Travel through time in your tests.
A quick example:
import datetime as dt from zoneinfo import ZoneInfo import time_machine hill_valley_tz = ZoneInfo("America/Los_Angeles") @time_machine.travel(dt.datetime(1985, 10, 26, 1, 24, tzinfo=hill_valley_tz)) def test_delorean(): assert dt.date.today().isoformat() == "1985-10-26"
python -m pip install time-machine
Python 3.6 to 3.9 supported. Only CPython is supported at this time because time-machine directly hooks into the C-level API.
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travel(destination, *, tick=True)
travel() is a class that allows time travel, to the datetime specified by
destination. It does so by mocking all functions from Python's standard library that return the current date or datetime. It can be used independently, as a function decorator, or as a context manager.
destination specifies the datetime to move to. It may be:
datetime.datetime. If it is naive, it will be assumed to have the UTC timezone. If it has
tzinfoset to a
zoneinfo.ZoneInfoinstance, the current timezone will also be mocked.
datetime.date. This will be converted to a UTC datetime with the time 00:00:00.
intspecifying a Unix timestamp
- A string, which will be parsed with dateutil.parse and converted to a timestamp.
Additionally, you can provide some more complex types:
- A generator, in which case
next()will be called on it, with the result treated as above.
- A callable, in which case it will be called with no parameters, with the result treated as above.
tick defines whether time continues to "tick" after travelling, or is frozen. If
True, the default, successive calls to mocked functions return values increasing by the elapsed real time since the first call. So after starting travel to
0.0 (the UNIX epoch), the first call to any datetime function will return its representation of
1970-01-01 00:00:00.000000 exactly. The following calls "tick," so if a call was made exactly half a second later, it would return
All datetime functions in the standard library are mocked to move to the destination current datetime:
The mocking is done at the C layer, replacing the function pointers for these built-ins. Therefore, it automatically affects everywhere those functions have been imported, unlike use of
To use independently, create and instance, use
start() to move to the destination time, and
stop() to move back. For example:
import datetime as dt import time_machine traveller = time_machine.travel(dt.datetime(1985, 10, 26)) traveller.start() # It's the past! assert dt.date.today() == dt.date(1985, 10, 26) traveller.stop() # We've gone back to the future! assert dt.date.today() > dt.date(2020, 4, 29)
travel() instances are nestable, but you'll need to be careful when manually managing to call their
stop() methods in the correct order, even when exceptions occur. It's recommended to use the decorator or context manager forms instead, to take advantage of Python features to do this.
When used as a function decorator, time is mocked during the wrapped function's duration:
import time import time_machine @time_machine.travel("1970-01-01 00:00 +0000") def test_in_the_deep_past(): assert 0.0 < time.time() < 1.0
You can also decorate asynchronous functions (coroutines):
import time import time_machine @time_machine.travel("1970-01-01 00:00 +0000") async def test_in_the_deep_past(): assert 0.0 < time.time() < 1.0
Beware: time is a global state - see below.
When used as a context manager, time is mocked during the
import time import time_machine def test_in_the_deep_past(): with time_machine.travel(0.0): assert 0.0 < time.time() < 1.0
unittest.TestCase subclasses are supported. When applied as a class decorator to such classes, time is mocked from the start of
setUpClass() to the end of
import time import time_machine import unittest @time_machine.travel(0.0) class DeepPastTests(TestCase): def test_in_the_deep_past(self): assert 0.0 < time.time() < 1.0
Note this is different to
unittest.mock.patch()'s behaviour, which is to mock only during the test methods.
destination passed to
Coordinates.move_to() has its
tzinfo set to a
zoneinfo.ZoneInfo instance, the current timezone will be mocked. This will be done by calling
time.tzset(), so it is only available on Unix. The
zoneinfo module is new in Python 3.8 - on older Python versions use the
backports.zoneinfo package, by the original
time.tzset() changes the
time module’s timezone constants and features that rely on those, such as
time.localtime(). It won’t affect other concepts of “the current timezone”, such as Django’s (which can be changed with its
Here’s a worked example changing the current timezone:
import datetime as dt import time from zoneinfo import ZoneInfo import time_machine hill_valley_tz = ZoneInfo("America/Los_Angeles") @time_machine.travel(dt.datetime(2015, 10, 21, 16, 29, tzinfo=hill_valley_tz)) def test_hoverboard_era(): assert time.tzname == ("PST", "PDT") now = dt.datetime.now() assert (now.hour, now.minute) == (16, 29)
start() method and entry of the context manager both return a
Coordinates object that corresponds to the given "trip" in time. This has a couple methods that can be used to travel to other times.
move_to() moves the current time to a new destination.
destination may be any of the types supported by
tick may be set to a boolean, to change the
tick flag of
import datetime as dt import time import time_machine with time_machine.travel(0, tick=False) as traveller: assert time.time() == 0 traveller.move_to(234) assert time.time() == 234
shift() takes one argument,
delta, which moves the current time by the given offset.
