Write to file created with add_real_file does not truncate

[ian-h-chamberlain]

Describe the bug
Adding a file with add_real_file(file, read_only=False), then writing to the file, does not properly truncate the fake in-memory file – instead it appears to continue reading from the real file on disk.
I'm not sure if this is related to #277, but I can reproduce with much simpler test code.

How To Reproduce
Using pytest:

def test_pyfakefs_real_file(fs):
    fs.add_real_file('foo.txt', read_only=False)

    with open('foo.txt', 'w') as foo:
        foo.write('Goodbye')

    with open('foo.txt', 'r') as foo:
        assert foo.read() == 'Goodbye'

foo.txt contents:

Hello,  world!

Output:

$ pytest test.py
============================================================= test session starts ==============================================================
platform darwin -- Python 3.6.5, pytest-4.2.1, py-1.7.0, pluggy-0.8.1
rootdir: /Users/ichamberlain/Documents/workspace, inifile:
plugins: pyfakefs-3.5.7
collected 1 item

test.py F                                                                                                                                [100%]

=================================================================== FAILURES ===================================================================
___________________________________________________________ test_pyfakefs_real_file ____________________________________________________________

fs = <pyfakefs.fake_filesystem.FakeFilesystem object at 0x1053f78d0>

    def test_pyfakefs_real_file(fs):
        fs.add_real_file('foo.txt', read_only=False)

        with open('foo.txt', 'w') as foo:
            foo.write('Goodbye')

        with open('foo.txt', 'r') as foo:
>           assert foo.read() == 'Goodbye'
E           AssertionError: assert 'Goodbye world!\n' == 'Goodbye'
E             - Goodbye world!
E             + Goodbye

/Users/ichamberlain/Documents/workspace/test.py:8: AssertionError
=========================================================== 1 failed in 0.37 seconds ===========================================================

Your enviroment
Using pyenv with pyfakefs and pytest installed in the "system" python provided by pyenv.

Darwin-18.2.0-x86_64-i386-64bit
Python 3.6.5 (default, Jan 29 2019, 13:07:11)
[GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.5)]
pyfakefs 3.5.7

[mrbean-bremen]

Thanks - I will check this. And this is not related to #277 - that happens only under Python 2, and only on MacOS, while I could reproduce this one also under Windows.

[mrbean-bremen]

Good catch - this has to do with lazily reading the file contents, which has been done even after writing to the file.

[ian-h-chamberlain]

Wow, thanks for the quick fix! For reference in case anyone else needs a temporary workaround, what I did in the meantime is pretty simple: you can just remove the file, then use create_file() to replace the contents and everything works as expected.