# Mocking dates and times

Testing code that involves dates and times or which has behaviour dependent on the date or time it is executed at has historically been tricky. Mocking lets you perform tests on this type of code and TestFixtures provides three specialised mock objects to help with this.

## Dates

TestFixtures provides the `test_date()` function that returns a subclass of `datetime.date` with a `today()` method that will return a consistent sequence of dates each time it is called.

This enables you to write tests for code such as the following, from the `testfixtures.tests.sample1` package:

    from datetime import datetime, date

    def str_today_1():
        return str(date.today())

`Replace` can be used to apply the mock as shown in the following example, which could appear in either a unit test or a doc test:

    >>> from testfixtures import Replace, test_date
    >>> from testfixtures.tests.sample1 import str_today_1
    >>> with Replace('testfixtures.tests.sample1.date', test_date()):
    ...    str_today_1()
    ...    str_today_1()
    '2001-01-01'
    '2001-01-02'

If you need a specific date to be returned, you can specify it:

    >>> with Replace('testfixtures.tests.sample1.date', test_date(1978,6,13)):
    ...    str_today_1()
    '1978-06-13'

If you need to test with a whole sequence of specific dates, this can be done as follows:

    >>> with Replace('testfixtures.tests.sample1.date', test_date(None)) as d:
    ...    d.add(1978,6,13)
    ...    d.add(2009,11,12)
    ...    str_today_1()
    ...    str_today_1()
    '1978-06-13'
    '2009-11-12'

Another way to test with a specific sequence of dates is to use the `delta_type` and `delta` parameters to `test_date()`. These parameters control the type and size, respectively, of the difference between each date returned.

For example, where 2 days elapse between each returned value:

    >>> with Replace('testfixtures.tests.sample1.date',
    ...              test_date(1978, 6, 13, delta=2, delta_type='days')) as d:
    ...    str_today_1()
    ...    str_today_1()
    ...    str_today_1()
    '1978-06-13'
    '1978-06-15'
    '1978-06-17'

The `delta_type` can be any keyword parameter accepted by the `timedelta` constructor. Specifying a `delta` of zero can be an effective way of ensuring that all calls to the `today()` method return the same value:

    >>> with Replace('testfixtures.tests.sample1.date',
    ...              test_date(1978, 6, 13, delta=0)) as d:
    ...    str_today_1()
    ...    str_today_1()
    ...    str_today_1()
    '1978-06-13'
    '1978-06-13'
    '1978-06-13'

When using `test_date()`, you can, at any time, set the next date to be returned using the `set()` method. The date returned after this will be the set date plus the `delta` in effect:

    >>> with Replace('testfixtures.tests.sample1.date', test_date(delta=2)) as d:
    ...    str_today_1()
    ...    d.set(1978,8,1)
    ...    str_today_1()
    ...    str_today_1()
    '2001-01-01'
    '1978-08-01'
    '1978-08-03'

## Datetimes

TextFixtures provides the `test_datetime()` function that returns a subclass of `datetime.datetime` with a `now()` method that will return a consistent sequence of `datetime` objects each time it is called.

This enables you to write tests for code such as the following, from the `testfixtures.tests.sample1` package:

    from datetime import datetime, date

    def str_now_1():
        return str(datetime.now())

We use a `Replace` as follows, which could appear in either a unit test or a doc test:

    >>> from testfixtures import Replace, test_datetime
    >>> from testfixtures.tests.sample1 import str_now_1
    >>> with Replace('testfixtures.tests.sample1.datetime', test_datetime()):
    ...    str_now_1()
    ...    str_now_1()
    '2001-01-01 00:00:00'
    '2001-01-01 00:00:10'

If you need a specific datetime to be returned, you can specify it:

    >>> with Replace('testfixtures.tests.sample1.datetime',
    ...              test_datetime(1978,6,13,1,2,3)):
    ...    str_now_1()
    '1978-06-13 01:02:03'

If you need to test with a whole sequence of specific datetimes, this can be done as follows:

    >>> with Replace('testfixtures.tests.sample1.datetime',
    ...              test_datetime(None)) as d:
    ...    d.add(1978,6,13,16,0,1)
    ...    d.add(2009,11,12,11,41,20)
    ...    str_now_1()
    ...    str_now_1()
    '1978-06-13 16:00:01'
    '2009-11-12 11:41:20'

Another way to test with a specific sequence of datetimes is to use the `delta_type` and `delta` parameters to `test_datetime()`. These parameters control the type and size, respectively, of the difference between each datetime returned.

