libfaketime 0.5.0

Installation

$ pip install libfaketime

Usage

import datetime
from libfaketime import fake_time, reexec_if_needed

# libfaketime needs to be preloaded by the dynamic linker.
# This will exec the same command, but with the proper environment variables set.
# You can also skip this and manually manage your env (see "How to avoid re-exec").
reexec_if_needed()

def test_datetime_now():

    # fake_time can be used as a context_manager
    with fake_time('1970-01-01 00:00:01'):

        # Every calls to a date or datetime function returns the mocked date
        assert datetime.datetime.utcnow() == datetime.datetime(1970, 1, 1, 0, 0, 1)
        assert datetime.datetime.now() == datetime.datetime(1970, 1, 1, 0, 0, 1)
        assert time.time() == 1


# fake_time can also be used as a decorator
@fake_time('1970-01-01 00:00:01', tz_offset=12)
def test_datetime_now_with_offset():

    # datetime.utcnow returns the mocked datetime without offset
    assert datetime.datetime.utcnow() == datetime.datetime(1970, 1, 1, 0, 0, 1)

    # datetime.now returns the mocked datetime with the offset passed to fake_time
    assert datetime.datetime.now() == datetime.datetime(1970, 1, 1, 12, 0, 1)

Performances

libfaketime serves as a fast drop-in replacement for freezegun. Here’s the output of a totally unscientific benchmark on my laptop:

$ python benchmark.py
re-exec with libfaketime dependencies
timing 1000 executions of <class 'libfaketime.fake_time'>
0.021755 seconds

$ python benchmark.py freezegun
timing 1000 executions of <function freeze_time at 0x10aaa1140>
6.561472 seconds

Use with py.test

The easiest way is to use the pytest-libfaketime plugin. This way you wont have to edit your conftest.py or call reexec_if_needed yourself. Just plug and play.

$ pip install pytest-libfaketime

You can also reexec from inside the pytest_configure hook:

# conftest.py
import os
import libfaketime

def pytest_configure():
    libfaketime.reexec_if_needed()
    _, env_additions = libfaketime.get_reload_information()
    os.environ.update(env_additions)

Migration from freezegun

find . -type f -name "*.py" -exec sed -i 's/freezegun/libfaketime/g' "{}" \;

How to avoid re-exec

Sometimes, re-exec does unexpected things. You can avoid those problems by preloading libfaketime yourself. The environment variables you need can be found by running python-libfaketime on the command line:

$ python-libfaketime
export LD_PRELOAD="/home/foo/<snip>/vendor/libfaketime/src/libfaketime.so.1"
export FAKETIME_DID_REEXEC=true

You can use them as such:

$ eval $(python-libfaketime)
$ pytest  # ...or any other code that imports libfaketime

Contributing and testing

Contributions are highly welcomed. You should compile libfaketime before running tests:

make -C libfaketime/vendor/libfaketime

Then you can install requirements with pip install -r requirements.txt and use pytest and tox to run the tests.

Known Issues

It was found that calling uuid.uuid1() multiple times while in a fake_time context could result in a deadlock. This situation only occured for users with a system level uuid1 library. In order to combat this issue, python-libfaketime temporarily disables the system level library by patching _uuid_generate_time to None while in the fake_time context.

Changelog

Semantic versioning is used.