Skip to content

Latest commit

 

History

History
220 lines (157 loc) · 7.32 KB

devmode.rst

File metadata and controls

220 lines (157 loc) · 7.32 KB

Python Development Mode

.. versionadded:: 3.7

The Python Development Mode introduces additional runtime checks that are too expensive to be enabled by default. It should not be more verbose than the default if the code is correct; new warnings are only emitted when an issue is detected.

It can be enabled using the :option:`-X dev <-X>` command line option or by setting the :envvar:`PYTHONDEVMODE` environment variable to 1.

See also :ref:`Python debug build <debug-build>`.

Effects of the Python Development Mode

Enabling the Python Development Mode is similar to the following command, but with additional effects described below:

PYTHONMALLOC=debug PYTHONASYNCIODEBUG=1 python -W default -X faulthandler

Effects of the Python Development Mode:

The Python Development Mode does not enable the :mod:`tracemalloc` module by default, because the overhead cost (to performance and memory) would be too large. Enabling the :mod:`tracemalloc` module provides additional information on the origin of some errors. For example, :exc:`ResourceWarning` logs the traceback where the resource was allocated, and a buffer overflow error logs the traceback where the memory block was allocated.

The Python Development Mode does not prevent the :option:`-O` command line option from removing :keyword:`assert` statements nor from setting :const:`__debug__` to False.

The Python Development Mode can only be enabled at the Python startup. Its value can be read from :data:`sys.flags.dev_mode <sys.flags>`.

.. versionchanged:: 3.8 The :class:`io.IOBase` destructor now logs ``close()`` exceptions. 
.. versionchanged:: 3.9 The *encoding* and *errors* arguments are now checked for string encoding and decoding operations.

ResourceWarning Example

Example of a script counting the number of lines of the text file specified in the command line:

import sys def main(): fp = open(sys.argv[1]) nlines = len(fp.readlines()) print(nlines) # The file is closed implicitly if __name__ == "__main__": main()

The script does not close the file explicitly. By default, Python does not emit any warning. Example using README.txt, which has 269 lines:

$ python script.py README.txt 269

Enabling the Python Development Mode displays a :exc:`ResourceWarning` warning:

$ 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 :mod:`tracemalloc` shows the line where the file was opened:

$ 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:

def main(): # Close the file explicitly when exiting the with block with open(sys.argv[1]) as fp: nlines = len(fp.readlines()) print(nlines)

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.

Bad file descriptor error example

Script displaying the first line of itself:

import os def main(): fp = open(__file__) firstline = fp.readline() print(firstline.rstrip()) os.close(fp.fileno()) # The file is closed implicitly main()

By default, Python does not emit any warning:

$ python script.py import os

The Python Development Mode shows a :exc:`ResourceWarning` and logs a "Bad file descriptor" error when finalizing the file object:

$ python -X dev script.py import os script.py:10: ResourceWarning: unclosed file <_io.TextIOWrapper name='script.py' mode='r' encoding='UTF-8'> main() ResourceWarning: Enable tracemalloc to get the object allocation traceback Exception ignored in: <_io.TextIOWrapper name='script.py' mode='r' encoding='UTF-8'> Traceback (most recent call last): File "script.py", line 10, in <module> main() OSError: [Errno 9] Bad file descriptor

os.close(fp.fileno()) closes the file descriptor. When the file object finalizer tries to close the file descriptor again, it fails with the Bad file descriptor error. A file descriptor must be closed only once. In the worst case scenario, closing it twice can lead to a crash (see :issue:`18748` for an example).

The fix is to remove the os.close(fp.fileno()) line, or open the file with closefd=False.

close