date, time and datetime alternate constructors should take fast construction path

[pganssle]

BPO	32403
Nosy	@abalkin, @pganssle
PRs	#4993 #5814 #5929

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-01-16.18:07:46.382>
created_at = <Date 2017-12-21.21:51:14.443>
labels = ['3.8', 'type-feature', '3.7']
title = 'date, time and datetime alternate constructors should take fast construction path'
updated_at = <Date 2018-02-27.19:58:30.646>
user = 'https://github.com/pganssle'

bugs.python.org fields:

activity = <Date 2018-02-27.19:58:30.646>
actor = 'belopolsky'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-16.18:07:46.382>
closer = 'belopolsky'
components = []
creation = <Date 2017-12-21.21:51:14.443>
creator = 'p-ganssle'
dependencies = []
files = []
hgrepos = []
issue_num = 32403
keywords = ['patch']
message_count = 4.0
messages = ['308907', '310099', '313019', '313021']
nosy_count = 2.0
nosy_names = ['belopolsky', 'p-ganssle']
pr_nums = ['4993', '5814', '5929']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32403'
versions = ['Python 3.7', 'Python 3.8']

[pganssle]

In the addition of the fromisoformat() alternate constructor (bpo-15873: #4699), I noted that I was able to get some significant speedup by special-casing the datetime baseclass in the C code so that it bypasses the Python constructor, by replacing code that looks like this:

return PyObject_CallFunction(cls, "iii", year, month, day);

With code that looks like this:

    PyObject *result;
    if ( (PyTypeObject *)cls == & PyDateTime_DateType ) {
        result = new_date_ex(year, month, day, (PyTypeObject *)cls);
    } else {
        result = PyObject_CallFunction(cls, "iii", year, month, day);
    }

return result;

(This is for date, but the results are even more striking for datetime).

In my initial proof of concept implementation of a new_date_subclass_ex method, I've seen (this is not compiled with optimizations on, mind you) speedups for the other constructors as well:

Old constructor:
\================
Class: date
constructor:        940.5ns
date.fromordinal:   1544.8ns
date.fromtimestamp: 1941.9ns

Class: DateSubclass
constructor:        1016.6ns
date.fromordinal:   1760.3ns
date.fromtimestamp: 2295.3ns


With fastpath:
\==============
Class: date
constructor:        964.3ns
date.fromordinal:   997.6ns
date.fromtimestamp: 1130.2ns

Class: DateSubclass
constructor:        1086.9ns
date.fromordinal:   1818.5ns
date.fromtimestamp: 2129.9ns

As you can see, this is a fairly significant speedup in the common case with no cost in the unusual case and no change in behavior. I propose that we switch over all the C constructors where it makes sense to do so in date, time and datetime.

I'll have a PR forthcoming soon.

[abalkin]

New changeset 9f1b7b9 by Alexander Belopolsky (Paul Ganssle) in branch 'master':
bpo-32403: Faster date and datetime constructors (bpo-4993)
9f1b7b9

[abalkin]

New changeset 5bd04f9 by Alexander Belopolsky (Paul Ganssle) in branch 'master':
bpo-10381, bpo-32403: What's new entries for changes to datetime (gh-5814)
5bd04f9

[abalkin]

New changeset fff596f by Alexander Belopolsky (Miss Islington (bot)) in branch '3.7':
bpo-10381, bpo-32403: What's new entries for changes to datetime (gh-5814) (gh-5929)
fff596f