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gh-148085: datetime cache time module lookups #148088

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36c494b
simple caching
maurycy Apr 4, 2026
48e9ea1
news
maurycy Apr 4, 2026
ae32bfd
oops, lint
maurycy Apr 4, 2026
7e7b739
lazy
maurycy Apr 4, 2026
cd247ca
i thought it existed... ok, now. no warnings?
maurycy Apr 4, 2026
2f3dbe0
mv to library
maurycy Apr 4, 2026
d3944fe
stan is right; just call PyTime_Time directly?
maurycy Apr 6, 2026
33c64f2
revert lazy
maurycy Apr 7, 2026
ff27dc4
news
maurycy Apr 7, 2026
056cf08
mv the comment
maurycy Apr 8, 2026
e00aa62
mea culpa!
maurycy Apr 8, 2026
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-04-04-14-25-43.gh-issue-148085.o97yTo.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Cache ``time.time``, ``time.struct_time``, and ``time.strftime`` imports,
speeding up :meth:`~datetime.date.timetuple` and
:meth:`~datetime.date.strftime` by up to 2x. Patch by Maurycy
Pawłowski-Wieroński.
72 changes: 56 additions & 16 deletions Modules/_datetimemodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -52,6 +52,10 @@ typedef struct {

/* The interned Unix epoch datetime instance */
PyObject *epoch;

PyObject *time_time;
PyObject *time_struct_time;
PyObject *time_strftime;
} datetime_state;

/* The module has a fixed number of static objects, due to being exposed
Expand Down Expand Up @@ -1879,10 +1883,19 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
assert(object && format && timetuple);
assert(PyUnicode_Check(format));

PyObject *strftime = PyImport_ImportModuleAttrString("time", "strftime");
if (strftime == NULL) {
PyObject *current_mod = NULL;
datetime_state *st = GET_CURRENT_STATE(current_mod);
if (st == NULL) {
return NULL;
}
if (st->time_strftime == NULL) {
st->time_strftime = PyImport_ImportModuleAttrString("time", "strftime");
if (st->time_strftime == NULL) {
RELEASE_CURRENT_STATE(st, current_mod);
return NULL;
}
}
PyObject *strftime = st->time_strftime;

/* Scan the input format, looking for %z/%Z/%f escapes, building
* a new format. Since computing the replacements for those codes
Expand Down Expand Up @@ -2042,7 +2055,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
Py_XDECREF(zreplacement);
Py_XDECREF(colonzreplacement);
Py_XDECREF(Zreplacement);
Py_XDECREF(strftime);
RELEASE_CURRENT_STATE(st, current_mod);
return result;

Error:
Expand All @@ -2059,13 +2072,20 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
static PyObject *
time_time(void)
{
PyObject *result = NULL;
PyObject *time = PyImport_ImportModuleAttrString("time", "time");

if (time != NULL) {
result = PyObject_CallNoArgs(time);
Py_DECREF(time);
PyObject *current_mod = NULL;
datetime_state *st = GET_CURRENT_STATE(current_mod);
if (st == NULL) {
return NULL;
}
if (st->time_time == NULL) {
st->time_time = PyImport_ImportModuleAttrString("time", "time");
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@StanFromIreland StanFromIreland Apr 4, 2026

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Thinking more about this, I think we could just replace this with a call to PyTime_Time directly, I assume this predates the existence of the C-API.

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@maurycy maurycy Apr 6, 2026 •
edited
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@StanFromIreland Thank you for taking a look. This is definitely a great simplification:

d3944fe

I updated the benchmark:

import datetime
import pyperf

r = pyperf.Runner()
dt = datetime.datetime(2026, 4, 4, 12, 30, 45)
d = datetime.date(2026, 4, 4)

class MyDate(datetime.date):
    pass

r.bench_func("datetime.timetuple", dt.timetuple)
r.bench_func("datetime.strftime", dt.strftime, "%Y-%m-%d")
r.bench_func("date.timetuple", d.timetuple)
r.bench_func("date.today", datetime.date.today)
r.bench_func("date_subclass.today", MyDate.today)

This is the result:

Benchmark main-fbdbea9 datetime-cache-time-module-lookups-d3944fe
datetime.timetuple 264 ns 130 ns: 2.04x faster
datetime.strftime 776 ns 524 ns: 1.48x faster
date.timetuple 263 ns 129 ns: 2.04x faster
date.today 78.4 ns 76.0 ns: 1.03x faster
date_subclass.today 306 ns 149 ns: 2.05x faster
Geometric mean (ref) 1.67x faster

date.today is expected: a fast hot path, but very good result in bypassing import in date_subclass.today

The downside is - as per @picnixz comment - that it now definitely breaks removing the module from sys.modules.

