[Python-ideas] strptime without second argument as an inverse to __str__

[Wolfgang Maier]

On 05.08.2014 03:39, Steven D'Aprano wrote:
>
> Since str(d) is documented to use a well-defined format, then I agree
> that it makes sense to make the second argument to d.strptime optional,
> and default to that same format. The concern I had was the sort of
> scenario Skip suggested: I might write out a datetime object as a string
> on one machine, where the format is X, and read it back elsewhere, where
> the format is Y, leading to at best an exception and at worse incorrect
> data.
>
> +1 on the suggestion.
>

After looking a bit into the code of the datetime module, I am not 
convinced anymore that strptime() is the right place for the 
functionality for the following reasons:

1) strptime already has a clear counterpart and that's strftime.

2) strftime/strptime use explicit format strings, not any more 
sophisticated parsing (as would be required to parse the different 
formats that datetime.__str__ can produce) and they try, intentionally, 
to mimick the behavior of their C equivalents.

In other words, strftime/strptime have a very clear underlying concept, 
which IMO should not be given up just because we are trying to stuff 
some extra-functionality into them.

That said, I still think that the basic idea - being able to 
reverse-parse the output of datetime.__str__ - is right.

I would suggest that a better place for this is an additional 
classmethod constructor (the datetime class already has quite a number 
of them). Maybe fromisostring() could be a suitable name ?
With this you could even pass an extra-argument for the date-time 
separator just like with the current isoformat.
This constructor would then be more like a counterpart to 
datetime.isoformat(), but it could simply be documented that calling it 
with fromisostring(datestring, sep=" ") can be used to parse strings 
written with datetime.str().

-1 on the specifics of the proposal,
+1 on the general idea.