Creating pytz `timezone` always returns `UnknownTimeZoneError`

[lsfischer]

Describe the bug
When using TestCase from from pyfakefs.fake_filesystem_unittest import TestCase calling self.setUpPyfakefs on setUp, trying to create a pytz timezone object will always result in ÙnknownTimeZoneError` no matter the timezone string.

In contrast, omitting the call to self.setUpPyfakefs or using TestCase from unittest does not result in this error.

How To Reproduce

from pyfakefs.fake_filesystem_unittest import TestCase
from pytz import timezone


class ExampleTestCase(TestCase):
    def setUp(self):
        self.setUpPyfakefs()

    def test_timezone(self):
        timezone("Europe/Copenhagen")
        # raises pytz.exceptions.UnknownTimeZoneError: 'Europe/Copenhagen'

However not setting up the fake fs (or equivalently, using TestCase from unittest) does not yield this error

from pyfakefs.fake_filesystem_unittest import TestCase
from pytz import timezone


class ExampleTestCase(TestCase):
    def test_timezone(self):
        timezone("Europe/Copenhagen")
        # test passes

Your environment

python -c "import platform; print(platform.platform())"
>>> Linux-6.5.0-14-generic-x86_64-with-glibc2.38

python -c "import sys; print('Python', sys.version)"
>>> Python 3.11.5 (main, Sep 11 2023, 13:54:46) [GCC 11.2.0]

python -c "from pyfakefs import __version__; print('pyfakefs', __version__)"
>>> pyfakefs 5.2.4

python -c "import pytest; print('pytest', pytest.__version__)"
>>> pytest 7.4.3

pip freeze | grep "^pytz"
>>> pytz==2023.3.post1

I'd be interested in helping out on a solution to this if required!
Thanks a lot!

[mrbean-bremen]

The problem here is that pytz uses time zone information data files inside its package to initialize the time zone, and won't find them in the fake fs.
To fix this, you have to map the timezone files into the fake filesystem, something like this (out of my head):

from pathlib import Path
import pytz

class ExampleTestCase(TestCase):
    def setUp(self):
        self.setUpPyfakefs()
        info_dir = Path(pytz.__file__).parent / "zoneinfo"
        self.fs.add_real_directory(info_dir)

See also this entry in the troubleshooting section of the documentation. It probably would make sense to add this concrete example there.

[mrbean-bremen]

I'm converting this into a discussion issue - it is probably helpful for others.