Commit 58019674 authored by Paul Sokolovsky's avatar Paul Sokolovsky
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docs/speed_python: Add many more details on memoryviews.

parent 47f9b10b
......@@ -85,18 +85,31 @@ elements in contiguous memory locations. Once again to avoid memory allocation i
code these should be pre-allocated and passed as arguments or as bound objects.
When passing slices of objects such as ``bytearray`` instances, Python creates
a copy which involves allocation. This can be avoided using a ``memoryview``
a copy which involves allocation of the size proportional to the size of slice.
This can be alleviated using a ``memoryview`` object. ``memoryview`` itself
is allocated on heap, but is a small, fixed-size object, regardless of the size
of slice it points too.
.. code:: python
ba = bytearray(100)
func(ba[3:10]) # a copy is passed
mv = memoryview(ba)
func(mv[3:10]) # a pointer to memory is passed
ba = bytearray(10000) # big array
func(ba[30:2000]) # a copy is passed, ~2K new allocation
mv = memoryview(ba) # small object is allocated
func(mv[30:2000]) # a pointer to memory is passed
A ``memoryview`` can only be applied to objects supporting the buffer protocol - this
includes arrays but not lists.
includes arrays but not lists. Small caveat is that while memoryview object is live,
it also keeps alive the original buffer object. So, memoryviews isn't universal
panacea. For instance, in the example above, if you are done with 10K buffer and
just need those bytes 30:2000 from it, it may be better to make a slice, and let
the 10K buffer go (be ready for garbage collection), instead of making a
long-living memoryview and keeping 10K blocked for GC.
Nonetheless, ``memoryview`` is indispensable for advanced preallocated buffer
management. ``.readinto()`` method discussed above puts data at the beginning
of buffer and fills in entire buffer. What if you need to put data in the
middle of existing buffer? Just create a memoryview into the needed section
of buffer and pass it to ``.readinto()``.
Identifying the slowest section of code
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