Python Development Mode

New in version 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 -X dev command line option or by setting the PYTHONDEVMODE environment variable to 1.

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 python3 -W default -X faulthandler

Effects of the Python Development Mode:

Add default warning filter. The following warnings are shown:
Normally, the above warnings are filtered by the default warning filters.

It behaves as if the -W default command line option is used.

Use the -W error command line option or set the PYTHONWARNINGS environment variable to error to treat warnings as errors.
Install debug hooks on memory allocators to check for:
- Buffer underflow
- Buffer overflow
- Memory allocator API violation
- Unsafe usage of the GIL
See the PyMem_SetupDebugHooks() C function.

It behaves as if the PYTHONMALLOC environment variable is set to debug.

To enable the Python Development Mode without installing debug hooks on memory allocators, set the PYTHONMALLOC environment variable to default.
Call faulthandler.enable() at Python startup to install handlers for the SIGSEGV, SIGFPE, SIGABRT, SIGBUS and SIGILL signals to dump the Python traceback on a crash.

It behaves as if the -X faulthandler command line option is used or if the PYTHONFAULTHANDLER environment variable is set to 1.
Enable asyncio debug mode. For example, asyncio checks for coroutines that were not awaited and logs them.

It behaves as if the PYTHONASYNCIODEBUG environment variable is set to 1.
Check the encoding and errors arguments for string encoding and decoding operations. Examples: open(), str.encode() and bytes.decode().

By default, for best performance, the errors argument is only checked at the first encoding/decoding error and the encoding argument is sometimes ignored for empty strings.
The io.IOBase destructor logs close() exceptions.
Set the dev_mode attribute of sys.flags to True.

The Python Development Mode does not enable the tracemalloc module by default, because the overhead cost (to performance and memory) would be too large. Enabling the tracemalloc module provides additional information on the origin of some errors. For example, 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 -O command line option from removing assert statements nor from setting __debug__ to False.

Changed in version 3.8: The io.IOBase destructor now logs close() exceptions.

Changed in version 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:

$ python3 script.py README.txt
269

Enabling the Python Development Mode displays a ResourceWarning warning:

$ python3 -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:

$ python3 -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:

$ python3 script.py
import os

The Python Development Mode shows a ResourceWarning and logs a “Bad file descriptor” error when finalizing the file object:

$ python3 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 bpo-18748 for an example).

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