numpy.memmap advice?

Carl Banks pavlovevidence at
Wed Feb 18 15:35:55 EST 2009

On Feb 18, 10:48 am, Lionel < at> wrote:
> Thanks Carl, I like your solution. Am I correct in my understanding
> that memory is allocated at the slicing step in your example i.e. when
> "reshaped_data" is sliced using "interesting_data = reshaped_data[:,
> 50:100]"? In other words, given a huge (say 1Gb) file, a memmap object
> is constructed that memmaps the entire file. Some relatively small
> amount of memory is allocated for the memmap operation, but the bulk
> memory allocation occurs when I generate my final numpy sub-array by
> slicing, and this accounts for the memory efficiency of using memmap?

No, what accounts for the memory efficienty is there is no bulk
allocation at all.  The ndarray you have points to the memory that's
in the mmap.  There is no copying data or separate array allocation.

Also, it's not any more memory efficient to use the offset parameter
with numpy.memmap than it is to memmap the whole file and take a

Carl Banks

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