CODEWITHSUNDEEP
pythoniofifowith-statement
Constantine
Constantine

Reputation: 2032

Python tail-like file read

In test I redirected stderr into file and trying to read it in tail-like way. So here is redirection:

class RedirectedEnv(object):
    def __init__(self, stderr=None):
        self._stderr = stderr or sys.stderr

    def __enter__(self):
        self.old_stderr = sys.stderr
        self.old_stderr.flush()
        self.old_exit = getattr(sys, 'exit')
        sys.stderr = self._stderr

        def mock_exit():
            raise Exception
        sys.exit = mock_exit

    def __exit__(self, exc_type, exc_val, exc_tb):
        self._stderr.flush()
        sys.stderr = self.old_stderr
        sys.exit = self.old_exit

        self._stderr.close()

and usage of it:

def test_invalid_args(self):
    with RedirectedEnv(stderr=tempfile.NamedTemporaryFile()):
        for args in [['-id', '123456789.00'], ['-i'], ['-i', '0'], ['-i', '../some_invalid_path/not_exist.json']]:
            try:
                self.parser.parse(args)
            except Exception:
                sys.stderr.flush()
                self.assertTrue(sys.stderr.readlines()[0].startswith("usage:")) <<-- here I need help

Is there some elegant way to do this? (Not making mess with enumerate and tell() with seek()) Maybe there some FIFO queue with file interface ?

Upvotes: 0

Views: 1757

Answers (1)

tdelaney
tdelaney

Reputation: 77347

You can just create a new file for each test and do a seek to the front to get its value. I moved to TemporaryFile since you don't use the file name.

def test_invalid_args(self):
    for args in [['-id', '123456789.00'], ['-i'], ['-i', '0'], ['-i', '../some_invalid_path/not_exist.json']]:
        with RedirectedEnv(stderr=tempfile.TemporaryFile()):
            try:
                self.parser.parse(args)
                raise AssertionError("should have failed")
            except Exception:
                sys.stderr.seek(0)
                self.assertTrue(sys.stderr.readline().startswith("usage:"))

Upvotes: 1

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