[issue42152] Use PyDict_Contains() and PyDict_SetDefault() instead of PyDict_GetItemWithError()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Sure. Thanks for reminder.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue42152] Use PyDict_Contains() and PyDict_SetDefault() instead of PyDict_GetItemWithError()

[Inada Naoki]

Inada Naoki  added the comment:

May I close this issue?

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[issue42152] Use PyDict_Contains() and PyDict_SetDefault() instead of PyDict_GetItemWithError()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset b510e101f8b5b31276bf97b921ca9247162881d2 by Serhiy Storchaka in 
branch 'master':
bpo-42152: Use PyDict_Contains and PyDict_SetDefault if appropriate. (GH-22986)
https://github.com/python/cpython/commit/b510e101f8b5b31276bf97b921ca9247162881d2


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[issue42152] Use PyDict_Contains() and PyDict_SetDefault() instead of PyDict_GetItemWithError()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The code uses also _PyDict_ContainsId() added in issue42006 instead of 
_PyDict_GetItemIdWithError().

Few uses of _PyDict_GetItemStringWithError() are left as is. I do not think it 
is worth to introduce _PyDict_ContainsString(). They could be rewritten using 
_PyDict_ContainsId() if we find it worth.

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[issue42152] Use PyDict_Contains() and PyDict_SetDefault() instead of PyDict_GetItemWithError()

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
keywords: +patch
pull_requests: +21900
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/22986

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[issue42152] Use PyDict_Contains() and PyDict_SetDefault() instead of PyDict_GetItemWithError()

[Serhiy Storchaka]

New submission from Serhiy Storchaka :

It was common to use the code PyDict_GetItem(dict, key) == NULL to check 
whether the key is in the dict. PyDict_GetItem() returns borrowed reference, so 
no clean up is needed. And if we need to check only existence of the key, we do 
not need to store a value.

But PyDict_GetItem() which suppresses all internal exceptions is considered not 
safe, so PyDict_GetItemWithError() should be used. The code looks like:

if (PyDict_GetItemWithError(dict, key) == NULL) {
if (PyErr_Occurred()) {
goto error;
}
// does not have the key
}
else {
// has the key
}

It can be written with using PyDict_Contains():

int r = PyDict_Contains(dict, key);
if (r < 0) {
goto error;
}
if (r == 0) {
// does not have the key
}
else {
// has the key
}

Advantages:

* It is more clear expression of what we do.
* It is more simple and efficient in PyPy, because it has to keep borrowed 
references and track their lifetime.
* It may be more efficient in CPython, because calling PyErr_Occurred() has 
small but non-zero cost.
* PyErr_Occurred() would not be fooled by exception raised before. So you can 
use this code even if an exception is set.

Disadvantages:

* You need to use an int variable.

In some cases, when this check is used in combinations with PyDict_SetItem(), 
the code can be replaced with PyDict_SetDefault(), which is bot more terse and 
efficient. And when it is used in combinations with PyDict_DelItem(), the code 
can be replaced with _PyDict_Pop().

The proposed patch makes the code using PyDict_Contains() and 
PyDict_SetDefault() if appropriate. All use cases for _PyDict_Pop() were 
already covered by previous changes.

--
components: +Interpreter Core
nosy: +methane, vstinner
title: Use PyDict_Contains() and PyDict_SetDefault() instead of 
PyDict_GetItemIdWithError -> Use PyDict_Contains() and PyDict_SetDefault() 
instead of PyDict_GetItemWithError()
type:  -> enhancement
versions: +Python 3.10

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