[issue40799] Create Lib/_pydecimal.py file to optimize "import datetime" when _decimal is available

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

What do decimals have to datetime?

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[issue40799] Create Lib/_pydecimal.py file to optimize "import datetime" when _decimal is available

[STINNER Victor]

Change by STINNER Victor :


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[issue40799] Create Lib/_pydecimal.py file to optimize "import datetime" when _decimal is available

[STINNER Victor]

Change by STINNER Victor :


--
keywords: +patch
pull_requests: +19724
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/20472

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[issue40799] Create Lib/_pydecimal.py file to optimize "import datetime" when _decimal is available

[Ezio Melotti]

Change by Ezio Melotti :


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[issue40799] Create Lib/_pydecimal.py file to optimize "import datetime" when _decimal is available

[STINNER Victor]

New submission from STINNER Victor :

Currently, "import datetime" starts by importing time, math, sys and operator 
modules, and then execute 2500 lines of Python code, define 7 classes, etc. For 
what? Just to remove all classes, functions, etc. to replace them with symbols 
from _decimal module:
---
try:
from _datetime import *
except ImportError:
pass
else:
# Clean up unused names
del (_DAYNAMES, _DAYS_BEFORE_MONTH, _DAYS_IN_MONTH, _DI100Y, _DI400Y,
 _DI4Y, _EPOCH, _MAXORDINAL, _MONTHNAMES, _build_struct_time,
 _check_date_fields, _check_time_fields,
 _check_tzinfo_arg, _check_tzname, _check_utc_offset, _cmp, _cmperror,
 _date_class, _days_before_month, _days_before_year, _days_in_month,
 _format_time, _format_offset, _index, _is_leap, _isoweek1monday, _math,
 _ord2ymd, _time, _time_class, _tzinfo_class, _wrap_strftime, _ymd2ord,
 _divide_and_round, _parse_isoformat_date, _parse_isoformat_time,
 _parse_hh_mm_ss_ff, _IsoCalendarDate)
# XXX Since import * above excludes names that start with _,
# docstring does not get overwritten. In the future, it may be
# appropriate to maintain a single module level docstring and
# remove the following line.
from _datetime import __doc__
---

I would prefer to use the same approach than the decimal module which also has 
large C and Python implementation. Lib/decimal.py is just:
---
try:
from _decimal import *
from _decimal import __doc__
from _decimal import __version__
from _decimal import __libmpdec_version__
except ImportError:
from _pydecimal import *
from _pydecimal import __doc__
from _pydecimal import __version__
from _pydecimal import __libmpdec_version__
---

Advantages:

* Faster import time
* Avoid importing indirectly time, math, sys and operator modules, whereas they 
are not used

IMO it also better separate the C and the Python implementations.


Attached PR implements this idea.


Currently, "import datetime" imports 4 modules:

  ['_operator', 'encodings.ascii', 'math', 'operator']

With the PR, "import datetime"  imports only 1 module:

  ['encodings.ascii']

Import performance:

  [ref] 814 us +- 32 us -> [change] 189 us +- 4 us: 4.31x faster (-77%)

Measured by:

  env/bin/python -m pyperf timeit -s 'import sys' 'import datetime; del 
sys.modules["datetime"]; del sys.modules["_datetime"]; del datetime'


Note: I noticed that "import datetime" imports the math module while working on 
minimizing "import test.support" imports, bpo-40275.

--
components: Library (Lib)
messages: 370153
nosy: vstinner
priority: normal
severity: normal
status: open
title: Create Lib/_pydecimal.py file to optimize "import datetime" when 
_decimal is available
versions: Python 3.10

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