[ python-Bugs-1289118 ] timedelta multiply and divide by floating point

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Bugs item #1289118, was opened at 2005-09-12 17:41
Message generated for change (Comment added) made by tim_one
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Category: Python Library
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Daniel Stutzbach (agthorr)
>Assigned to: Nobody/Anonymous (nobody)
Summary: timedelta multiply and divide by floating point

Initial Comment:
In python 2.4.1, the datetime.timedelta type allows for
the multiplication and division by integers.  However,
it raises a TypeError for multiplication or division by
floating point numbers.  This is a counterintutive
restriction and I can't think of any good reason for it.

For example:

>>> import datetime
>>> datetime.timedelta(minutes=5)/2
datetime.timedelta(0, 150)
>>> datetime.timedelta(minutes=5)*0.5
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for *:
'datetime.timedelta' and 'float'


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>Comment By: Tim Peters (tim_one)
Date: 2005-09-15 17:04

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timedelta arithmetic is 100% portable now, and wholly 
explainable in terms of universally understood integer 
arithmetic.  Throw floats into it, and that's lost.

That said, I don't have a strong objection to complicating the 
implementation if there _are_ strong use cases.  The OP's 
example isn't "a use case":  it's not worth anything to let 
someone multiply a timedelta by 0.5 instead of dividing by 2.  
I don't have a use case to offer in its place (never felt a need 
here).

If someone wants to work on it, note that a timedelta can 
contain more than 53 bits of information, so, e.g., trying to 
represent a timedelta as an IEEE double-precision number of 
microseconds can lose information.  This makes a high-
qualty "computed as if to infinite precision with one rounding 
at the end" implementation of mixed datetime/float arithmetic 
tricky to do right.

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Comment By: Michael Chermside (mcherm)
Date: 2005-09-15 12:03

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I, too, would like to know what Tim thinks, but for what it's 
worth (not much) I find Daniel's point fairly convincing... 
multiplication by floats is an operation that makes sense, has 
only one possible obvious meaning, and is not particularly 
likely to cause errors (the way multiplying Decimal's with 
floats does). So IF it's easy to implement, I say go for it.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2005-09-13 00:11

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Tim, do you prefer the current behavior?

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