Python Development Mode¶
Nouveau dans la 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.Définit l'attribut
dev_mode
desys.flags
àTrue
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
.
Modifié dans la version 3.8: The io.IOBase
destructor now logs close()
exceptions.
Modifié dans la 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
.