How to check if a datetime object is localized with pytz?

Question

I want to store a datetime object with a localized UTC timezone. The method that stores the datetime object can be given a non-localized datetime (naive) object or an object that already has been localized. How do I determine if localization is needed?

Code with missing if condition:

class MyClass:
  def set_date(self, d):
    # what do i check here?
    # if(d.tzinfo):
      self.date = d.astimezone(pytz.utc)
    # else:
      self.date = pytz.utc.localize(d)

Solution 1

How do I determine if localization is needed?

From datetime docs:

• a datetime object d is aware iff:

  d.tzinfo is not None and d.tzinfo.utcoffset(d) is not None
  

• d is naive iff:

  d.tzinfo is None or d.tzinfo.utcoffset(d) is None
  

Though if d is a datetime object representing time in UTC timezone then you could use in both cases:

self.date = d.replace(tzinfo=pytz.utc)

It works regardless d is timezone-aware or naive.

Note: don't use datetime.replace() method with a timezone with a non-fixed utc offset (it is ok to use it with UTC timezone but otherwise you should use tz.localize() method).

Solution 2

if you want to check if a datetime object 'd' is localized, check the d.tzinfo, if it is None, no localization.

Solution 3

Here is a function wrapping up the top answer.

def tz_aware(dt):
    return dt.tzinfo is not None and dt.tzinfo.utcoffset(dt) is not None

Solution 4

Here's a more complete function to convert or coerce a timestamp obj to utc. If it reaches the exception this means the timestamp is not localized. Since it's good practice to always work in UTC within the code, this function is very useful at the entry level from persistence.

def convert_or_coerce_timestamp_to_utc(timeobj):
        out = timeobj
        try:
            out = timeobj.astimezone(pytz.utc) # aware object can be in any timezone
        except (ValueError,TypeError) as exc: # naive
            out = timeobj.replace(tzinfo=pytz.utc)
        return out

The small addition from the 'try catch' in the answer by J.F. Sebastian is the additional catch condition, without which not all naive cases will be caught by the function.

View more solutions

Comments

[chiborg]

But what if it has a tzinfo object that is not from pytz?

[jfs]

note: it is not enough to check if d.tzinfo is not None. Also, d.tzinfo.utcoffset(d) should not be None for d to be called an aware datetime object.

[Geekfish]

I know that's what the docs say, but in what scenario can you have a tzinfo object that is not really specifying a timezone and thus d.tzinfo.utcoffset(d) is None is needed?

[jfs]

@Geekfish: no idea. All sane implementations should either return non-None value or raise an exception.

[Daenyth]

Why do you catch TypeError as well? I don't see that mentioned in the other answers.

[eiTan LaVi]

As I wrote, without the additional catch not all naive cases will be caught. This is simply an addition based on direct experience. I had originally worked with the first catch alone, and some naive timestamps got through :)

[Daenyth]

The answer could be improved if you show a specific example of a naive object that throws TypeError here.

[Mitar]

Since Python 3.6 astimezone can be called on naive instances and it assumes system timezone.

[jfs]

@Mitar: yes. It is unfortunate.

[Stephen Rauch]

And return None the rest of the time?

[Gonzalo]

This answer it is not true, here is the definition of when it is naive or aware , from datetime official docs : "An object of type time or datetime may be naive or aware. A datetime object d is aware if d.tzinfo is not None and d.tzinfo.utcoffset(d) does not return None. If d.tzinfo is None, or if d.tzinfo is not None but d.tzinfo.utcoffset(d) returns None, d is naive. A time object t is aware if t.tzinfo is not None and t.tzinfo.utcoffset(None) does not return None. Otherwise, t is naive."

[gimboland]

It's sufficient to just say return dt.tzinfo is not None and dt.tzinfo.utcoffset(dt) is not None

[pablete]

Why is this not a method of datetime.datetime and datetime.time? why?!

[jfs]

@pablete: It is a valid question. If you feel strongly about it, you could submit a patch such as: def is_naive(self): return self.tzinfo is None or self.tzinfo.utcoffset(self) is None devguide.python.org

[pablete]

is there a discussion about that? that you are aware of? I would like to understand why such a method is not implemented in a battle tested module as datetime

[Nikolay Prokopyev]

As far it has been already mentioned - Since Python 3.6 astimezone can be called on naive instances and it assumes system timezone. So your implementation is dangerous without specific checks.

[ryanjdillon]

When calling utcoffset from a datetime instance, no parameters are passed, so your example should rather be: d.tzinfo is not None and d.tzinfo.utcoffset() is not None, same for the naive example.

[JKillian]

@ryanjdillon: The utcoffset method on tzinfo (as opposed to on a datetime) requires a datetime argument and so the code in the answer is correct.

[ryanjdillon]

I missed that. Thanks @JKillian!