anybox.testing.datetime
=======================

This package allows to **cheat with current time** in tests.
It has initially been used in OpenERP to test workflows spanning over a long period of time.

Currently it mainly provides a ``datetime.set_now()`` method to fake the current time.

Usage
~~~~~

Before anything, the package must be imported in order to replace the
regular ``datetime`` module with the modified one::

  >>> import anybox.testing.datetime
  >>> from datetime import datetime
  >>> import time

Let's keep the real value of ``now`` around::

  >>> start = datetime.now()
  >>> start_t = time.time()

Then you can change the current time::

  >>> datetime.set_now(datetime(2001, 01, 01, 3, 57, 0))
  >>> datetime.now()
  datetime(2001, 1, 1, 3, 57)
  >>> datetime.today()
  datetime(2001, 1, 1, 3, 57)

The time module goes along::

  >>> datetime.fromtimestamp(time.time())
  datetime(2001, 1, 1, 3, 57)

Note that you can expect a few microseconds difference (not displayed
here because ``datetime.fromtimestamp`` ignores them).


Don't forget afterwards to get back to the regular system clock, otherwise
many pieces of code might get very suprised if the system clock looks as if 
it's frozen::

  >>> datetime.real_now()

Now let's check it worked::

  >>> now = datetime.now()
  >>> now > start
  True
  >>> from datetime import timedelta
  >>> now - start < timedelta(0, 0, 10000) # 10 ms
  True

And with the ``time`` module::

  >>> now_t = time.time()
  >>> now_t > start_t
  True
  >>> now_t - start_t < 0.01 # 10 ms again
  True

Other constructors are still available (this is a non regression
test)::

  >>> import datetime
  >>> datetime.time(3, 57, 0)
  datetime.time(3, 57)
  >>> datetime.datetime(2013, 1, 1, 3, 57, 0)
  datetime(2013, 1, 1, 3, 57)
  >>> datetime.date(2013, 1, 1)
  datetime.date(2013, 1, 1)

Behind the hood
~~~~~~~~~~~~~~~

Our replacement class is the one loaded from the ``datetime`` module,
but instances of the original ``datetime`` class behave exactly as
instances of our ``datetime.datetime``. This is needed because most
computational methods, actually return an object of the original
``datetime`` class. This works with python >= 2.6 only.

First let's check that our class is a subclass of the original
one. If this fails, this test does not mean anything anymore::

  >>> datetime.datetime is datetime.original_datetime
  False
  >>> issubclass(datetime.datetime, datetime.original_datetime)
  True

Then let's demonstrate the behaviour::

  >>> odt = datetime.original_datetime(2012, 1, 1)
  >>> isinstance(odt, datetime.datetime)
  True
  >>> issubclass(datetime.original_datetime, datetime.datetime)
  True

We'll need a ``tzinfo`` subclass from now on.

  >>> from datetime import tzinfo
  >>> class mytzinfo(tzinfo):
  ...     def utcoffset(self, dt):
  ...         return timedelta(hours=2)
  ...     def dst(self, dt):
  ...         return timedelta(0)

Compatibility
~~~~~~~~~~~~~

Over the lifespan of this development toolkit module, we've had to ensure
compatibility with several subsystems

SQLite
------

Also, ``sqlite3`` does recognize our ``datetime`` and ``date`` classes as
if they were the original ones::

  >>> import sqlite3
  >>> cnx = sqlite3.connect(':memory:')
  >>> cr = cnx.cursor()
  >>> cr = cr.execute("CREATE TABLE dates (dt text, d text)")
  >>> dt = datetime.datetime(2013, 1, 25, 12, 34, 0)
  >>> d = datetime.date(2013, 4, 7)
  >>> cr = cr.execute("INSERT INTO dates VALUES (?, ?)", (dt, d))
  >>> cr = cr.execute("SELECT dt, d from dates")
  >>> cr.fetchall()
  [(u'2013-01-25 12:34:00', u'2013-04-07')]

Now let's try this again with the original ones::

  >>> dt = datetime.datetime.now()
  >>> isinstance(dt, datetime.original_datetime)
  True
  >>> d = datetime.date.today()
  >>> cr = cr.execute("INSERT INTO dates VALUES (?, ?)", (dt, d))
  >>> cr = cr.execute("SELECT dt, d from dates")
  >>> res = cr.fetchall() # can't check the value, it changes a lot !

Test
~~~~

This README is also a doctest. To it and other doctests for this package,
simply do::

  nosetests --with-doctest --doctest-extension=txt

