[Python-Dev] dateutil (original) (raw)

Moore, Paul Paul.Moore at atosorigin.com
Mon Mar 15 08:49:13 EST 2004

From: Greg Ewing

This makes operations like "move to the end of the current month" non-trivial (not hard, just non-trivial). If I were designing something like this, I think I would approach it quite differently, and provide a bunch of separate functions or methods that transform dates in specific ways. e.g. endofmonth(d) forwardmonths(n, d) then to "go to the end of the month 3 months from now" would be endofmonth(forwardmonths(3, d)) or, if you prefer a method-chaining style, d.forwardmonths(3).endofmonth()

In dateutil, I think it's d + relativedelta(months=+1, day=1, days=-1). Arguably, this is fairly trivial (highly non-obvious, but trivial...)

It's a trade-off, power at the cost of comprehensibility. I've always been a fan of flexible, powerful tools (and I'm sure the readability of my code suffers for it :-)). So I prefer relativedelta.

Some use cases that I find hard with "bare" datetime (all are real requirements for me):

  1. For a date d, get the start and end of the month containing it. (many "report this month" jobs)
  2. For a date d, get the start and end of the month before it. (equivalent "report last month" jobs). This is just (1) plus the need to subtract "roughly" a month from a date.
  3. 12 noon on the first Thursday of next month. (Don't blame me, that's when we have a maintenance slot on one of our systems).
  4. The last Friday of the month. (End-of-month reporting time).

The key functionality here is "add N months" and "go to weekday N (forward or backward)". Both things that the core datetime avoids for well-explained reasons.

I'm happy with dateutil as it stands, outside of the core. If it's to go into the core, I agree that a PEP would be the right thing to do. And probably not just one - separate PEPS for the different areas would focus discussion better:

  1. Expanded datetime "delta" functionality
  2. Recurrence rules
  3. Flexible date string parser
  4. Special date calculations (Easter, any others?)

Paul.

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