[Tutor] modify values for object derived from datetime.datetime

[rail shafigulin]

On Fri, Dec 16, 2011 at 5:26 AM, Steven D'Aprano <steve at pearwood.info>wrote:

> rail shafigulin wrote:
>
>> i writing some code to do device testing at my work. testing is related to
>> date and time, so naturally i decided to create a class that inherits from
>> datetime.datetime. main reason is that i need to add, subtract and compare
>> datetime objects and datetime.datetime allows me to do that. here is the
>> code:
>>
>> class LTime(datetime.datetime):
>>  TOLERANCE = 10
>>
>>  def __new__(self, year, month, day, *args):
>>    if year == 0:
>>      year = 2000
>>    return super().__new__(self, year, month, day, *args)
>>
>
>
> By convention, the first argument of __new__ is normally spelled as "cls",
> short for class, because the instance hasn't been created yet.
>
>
>
>   def modify(self):
>>    self = self.replace(2012, 12, 12)
>>    print(self)
>>
>
> The replace method creates a new datetime object. Just because you assign
> it to the name "self" doesn't mean you can change the existing datetime
> object. That is simply impossible: datetime objects are immutable, like
> ints.
>
> You might not quite understand why modifying immutable objects would be
> bad (which is why Python doesn't allow it). I can simulate the effect with
> this simple wrapper class:
>
> >>> class Mutable:
> ...     def __init__(self, value):
> ...             self.value = value
> ...     def __str__(self):
> ...             return str(self.value)
> ...     __repr__ = __str__
> ...     def add(self, value):
> ...             self.value += value
> ...
> >>> one = Mutable(1)  # Pretend this is the int 1
> >>> print(one)
> 1
> >>> x = one
> >>> x.set(1)  # pretend this was x += 1
> >>> one  # the int 1 has been modified in place
> 2
>
> So if ints (and datetime objects) could be changed in place, you could
> never be sure what value a literal like 1 would have.
>
> Obviously this is useful in some situations, which is why we have mutable
> objects like lists. But datetime objects are not mutable, a design choice
> made by the creator of the module, so you cannot change it.
>
> So you have to change your design. Instead of writing code like this:
>
>
> today = LTime(2011, 12, 16)
> # ...
> # ... do stuff with today
> # ...
> today.modify(day=17)  # fast forward in time to tomorrow
> # ...
> # ... do stuff with today, which is actually tomorrow
> # ...
>
> you need to change your code to be more like this:
>
>
> today = LTime(2011, 12, 16)
> # ...
> # ... do stuff with today
> # ...
> tomorrow = today.modify(day=17)
> # ...
> # ... do stuff with tomorrow
> # ...
>
>
>
>
> --
> Steven
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Thanks for the explanation.
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