delta may be a
timedelta or a number of seconds, which will be added to destination. It may be negative, in which case time will move to an earlier point.
import datetime as dt import time import time_machine with time_machine.travel(0, tick=False) as traveller: assert time.time() == 0 traveller.shift(dt.timedelta(seconds=100)) assert time.time() == 100 traveller.shift(-dt.timedelta(seconds=10)) assert time.time() == 90
time-machine also works as a pytest plugin. It provides a function-scoped fixture called
time_machine that has one method,
move_to(), which has the same signature as
Coordinates.move_to(). This can be used to mock your test at different points in time and will automatically be un-mock when the test is torn down.
import datetime as dt def test_delorean(time_machine): time_machine.move_to(dt.datetime(1985, 10, 26)) assert dt.date.today().isoformat() == "1985-10-26" time_machine.move_to(dt.datetime(2015, 10, 21)) assert dt.date.today().isoformat() == "2015-10-21"
Time is a global state. Any concurrent threads or asynchronous functions are also be affected. Some aren't ready for time to move so rapidly or backwards, and may crash or produce unexpected results.
Also beware that other processes are not affected. For example, if you use SQL datetime functions on a database server, they will return the real time.
There are some prior libraries that try to achieve the same thing. They have their own strengths and weaknesses. Here's a quick comparison.
The standard library's unittest.mock can be used to target imports of
time to change the returned value for current time. Unfortunately, this is fragile as it only affects the import location the mock targets. Therefore, if you have several modules in a call tree requesting the date/time, you need several mocks. This is a general problem with unittest.mock - see Why Your Mock Doesn't Work.
It's also impossible to mock certain references, such as function default arguments:
def update_books(_now=time.time): # set as default argument so faster lookup for book in books: ...
Although such references are rare, they are occasionally used to optimize highly repeated loops.
Steve Pulec's freezegun library is a popular solution. It provides a clear API which was much of the inspiration for time-machine.
The main drawback is its slow implementation. It essentially does a find-and-replace mock of all the places that the
time modules have been imported. This gets around the problems with using unittest.mock, but it means the time it takes to do the mocking is proportional to the number of loaded modules. In large projects, this can take several seconds, an impractical overhead for an individual test.
It's also not a perfect search, since it searches only module-level imports. Such imports are definitely the most common way projects use date and time functions, but they're not the only way. freezegun won’t find functions that have been “hidden” inside arbitrary objects, such as class-level attributes.
It also can't affect C extensions that call the standard library functions, including (I believe) Cython-ized Python code.
Simon Weber's python-libfaketime wraps the libfaketime library. libfaketime replaces all the C-level system calls for the current time with its own wrappers. It's therefore a "perfect" mock for the current process, affecting every single point the current time might be fetched, and performs much faster than freezegun.
Unfortunately python-libfaketime comes with the limitations of
LD_PRELOAD. This is a mechanism to replace system libraries for a program as it loads (explanation). This causes two issues in particular when you use python-libfaketime.
LD_PRELOAD is only available on Unix platforms, which prevents you from using it on Windows.
Second, you have to help manage
LD_PRELOAD. You either use python-libfaketime's
reexec_if_needed() function, which restarts (re-execs) your test process while loading, or manually manage the
LD_PRELOAD environment variable. Neither is ideal. Re-execing breaks anything that might wrap your test process, such as profilers, debuggers, and IDE test runners. Manually managing the environment variable is a bit of overhead, and must be done for each environment you run your tests in, including each developer's machine.
time-machine is intended to combine the advantages of freezegun and libfaketime. It works without
LD_PRELOAD but still mocks the standard library functions everywhere they may be referenced. Its weak point is that other libraries using date/time system calls won't be mocked. Thankfully this is rare. It's also possible such python libraries can be added to the set mocked by time-machine.
One drawback is that it only works with CPython, so can't be used with other Python interpreters like PyPy. However it may possible to extend it to support other interpreters through different mocking mechanisms.
Migrating from libfaketime or freezegun
freezegun has a useful API, and python-libfaketime copies some of it, with a different function name. time-machine also copies some of freezegun's API, in
tick arguments, and the
shift() method. There are a few differences:
tickargument defaults to
True, because code tends to make the (reasonable) assumption that time progresses between function calls, and should normally be tested as such. Testing with time frozen can make it easy to write complete assertions, but it's quite artificial.
tick()method has been implemented as
shift(), to avoid confusion with the
tickargument. It also requires an explicit delta rather than defaulting to 1 second.
tz_offsetargument only partially mocks the current time zone. Time zones are more complicated than a single offset from UTC, and freezegun only uses the offset in
time.localtime(). Instead, time-machine will mock the current time zone if you give it a
Some features aren't supported like the
auto_tick_seconds argument. These may be added in a future release.
If you are only fairly simple function calls, you should be able to migrate by replacing calls to