Roundup Robot added the comment:
New changeset 39f475aa0163 by Raymond Hettinger in branch 'default':
Issue 21101: Internal API for dict getitem and setitem where the hash value is
known.
http://hg.python.org/cpython/rev/39f475aa0163
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nosy: +python-dev
Roundup Robot added the comment:
New changeset 592a57682ced by Raymond Hettinger in branch 'default':
Issue #21101: Eliminate double hashing in the C code for collections.Counter().
http://hg.python.org/cpython/rev/592a57682ced
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Python tracker
Changes by Raymond Hettinger raymond.hettin...@gmail.com:
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resolution: - fixed
status: open - closed
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http://bugs.python.org/issue21101
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Raymond Hettinger added the comment:
Antoine, do you support adding these as part of the public API? If not, I can
make them private.
I think the functions are broadly useful, but no one has ever asked for this
functionality either.
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Python
Antoine Pitrou added the comment:
I think we can start with making them private. Do you know of any third-party
code bases which may be interested in the speedup?
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http://bugs.python.org/issue21101
Terry J. Reedy added the comment:
While the question is reasonable, I agree with Raymond's answer. As a python
programmer, I would not like to see
d.setitem_known_hash(key, hash, d.getitem_known_hash(key, hash) + 1)
Of course, d[key] += 1 already solves the double lookup issue at the Python
Alex Gaynor added the comment:
d[key] += 1 still does two dict lookups, and invokes the hash function twice:
class X(object):
... def __hash__(self):
... print hashed
... return 0
... def __eq__(self, other):
... return True
...
d = {X(): 0}
hashed
d[X()]
hashed
0
d[X()] = 3
Antoine Pitrou added the comment:
Of course, d[key] += 1 already solves the double lookup issue at the
Python level.
What the hell are you talking about?
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Terry J. Reedy added the comment:
What the hell I am talking about is what the doc says. 'd[key]' is written
just once and is evaluated just once.
https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements
PS: Try being a bit more polite.
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Antoine Pitrou added the comment:
PS: Try being a bit more polite.
You could definitely do some research before posting erroneous
statements (this one isn't difficult to check, as Alex showed).
Especially when the other posters (Alex and Raymond) are a lot more
competent than you on the
R. David Murray added the comment:
Antoine, being polite never hurts. Terry is a valuable member of the community
and sure, he sometimes makes mistakes (or trusts the docs too much?). So do
the the rest of us.
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Python
Terry J. Reedy added the comment:
Raymond identified a need and a possible solution. The important part of my
post was suggesting another possible solution. Please focus on that.
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Changes by Arfrever Frehtes Taifersar Arahesis arfrever@gmail.com:
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nosy: +Arfrever
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http://bugs.python.org/issue21101
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Changes by Josh Rosenberg shadowranger+pyt...@gmail.com:
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nosy: +josh.rosenberg
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Changes by Raymond Hettinger raymond.hettin...@gmail.com:
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components: Interpreter Core
files: known_hash.diff
keywords: patch
nosy: rhettinger
priority: normal
severity: normal
stage: patch review
status: open
title: Extend the PyDict C API to handle cases where the hash value in
Changes by Raymond Hettinger raymond.hettin...@gmail.com:
Added file: http://bugs.python.org/file34667/applied_known_hash.diff
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New submission from Raymond Hettinger:
Propose adding two functions, PyDict_GetItem_KnownHash() and
PyDict_SetItem_KnownHash().
It is reasonably common to make two successive dictionary accesses with the
same key. This results in calling the hash function twice to compute the same
result.
Changes by Raymond Hettinger raymond.hettin...@gmail.com:
Added file: http://bugs.python.org/file34668/double_counter_hash.py
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Changes by Serhiy Storchaka storch...@gmail.com:
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nosy: +serhiy.storchaka
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Antoine Pitrou added the comment:
Is there any benefit in making them public API functions?
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Raymond Hettinger added the comment:
Is there any benefit in making them public API functions?
Originally, I was going to suggest them as internal functions, but the variety
of use cases in the standard library suggested that third-party C extensions
would benefit as well.
Since this
Alex Gaynor added the comment:
Would it be reasonable to develop a Python API for this? If C functions have a
need to do this, surely Python code does as well.
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Changes by Raymond Hettinger raymond.hettin...@gmail.com:
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title: Extend the PyDict C API to handle cases where the hash value in known -
Extend the PyDict C API to handle cases where the hash value is known
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Python tracker
Raymond Hettinger added the comment:
Would it be reasonable to develop a Python API for this?
I suspect that in pure Python, the overhead would exceed the benefit.
Current code:
d[key] = d[key] + 1
Effort to save a double hash:
h = hash(key)
c = d.getitem_known_hash(key, hash)
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