform into a binary string. The last question on the output when then be to test the speed of using generic Python arrays, which have append methods as well. Then, there would still only be the binary string conversion as apposed to list-->numpy array-->binary string Thanks to all for your input.... MJ ________________________________ From: numpy-discussion-bounces@scipy.org [mailto:numpy-discussion-bounces@scipy.org] On Behalf Of Charles R Harris Sent: Tuesday, February 13, 2007 12:44 PM To: Discussion of Numerical Python Subject: Re: [Numpy-discussion] fromstring, tostring slow? I am going to guess that a list would be faster for appending. Concat and, I suspect, append make new arrays for each use, rather like string concatenation in Python. A list, on the other hand, is no doubt optimized for adding new values. Another option might be using PyTables with extensible arrays. In any case, a bit of timing should show the way if the performance is that crucial to your application. Chuck