For example, where 2 hours elapse between each returned value:

    >>> with Replace(
    ...    'testfixtures.tests.sample1.datetime',
    ...    test_datetime(1978, 6, 13, 16, 0, 1, delta=2, delta_type='hours')
    ... ) as d:
    ...    str_now_1()
    ...    str_now_1()
    ...    str_now_1()
    '1978-06-13 16:00:01'
    '1978-06-13 18:00:01'
    '1978-06-13 20:00:01'

The `delta_type` can be any keyword parameter accepted by the `timedelta` constructor. Specifying a `delta` of zero can be an effective way of ensuring that all calls to the `now()` method return the same value:

    >>> with Replace('testfixtures.tests.sample1.datetime',
    ...              test_datetime(1978, 6, 13, 16, 0, 1, delta=0)) as d:
    ...    str_now_1()
    ...    str_now_1()
    ...    str_now_1()
    '1978-06-13 16:00:01'
    '1978-06-13 16:00:01'
    '1978-06-13 16:00:01'

When using `test_datetime()`, you can, at any time, set the next datetime to be returned using the `set()` method. The value returned after this will be the set value plus the `delta` in effect:

    >>> with Replace('testfixtures.tests.sample1.datetime',
    ...              test_datetime(delta=2)) as d:
    ...    str_now_1()
    ...    d.set(1978,8,1)
    ...    str_now_1()
    ...    str_now_1()
    '2001-01-01 00:00:00'
    '1978-08-01 00:00:00'
    '1978-08-01 00:00:02'

### Timezones

In many situations where you’re mocking out `now()` or `utcnow()` you’re not concerned about timezones, especially given that both methods will usually return `datetime` objects that have a tzinfo of `None`. However, in some applications it is important that `now()` and `utcnow()` return different times, as they would normally if the application is run anywhere other than the UTC timezone.

The best way to understand how to use `test_datetime()` in these situations is to think of the internal queue as being a queue of `datetime` objects at the current local time with a tzinfo of None, much as would be returned by `now()`. If you pass in a tz parameter to `now()` it will be applied to the value before it is returned in the same way as it would by `datetime.datetime.now()`.

If you pass in a tzinfo to `test_datetime()`, this will be taken to indicate the timezone you intend for the local times that `now()` simulates. As such, that timezone will be used to compute values returned from `utcnow()` such that they would be `test_datetime` objects in the UTC timezone with the tzinfo set to `None`, as would be the case for a normal call to `datetime.datetime.utcnow()`.

For example, lets take a timezone as defined by the following class:

    from datetime import tzinfo, timedelta

    class ATZInfo(tzinfo):

       def tzname(self, dt):
            return 'A TimeZone'

       def utcoffset(self, dt):
            # In general, this timezone is 5 hours behind UTC
            offset  = timedelta(hours=-5)
            return offset+self.dst(dt)

       def dst(self, dt):
            # However, between March and September, it is only
            # 4 hours behind UTC
            if 3 < dt.month < 9:
                return timedelta(hours=1)
            return timedelta()

If we create a `test_datetime` with this timezone and a delta of zero, so we can see affect of the timezone over multiple calls, the values returned by `now()` will be affected:

    >>> datetime = test_datetime(2001, 1, 1, delta=0, tzinfo=ATZInfo())

A normal call to `now()` will return the values passed to the constructor:

    >>> print(datetime.now())
    2001-01-01 00:00:00

If we now ask for this time but in the timezone we passed to `test_datetime`, we will get the same hours, minutes and seconds but with a `tzinfo` attribute set:

    >>> print(datetime.now(ATZInfo()))
    2001-01-01 00:00:00-05:00

If we call `utcnow()`, we will get the time equivalent to the values passed to the constructor, but in the UTC timezone:

    >>> print(datetime.utcnow())
    2001-01-01 05:00:00

The timezone passed in when the `test_datetime` is created has a similar effect on any items set:

    >>> datetime.set(2011,5,1,10)
    >>> print(datetime.now())
    2011-05-01 10:00:00
    >>> print(datetime.utcnow())
    2011-05-01 14:00:00

Likewise, `add()` behaves the same way:

    >>> datetime = test_datetime(None, delta=0, tzinfo=ATZInfo())
    >>> datetime.add(2011,1,1,10)
    >>> datetime.add(2011,5,1,10)
    >>> datetime.add(2011,10,1,10)
    >>> print(datetime.now())
    2011-01-01 10:00:00
    >>> print(datetime.utcnow())
    2011-05-01 14:00:00
    >>> print(datetime.now())
    2011-10-01 10:00:00

## Times

TextFixtures provides the `test_time()` function that, when called, returns a replacement for the `time.time()` function.

