Preserve file permissions in fakefs after add_real_file

[daviewales]

Describe the bug
Suppose I have a real file /usr/bin/env, owned by root:root, with permissions 0755.

If I copy this file into a fake filesystem the user and group are preserved, but the permissions are changed to 0444.

How To Reproduce
Note: This assumes you are on a *nix system with an executable file /usr/bin/env on your real filesystem.
If not, substitute any other file owned by root with permissions 0755.

import os
from pyfakefs.fake_filesystem import FakeFilesystem
from pyfakefs.fake_os import FakeOsModule

file = '/usr/bin/env'

fs = FakeFilesystem()
fs.add_real_file(file)

real_permissions = oct(os.stat(file).st_mode)
print(f"{real_permissions=}")

fake_os = FakeOsModule(fs)

fake_permissions = oct(fake_os.stat(file).st_mode)
print(f"{fake_permissions=}")

# output
# real_permissions='0o100755'
# fake_permissions='0o100444'

Your environment
Please run the following in the environment where the problem happened and
paste the output.

Linux-6.8.0-53-generic-x86_64-with-glibc2.39
Python 3.12.3 (main, Jan 17 2025, 18:03:48) [GCC 13.3.0]
pyfakefs 5.7.4
pytest 8.3.4

[mrbean-bremen]

Thanks, I will have a look!

[mrbean-bremen]

Ok, I completly forgot that this is expected behavior. By default, files from the real filesystem are mapped as read-only. From the documentation:

The access to the files is by default read-only, but even if you add them using read_only=False, the files are written only in the fake system (e.g. in memory).

Replacing fs.add_real_file(file) with fs.add_real_file(file, read_only=False) should give you the wanted behavior.
Please check if this is sufficient for you.