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The -X dev mode currently *hides* some ResourceWarning warnings:
$ cat x.py 
def func():
 open('/etc/issue')
func()
func()
$ ./python x.py 
x.py:2: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
 open('/etc/issue')
x.py:2: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
 open('/etc/issue')
haypo@selma$ ./python -X dev x.py 
x.py:2: ResourceWarning: unclosed file <_io.TextIOWrapper name='/etc/issue' mode='r' encoding='UTF-8'>
 open('/etc/issue')
The problem is that the "-W default" inserted by -X dev overrides the final filter on ResourceWarning:
$ ./python -X dev -c 'import warnings, pprint; pprint.pprint(warnings.filters)'
[('default',
 re.compile('', re.IGNORECASE),
 <class 'Warning'>,
 re.compile(''),
 0),
 ('ignore', None, <class 'BytesWarning'>, None, 0),
 ('always', None, <class 'ResourceWarning'>, None, 0)]