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 thePYTHONWARNINGS
environment variable toerror
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 todebug
.To enable the Python Development Mode without installing debug hooks on memory allocators, set the
PYTHONMALLOC
environment variable todefault
. -
Call
faulthandler.enable()
at Python startup to install handlers for theSIGSEGV
,SIGFPE
,SIGABRT
,SIGBUS
andSIGILL
signals to dump the Python traceback on a crash.It behaves as if the
-X faulthandler
command line option is used or if thePYTHONFAULTHANDLER
environment variable is set to1
. -
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 to1
. -
Check the encoding and errors arguments for string encoding and decoding operations. Examples:
open()
,str.encode()
andbytes.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 logsclose()
exceptions. - Set the
dev_mode
attribute ofsys.flags
toTrue
.
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 descriptorerror. 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
.
License
© 2001–2021 Python Software Foundation
Licensed under the PSF License.
https://docs.python.org/3.9/library/devmode.html