[issue17139] dateTime.now() return format missing decimal seconds.

[R. David Murray]

R. David Murray added the comment:

No, it is not "it's not broke because there's a workaround", it is not broken because it is *working as designed*.

The str of any object in python is intended to be a convenient representation of that object.  Note that this applies to *all* objects in Python, not just datetime.  (Thus, talking about deprecating __str__ doesn't make any sense.)

The most convenient representation of a datetime has been deemed to be yyyy-mm-dd hh:mm:ss if there microseconds is equal to zero, and yyy-mm-dd hh:mm:ss.MMMMMM if microseconds is not equal to zero, and in practice this has, in general, worked out very well.  It is too bad that Python can't read the programmer's mind and know whether or not zero microseconds are important in a given str call, but sadly it can't, so the above algorithm is the most convenient for the typical uses of str on a datetime.

Now, if you are writing an application and you are doing output that needs to be in a particular format, you should *format your output* to be the way you want it.  This applies to any data, not just datetimes.  It means using a format specification, to which the str of an object might or might not be an input.  In the case of datetime, you don't want to use its str as the input to the format, because that can loose information, as you have discovered.  Instead you want to specify an strftime format string that causes the data to be displayed *exactly as you want it to be*:

    >>> counter1 = 23360
    >>> x = datetime.datetime(2013, 2, 5, 20, 45)
    >>> x
    datetime.datetime(2013, 2, 5, 20, 45)
    >>> str(x)
    '2013-02-05 20:45:00'
    >>> "counter1 is: {:6d} time is: {:%Y-%m-%d %H:%M:%S.%f}".format(counter1, x)
    'counter 1 is:  23360 time is: 2013-02-05 20:45:00.000000'

str is a generic function that is a convenience.  A format specification is, well, *specific* to a particular application.  Note how it is necessary to also use a format for the counter in order to obtain output records with consistent spacing.

Now, as far as documentation goes, __str__ references isoformat, and isoformat says:

    datetime.isoformat(sep='T')

        Return a string representing the date and time in ISO 8601 format,
        YYYY-MM-DDTHH:MM:SS.mmmmmm or, if microsecond is 0,
        YYYY-MM-DDTHH:MM:SS

It is hard to see how the documentation can get clearer than that.  

(As an aside, I *can* see the argument, as was made in the other issue, that the isoformat method should provide a way to control the micorseconds display.  That would be a feature request, but given the history of this issue you should probably raise it on the python-ideas mailing list first in order to get buy-in before opening a new request for enhancement for it.  Not optimal, I know, but sometimes you have to deal with history when suggesting/requesting a change in a project.

Also note that datetime.now() does not have *any* format.  It is a datetime object.  Only when turning it in to a string by some method does formatting come in to play.

I want to thank you for your enthusiasm for making things better.  I hope that my more detailed explanation will help you understand the reasoning behind the design decisions that have been made here.

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resolution:  -> invalid
stage:  -> committed/rejected
status: open -> closed

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17139>
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