Why is `datetime.date.today` slower than `datetime.datetime.now` in Python?

Question

Out of curiosity, I did the following benchmark of these two functions:

In [12]: %timeit datetime.datetime.now()
100000 loops, best of 3: 5.09 µs per loop

In [13]: %timeit datetime.date.today()
100000 loops, best of 3: 6.4 µs per loop

I thought date object involved less information, so it should be the faster one, but it turned out to be slower.

What could be the reason?

Answer 1

I got nerd-sniped by this today so I'll show you what I found -- strap in.

first, the implementation of date.today() must go through a member function call -- this lookup appears to be the slow part:

https://github.com/python/cpython/blob/b2bf2bc1ece673d387341e06c8d3c2bc6e259747/Modules/_datetimemodule.c#L2886-L2892

excerpted here:

static PyObject *
date_today(PyObject *cls, PyObject *dummy)
{
    PyObject *time;
    PyObject *result;
    _Py_IDENTIFIER(fromtimestamp);

    time = time_time();
    if (time == NULL)
        return NULL;

    /* Note well:  today() is a class method, so this may not call
     * date.fromtimestamp.  For example, it may call
     * datetime.fromtimestamp.  That's why we need all the accuracy
     * time.time() delivers; if someone were gonzo about optimization,
     * date.today() could get away with plain C time().
     */
    result = _PyObject_CallMethodIdOneArg(cls, &PyId_fromtimestamp, time);
    Py_DECREF(time);
    return result;
}

notably it's always going through the slow path

so I figured, why not give it a fast path?

$ git diff -w
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index 8ef2dad37a..7eaa5d1740 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -2875,6 +2875,17 @@ date_fromtimestamp(PyObject *cls, PyObject *obj)
 static PyObject *
 date_today(PyObject *cls, PyObject *dummy)
 {
+    /* fast path, don't call fromtimestamp */
+    if ((PyTypeObject *)cls == &PyDateTime_DateType) {
+        struct tm tm;
+        time_t t;
+        time(&t);
+        localtime_r(&t, &tm);
+        return new_date_ex(tm.tm_year + 1900,
+                           tm.tm_mon + 1,
+                           tm.tm_mday,
+                           (PyTypeObject*)cls);
+    } else {
         PyObject *time;
         PyObject *result;
         _Py_IDENTIFIER(fromtimestamp);
@@ -2893,6 +2904,7 @@ date_today(PyObject *cls, PyObject *dummy)
         Py_DECREF(time);
         return result;
     }
+}
 
 /*[clinic input]
 @classmethod

zoom zoom

$ ./python -m timeit -s 'from datetime import date' 'date.today()'
500000 loops, best of 5: 407 nsec per loop
$ ./python -m timeit -s 'from datetime import datetime' 'datetime.now().date()'
500000 loops, best of 5: 764 nsec per loop