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pythonlinuxtemp

Copying a NamedTemporaryFile in Linux results in empty file

I am writing some contents to a tempfile.NamedTemporaryFile in Python 3 under Ubuntu 16.04. Under certain circumstances, I want to copy that file to a different location after the writing is done. The problem is reproduced with the following code:

import tempfile
import shutil

with tempfile.NamedTemporaryFile('w+t') as tmp_file:
    print('Hello, world', file=tmp_file)
    shutil.copy2(tmp_file.name, 'mytest.txt')

mytest.txt is empty once the execution is over. If I use delete=False when creating the NamedTemporaryFile I can inspect its content in /tmp/ and they are fine.

I know the file cannot be open again while open under Windows as per the documentation, but Linux should be fine, so I wouldn't expect it to be that.

What is happening and how can it be resolved?

Solution

  • The problem is that the print() calls are not being flushed, so when the file is copied nothing has yet been written to it.

    Using flush=True as a parameter of print() fixes the issue:

    import tempfile
import shutil

with tempfile.NamedTemporaryFile('w+t') as tmp_file:
    print('Hello, world', file=tmp_file, flush=True)
    shutil.copy2(tmp_file.name, 'mytest.txt')