Does the datetime.datetime.ctime() function return a fixed width string?

Question

If I were to run the following code:

import datetime
dt = datetime.datetime.now()
date_str = dt.ctime()

Will the result of len(date_str) always be the same, regardless of when I happen to run the code? I think it will be, but I'm not certain. I looked at the datetime module reference but the relevant section says:

Return a string representing the date:

 >>> from datetime import date >>> date(2002, 12, 4).ctime() 'Wed Dec 4 00:00:00 2002'

d.ctime() is equivalent to:

 time.ctime(time.mktime(d.timetuple()))

on platforms where the native C ctime() function (which time.ctime() invokes, but which date.ctime() does not invoke) conforms to the C standard.

But looking at the article on cppreference.com , the answer is less than clear to me. I did find a reference here which suggests that it will be a 24-character wide string, but I was hoping for a more official source or reference if possible. It's possible that I didn't delve far enough into the C-standard to divine the answer.

Also, based on that quote, does that mean the behavior might be platform-dependent? Or is it likely to be generally uniform outside of niche operating systems?

(yes I know that I can manually format to fixed-width, but I want to know whether the built-in one already is that way)

Answer 1

Well, cppreference clearly states that the C standard requires ctime to return a 25 characters strings ending with a newline.

The resulting string has the following format:

 Www Mmm dd hh:mm:ss yyyy\n

...

The function does not support localization.

The doc for the time module states (emphasize mine):

Most of the functions defined in this module call platform C library functions with the same name.

A ctime is required by C standard, it should be available on any platform. So it should be always used by time.ctime .

Let us go on:

time.ctime([secs])

...

If secs is not provided or None, the current time as returned by time() is used. ctime(secs) is equivalent to asctime(localtime(secs)) . Locale information is not used by ctime().

and

time.asctime([t])

Convert a tuple or struct_time representing a time as returned by gmtime() or localtime() to a string of the following form: 'Sun Jun 20 23:21:05 1993' . The day field is two characters long and is space padded if the day is a single digit, eg: 'Wed Jun 9 04:26:40 1993' . ... Note: Unlike the C function of the same name, asctime() does not add a trailing newline.

So instead of the 25 characters string returned by the C function, the ctime or asctime functions of the Python standard library time module return a non localized 24 characters string.

So if you use CPython, and except on a specific system where the C standard library would be non conformant, you can trust time.ctime to always return a 24 character string.

And even if you use a different implementation (Jython, PyPy, etc.) the Python standard library documentation states that this non localized 24 characters string should be returned. So I cannot imagine a system returning a different size...

Answer 2

I've looked up datetime<\/code> in the source code of cpython<\/a> on github. It is implemented as this:

    def ctime(self):
        "Return ctime() style string."
        weekday = self.toordinal() % 7 or 7
        return "%s %s %2d 00:00:00 %04d" % (
            _DAYNAMES[weekday],
            _MONTHNAMES[self._month],
            self._day, self._year)

Answer 3

It's fixed-width. You can see in the cppreference article you linked that the output of C ctime<\/code> follows the format

Www Mmm dd hh:mm:ss yyyy\n