Just looking at the attribute '.first' does not tell you what it is intended to mean. '.is_dst' of one spelling or another is unambiguous; "This datetime object is in the 'special' time." '.first' also makes the default 'True', and it might just be me, but I don't like that aesthetically.
 Arguments over if DST is the new standard time or not in some parts of the world not withstanding
On 7/30/2015 12:43, Alexander Belopolsky wrote:
I would like to make a specific proposal on a narrow topic of how to handle ambiguities inherent in representing time instances in common time zones. The following is a rough sketch where I attempted to identify the areas affected by the proposal and outline the proposed changes.
======= Rationale =======
In the most world locations there have been and will be times when local clocks are moved back. In those times intervals are introduced in which local clocks show the same time twice in the same day. In these situations, the information displayed on a local clock (or stored in a Python datetime instance) is insufficient to identify a particular instance in time. The proposed solution is to add a boolean flag to the datetime instances that will distinguish between the two ambiguous times.
======= Proposal =======
The "first" flag
We propose adding a boolean member called "first" to the instances of datetime.time and datetime.time classes. This member will have the value True for all instances except those that represent the second (chronologically) moment in time in an ambiguous case.
Instances of datetime.time and datetime.datetime will get a new boolean attribute called "first."
The __new__ methods of the datetime.time and datetime.datetime classes will get a new keyword-only argument first=True that will control the value of the "first" attribute in the returned instance.
The replace() methods methods of the datetime.time and datetime.datetime classes will get a new keyword-only argument first=True that will control the value of the "first" attribute in the returned instance.
Implementations of tzinfo .......................................
Subclasses of datetime.tzinfo will read the values of "first" in utcoffset() and dst() methods and set it appropriately in the instances returned by the fromutc() method. No change to the signatures of these methods is proposed.
Pickle sizes for the datetime.datetime and datetime.time objects will not change. The "first" flag will be encoded in the first bit of the byte that currently stores the 4th byte of the datetime.datetime pickle payload or the 1st byte of the datetime.time. In the current implementation  these bytes are used to store hour value (0-23) and the first bit is always 0. Note that first=True will be encoded as 0 in the first bit and first=False as 1. (This change only affects pickle format. In C implementation, the "first" member will get a full byte to store the actual boolean value.)
The value of "first" will be ignored in all operations except utcoffset() and dst() methods of tzinfo implementations and __eq__ and __hash__ methods of the datetime.datetime and datetime.time classes. The only methods that will be able to produce nonzero values of "first" are __new__ and replace() methods of the datetime.datetime and datetime.time classes and fromutc() method of some tzinfo implementations.
Backward and forward compatibility
This proposal will have no effect on the programs that do not set the "first" flag explicitly or use tzinfo implementations that do. Pickles produced by older programs will remain fully forward compatible. Only datetime/time instances with first=False pickled in the new versions will become unreadable by the older Python versions. Pickles of instances with first=False will remain unchanged.
================== Questions and Answers ==================
- Why not call the new flag "isdst"?
Alice: Bob - let's have a stargazing party at 01:30 AM tomorrow! Bob: Should I presume initially that summer time (for example, Daylight Saving Time) is or is not (respectively) in effect for the specified time? Allice: Huh?
Bob: Alice - let's have a stargazing party at 01:30 AM tomorrow! Alice: You know, Bob, 01:30 AM will happen twice tomorrow. Which one do you have in mind? Bob: I did not think about it, but let's pick the first.
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