time-machine 1.0.1

Warp time

Travel through time in your tests.

A quick example:

import datetime as dt
import time_machine

@time_machine.travel("1955-11-05 01:22")
def test_delorean():
    assert dt.date.today().isoformat() == "1955-11-05"

Installation

Use pip:

python -m pip install time-machine

Python 3.6 to 3.8 supported (CPython only).

Usage

travel(destination, *, tick=True, tz_offset=None)

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:

• A datetime.datetime. If it is naive, it will be assumed to have the UTC timezone.

• A datetime.date. This will be converted to a UTC datetime with the time 00:00:00.

• A float or int specifying 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 1970-01-01 00:00:00.500000.

tz_offset allows you to offset the given destination. It may be a timedelta or a number of seconds, which will be added to destination. It may be negative.

Mocked Functions

All datetime functions in the standard library are mocked to move to the destination current datetime:

• datetime.datetime.now()

• datetime.datetime.utcnow()

• time.gmtime()

• time.localtime()

• time.clock_gettime() (only for CLOCK_REALTIME)

• time.clock_gettime_ns() (only for CLOCK_REALTIME)

• time.strftime()

• time.time()

• time.time_ns()

This 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 unittest.mock.patch().

Usage with start() / stop()

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(1955, 11, 5))
traveller.start()
# It's the past!
assert dt.date.today() == dt.date(1955, 11, 5)
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.

Function Decorator

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

This also works for 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

Context Manager

When used as a context manager, time is mocked during the with block:

def test_in_the_deep_past():
    with time_machine.travel(0.0):
        assert 0.0 < time.time() < 1.0

Class Decorator

Only 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 tearDownClass():

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.

Caveats

Time is global state. Any concurrent threads or async 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 datetime functions in a client/server database they will still return the real time.

Comparison

There are some prior libraries that try to achieve the same thing. They have their own strengths and weaknesses. Here’s a quick comparison.

unittest.mock

The standard library’s unittest.mock can be used to target imports of datetime and 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 this is rare, it’s often used to optimize repeat loops.

freezegun

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 datetime and 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.

Also, it can’t affect C extensions that call the standard library functions, including Cython-ized Python code.

Additionally, it has the same problem that it can’t mock function default arguments and some other references.

python-libfaketime

Simon Weber’s python-libfaketime wraps the LD_PRELOAD library libfaketime. 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 it comes with the limitations of LD_PRELOAD (explanation). First, this is only available on Unix platforms, which prevents it from working on Windows. Seccond, you either use its reexec_if_needed() function, which restarts (re-execs) your tests’ process once while loading, or manually manage the LD_PRELOAD environment variable (everywhere you run your tests). Re-execing breaks profilers, use of python -m pdb and similar, and other things that might wrap your test process. Manually managing the environment variable is a bit of overhead for each environment you want to run your tests in.

time-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 travel()'s destination, tick, and tz_offset arguments. There is one difference - time-machine’s tick argument defaults to True, because code tends to make the (reasonable) assumption that time progresses between function calls, and should normally be tested as such.

Some arguments aren’t supported like auto_tick_seconds, or the move_to() and tick() methods. 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 freezegun.freeze_time() and libfaketime.fake_time() with time_machine.travel().