Time object subclassing datetime allowing diverting local clock mecanism
This packages provides a time abstraction mecanism, allowing code that would use it as reference to be diverted both on the local time zone and the real time.
This allows clock-dependent code to be tested. See Getting started section for a more thorough explanation.
Additionnaly, as an abstraction of the legacy datetime object, Time object provided in sact.epoch.Time provides some common helpers and force this object to always provide a timezone.
sact.epoch objects will be of some help:
- if your application manage 2+ different timezones (ie: UTC and local timezone), and you are tired of legacy “naive” datetime object on this matter.
- if you want to be able to divert system calls to local time, or local time zone of your application for test purpose.
subclass of datetime, represent an absolute instant of time (with timezone). It forces datetime usage to be aware of the timezone. Additionally, Time.now() will ask the Zope Component Architecture registry for a Clock Utility to provide the real time.
Clock objects are general reference for getting Time object. The default clock is our common normal clock, but ZCA allows to substiture the reference and provide other type of Clock as sact.epoch.ManageableClock which can be managed (this means it can be stopped, set, translated in the future or the past).
TimeZone objects represents a specific timezone, sact.epoch.TzLocal() will get the local system time zone. This call can be diverted also via the ZCA to provide another TimeZone. sact.epoch.testTimeZone is a common divert target timezone that is provided to help testing code.
sact.epoch.Time is meant as a replacement of datetime.datetime. As a subclass of this later one, it’ll provide the same functionality, and thus can be used almost anywhere you are using datetime.datetime.
Additionaly, using sact.epoch.Time ensures that:
- all instances will get a time zone. Which isn’t the case for datetime objects.
- .now() method will use the Zope Component Architecture registry to get a common Utility that is in charge of giving the real time. This allows simple overriding mecanism.
Get timestamp from a datetime
Let’s say you have some code using datetime.datetime.now():
>>> import datetime >>> now = datetime.datetime.now()
First issue: if this variable is meant to represent a time stored as UTC in a database.
How do you get UTC timestamp from datetime ? (cf: http://bugs.python.org/issue1457227)
This is a common question. (cf: http://stackoverflow.com/questions/5067218/get-utc-timestamp-in-python-with-datetime/5499906#5499906)
The answer is datetime.datetime objects can be naive, which means unaware of the timezone. Thus, it is impossible to get UTC timestamp from this form of datetime unless you can guess the timezone yourself.
Hopefully, the timezone of your system didn’t change between the datetime object creation and the moment you want to get a timestamp, if this is the case you can safely use:
>>> import sact.epoch.utils >>> utc_timestamp = sact.epoch.utils.dt2ts(now)
dt2ts will ask your system for the number of seconds between EPOCH in the current timezone and the provided datetime. This is why you must ensure that datetime object was created when the same TimeZone on the system than when you run this function.
No doctest is given to your eyes on the content of the variable utc_timestamp because the output depends on the current time. And this can be often an issue you’ll encounter: having complex code that depends on the current date, how do you test it ? This is the purpose of sact.epoch.Time.
Forcing to use only aware datetime
Quite quickly, you’ll ask yourself: but what use have I of naive datetime.datetime objects if they can’t be used in lots of cases ?
The answer is: there are no use of naive datetime.datetime.
Aware datetime objects as sact.epoch.Time contains all additional information allowing to:
- get an UTC timestamp
- compare two Time object whatever there timezone is.
Using naive datetime might even be concidered harmfull. sact.epoch.Time will ensure that all your objects are timezone aware. By default, TimeZone won’t even be dependent of your system local time, but will be stored in UTC timezone.
>>> from __future__ import print_function >>> print(repr(datetime.datetime.now().tzinfo)) None >>> print(repr(datetime.datetime(1970, 1, 1, 1, 1).tzinfo)) None
In comparison, sact.epoch.Time object will always set a timezone:
>>> print(repr(sact.epoch.Time.now().tzinfo)) <TimeZone: UTC> >>> print(repr(sact.epoch.Time(1970, 1, 1, 1, 1).tzinfo)) <TimeZone: UTC>
Of course, as Time object is aware, a simple timestamp property is available:
>>> epoch = sact.epoch.Time(1970, 1, 1, 0, 0) >>> epoch.timestamp 0
If you use sact.epoch.Time.now() in place of datetime.datetime.now(), your code will have seams to divert real time reference without touching the system clock.
Say your code is:
>>> db_timestamp = epoch.timestamp >>> def is_it_ok(): ... now = sact.epoch.Time.now().timestamp ... print(0 == ((now - db_timestamp) % 2))
is_it_ok function code should print True if number of seconds between now and epoch is odd.
This is the type of function which is quite difficult to test if you are using datetime.datetime.now(). Whole application will make extensive usage of the system clock, and will eventually be difficult to test unless you used sact.epoch.Time.now() in place of datetime.
Here’s the test of the function:
>>> clock = sact.epoch.clock.ManageableClock()
By default, the clock is following the system clock. Let’s stop it and set it to epoch (more on manageable clock in the docstring of the class ManageableClock):
>>> clock.stop() >>> clock.ts = 0
Now let’s use ZCA to declare this clock as new reference clock:
>>> from zope.component import globalSiteManager as gsm >>> gsm.registerUtility(clock)
We are ready to test the function:
>>> sact.epoch.Time.now().timestamp 0 >>> is_it_ok() True >>> clock.ts = 1 >>> sact.epoch.Time.now().timestamp 1 >>> is_it_ok() False
Please note that ManageableClock have a wait method:
>>> clock.wait(minutes=1) >>> sact.epoch.Time.now().timestamp 61 >>> is_it_ok() False
Of course, the execution of clock.wait is immediate. You can use a datetime.timedelta as argument of wait or any keyword args you would send to datetime.timedelta constructor (this includes days, seconds, microseconds, milliseconds, minutes, hours, weeks as of python version 2.7.1, cf: http://docs.python.org/library/datetime.html#datetime.timedelta)
Diverting timezone of system
When displaying times to the user, it is appreciated to show the time in local timezone:
>>> def what_time_is_it(): ... print(sact.epoch.Time.now().local.iso)
Notice the use of the property local which returns a new Time instance set to the same moment in time but in the system local timezone, and the iso property which returns the iso string representation of the Time object.
The local property uses sact.epoch.TzLocal() which is responsible of giving the system local timezone:
>>> sact.epoch.TzLocal() <TimeZone: System>
Let use the ZCA to divert the TzLocal mechanism to get the system local:
>>> from sact.epoch import testTimeZone >>> from sact.epoch.interfaces import ITimeZone >>> gsm.registerUtility(testTimeZone, ITimeZone, name='local')
Now we can test our function:
>>> clock.ts = 0 >>> what_time_is_it() 1970-01-01 00:05:00+00:05
The testTimeZone used is very special and recognizable on purpose: it has a constant +5 minute offset on UTC.
Internally, call to TzLocal() has been diverted:
>>> sact.epoch.TzLocal() <TimeZone: Test>