This enables you to write tests for code such as the following, from the `testfixtures.tests.sample1` package:

    from time import time

    def str_time():
        return str(time())

We use a `Replace` as follows, which could appear in either a unit test or a doc test:

    >>> from testfixtures import Replace, test_time
    >>> from testfixtures.tests.sample1 import str_time
    >>> with Replace('testfixtures.tests.sample1.time', test_time()):
    ...    str_time()
    ...    str_time()
    '978307200.0'
    '978307201.0'

If you need an integer representing a specific time to be returned, you can specify it:

    >>> with Replace('testfixtures.tests.sample1.time',
    ...              test_time(1978, 6, 13, 1, 2, 3)):
    ...    str_time()
    '266547723.0'

If you need to test with a whole sequence of specific timestamps, this can be done as follows:

    >>> with Replace('testfixtures.tests.sample1.time', test_time(None)) as t:
    ...    t.add(1978,6,13,16,0,1)
    ...    t.add(2009,11,12,11,41,20)
    ...    str_time()
    ...    str_time()
    '266601601.0'
    '1258026080.0'

Another way to test with a specific sequence of timestamps is to use the `delta_type` and `delta` parameters to `test_time()`. These parameters control the type and size, respectively, of the difference between each timestamp returned.

For example, where 2 hours elapse between each returned value:

    >>> with Replace(
    ...     'testfixtures.tests.sample1.time',
    ...     test_time(1978, 6, 13, 16, 0, 1, delta=2, delta_type='hours')
    ... ) as d:
    ...    str_time()
    ...    str_time()
    ...    str_time()
    '266601601.0'
    '266608801.0'
    '266616001.0'

The `delta_type` can be any keyword parameter accepted by the `timedelta` constructor. Specifying a `delta` of zero can be an effective way of ensuring that all calls to the `time()` function return the same value:

    >>> with Replace('testfixtures.tests.sample1.time',
    ...              test_time(1978, 6, 13, 16, 0, 1, delta=0)) as d:
    ...    str_time()
    ...    str_time()
    ...    str_time()
    '266601601.0'
    '266601601.0'
    '266601601.0'

When using `test_time()`, you can, at any time, set the next timestamp to be returned using the `set()` method. The value returned after this will be the set value plus the `delta` in effect:

    >>> with Replace('testfixtures.tests.sample1.time', test_time(delta=2)) as d:
    ...    str_time()
    ...    d.set(1978,8,1)
    ...    str_time()
    ...    str_time()
    '978307200.0'
    '270777600.0'
    '270777602.0'

## Gotchas with dates and times

Using these specialised mock objects can have some intricacies as described below:

### Local references to functions

There are situations where people may have obtained a local reference to the `today()` or `now()` methods, such as the following code from the `testfixtures.tests.sample1` package:

    from datetime import datetime, date

    now = datetime.now

    def str_now_2():
        return str(now())
    today = date.today

    def str_today_2():
        return str(today())

In these cases, you need to be careful with the replacement:

    >>> from testfixtures import Replacer, test_datetime
    >>> from testfixtures.tests.sample1 import str_now_2, str_today_2
    >>> with Replacer() as replace:
    ...    today = replace('testfixtures.tests.sample1.today', test_date().today)
    ...    now = replace('testfixtures.tests.sample1.now', test_datetime().now)
    ...    str_today_2()
    ...    str_now_2()
    '2001-01-01'
    '2001-01-01 00:00:00'

### Use with code that checks class types

When using the above specialist mocks, you may find code that checks the type of parameters passed may get confused. This is because, by default, `test_datetime` and `test_date` return instances of the real `datetime` and `date` classes:

    >>> from testfixtures import test_datetime
    >>> from datetime import datetime
    >>> tdatetime = test_datetime()
    >>> issubclass(tdatetime, datetime)
    True
    >>> tdatetime.now().__class__
    <...'datetime.datetime'>

The above behaviour, however, is generally what you want as other code in your application and, more importantly, in other code such as database adapters, may handle instances of the real `datetime` and `date` classes, but not instances of the `test_datetime` and `test_date` mocks.

That said, this behaviour can cause problems if you check the type of an instance against one of the mock classes. Most people might expect the following to return `True`:

    >>> isinstance(tdatetime(2011, 1, 1), tdatetime)
    False
    >>> isinstance(tdatetime.now(), tdatetime)
    False

If this causes a problem for you, then both `datetime` and `date` take a strict keyword parameter that can be used as follows:

    >>> tdatetime = test_datetime(strict=True)
    >>> tdatetime.now().__class__
    <class 'testfixtures.tdatetime.tdatetime'>
    >>> isinstance(tdatetime.now(), tdatetime)
    True

You will need to take care that you have replaced occurrences of the class where type checking is done with the correct `test_datetime` or `test_date`. Also, be aware that the `date()` method of `test_datetime` instances will still return a normal `date` instance. If type checking related to this is causing problems, the type the `date()` method returns can be controlled as shown in the following example:

    from testfixtures import test_date, test_datetime

    date_type = test_date(strict=True)
    datetime_type = test_datetime(strict=True, date_type=date_type)

With things set up like this, the `date()` method will return an instance of the `date_type` mock:

    >>> somewhen = datetime_type.now()
    >>> somewhen.date()
    tdate(2001, 1, 1)
    >>> _.__class__ is date_type
    True