On the other hand, we don't seem to particularly care about monkey-patching:

cpython/Modules/_pickle.c

Line 371 in fbdbea9

st->codecs_encode = PyImport_ImportModuleAttrString("codecs", "encode");

cpython/Modules/arraymodule.c

Line 2562 in fbdbea9

state->array_reconstructor = PyImport_ImportModuleAttrString(

cpython/Modules/_zoneinfo.c

Line 2765 in fbdbea9

state->_tzpath_find_tzfile =

cpython/Modules/_asynciomodule.c

Line 4257 in fbdbea9

state->asyncio_mod = PyImport_ImportModule("asyncio");

cpython/Modules/_decimal/_decimal.c

Line 3700 in fbdbea9

state->PyDecimal = PyImport_ImportModuleAttrString("_pydecimal", "Decimal");

etc.

if (st->time_time == NULL) {
RELEASE_CURRENT_STATE(st, current_mod);
return NULL;
}
}
PyObject *result = PyObject_CallNoArgs(st->time_time);
RELEASE_CURRENT_STATE(st, current_mod);
return result;
}

Expand All @@ -2075,21 +2095,27 @@ time_time(void)
static PyObject *
build_struct_time(int y, int m, int d, int hh, int mm, int ss, int dstflag)
{
PyObject *struct_time;
PyObject *result;

struct_time = PyImport_ImportModuleAttrString("time", "struct_time");
if (struct_time == NULL) {
PyObject *current_mod = NULL;
datetime_state *st = GET_CURRENT_STATE(current_mod);
if (st == NULL) {
return NULL;
}
if (st->time_struct_time == NULL) {
st->time_struct_time = PyImport_ImportModuleAttrString("time", "struct_time");
if (st->time_struct_time == NULL) {
RELEASE_CURRENT_STATE(st, current_mod);
return NULL;
}
}

result = PyObject_CallFunction(struct_time, "((iiiiiiiii))",
PyObject *result = PyObject_CallFunction(st->time_struct_time,
"((iiiiiiiii))",
y, m, d,
hh, mm, ss,
weekday(y, m, d),
days_before_month(y, m) + d,
dstflag);
Py_DECREF(struct_time);
RELEASE_CURRENT_STATE(st, current_mod);
return result;
}

Expand Down Expand Up @@ -7414,6 +7440,9 @@ init_state(datetime_state *st, PyObject *module, PyObject *old_module)
.us_per_week = Py_NewRef(st_old->us_per_week),
.seconds_per_day = Py_NewRef(st_old->seconds_per_day),
.epoch = Py_NewRef(st_old->epoch),
.time_time = Py_XNewRef(st_old->time_time),
.time_struct_time = Py_XNewRef(st_old->time_struct_time),
.time_strftime = Py_XNewRef(st_old->time_strftime),
};
return 0;
}
Expand Down Expand Up @@ -7458,6 +7487,10 @@ init_state(datetime_state *st, PyObject *module, PyObject *old_module)
return -1;
}

st->time_time = NULL;
st->time_struct_time = NULL;
st->time_strftime = NULL;

return 0;
}

Expand All @@ -7467,6 +7500,10 @@ traverse_state(datetime_state *st, visitproc visit, void *arg)
/* heap types */
Py_VISIT(st->isocalendar_date_type);

Py_VISIT(st->time_time);
Py_VISIT(st->time_struct_time);
Py_VISIT(st->time_strftime);

return 0;
}

Expand All @@ -7482,6 +7519,9 @@ clear_state(datetime_state *st)
Py_CLEAR(st->us_per_week);
Py_CLEAR(st->seconds_per_day);
Py_CLEAR(st->epoch);
Py_CLEAR(st->time_time);
Py_CLEAR(st->time_struct_time);
Py_CLEAR(st->time_strftime);
return 0;
}

Expand Down
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