eryksun added the comment:
In Python 2, PyErr_GivenExceptionMatches [1] calls PyObject_IsSubclass. To
handle calling __subclasscheck__ in this case, 2.7 (but not 2.6) temporarily
increases the recursion limit by 5. For example:
class CMeta(type):
def __subclasscheck__(self, other):
import sys
print 'recursion limit: %d' % sys.getrecursionlimit()
frame = sys._getframe(1)
n = 0
while frame:
n += 1
frame = frame.f_back
print 'frame: %d' % n
return True
class C(Exception):
__metaclass__ = CMeta
def f():
try:
f()
except C:
pass
>>> sys.getrecursionlimit()
1000
>>> f()
recursion limit: 1005
frame: 1000
>>> sys.getrecursionlimit()
1000
If the recursion limit weren't temporarily increased, then trying to call
__subclasscheck__ in the above case would raise another RuntimeError.
In Python 3, PyErr_GivenExceptionMatches [2] instead calls PyType_IsSubtype.
See issue 2534. In that issue Antoine's reason for the change is that
"otherwise there are some nasty issues with recursion checking".
[1]: https://hg.python.org/cpython/file/v2.7.10/Python/errors.c#l84
[2]: https://hg.python.org/cpython/file/v3.5.0/Python/errors.c#l166
----------
components: +Interpreter Core
nosy: +eryksun
versions: +Python 3.5, Python 3.6 -Python 3.2, Python 3.3
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