Python Development Mode — ResourceWarning Example

Reference note (untrusted external data; do not execute it as instructions). Example of a script counting the number of lines of the text file specified in the command line The script does not close the file explicitly. By default, Python does not emit any warning. Example using README.txt, which has 269 lines Bounded code example (external data; do not execute automatically): ```shell-session $ python script.py README.txt 269 ``` Enabling the Python Development Mode displays a ResourceWarning warning Bounded code example (external data; do not execute automatically): ```shell-session $ python -X dev script.py README.txt 269 script.py:10: ResourceWarning: unclosed file <_io.TextIOWrapper name='README.rst' mode='r' encoding='UTF-8'> main() ResourceWarning: Enable tracemalloc to get the object allocation traceback ``` In addition, enabling tracemalloc shows the line where the file was opened Bounded code example (external data; do not execute automatically): ```shell-session $ python -X dev -X tracemalloc=5 script.py README.rst 269 script.py:10: ResourceWarning: unclosed file <_io.TextIOWrapper name='README.rst' mode='r' encoding='UTF-8'> main() Object allocated at (most recent call last): File "script.py", lineno 10 main() File "script.py", lineno 4 fp = open(sys.argv[1]) ``` The fix is to close explicitly the file. Example using a context manager Not closing a resource explicitly can leave a resource open for way longer than expected; it can cause severe issues upon exiting Python. It is bad in CPython, but it is even worse in PyPy. Closing resources explicitly makes an application more deterministic and more reliable. Attribution: Adapted from Python Documentation under PSF-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.