Terry J. Reedy <[email protected]> added the comment:
This is not a crash because IDLE does not crash. It does not even exit with a
traceback. Some crash and freeze bugs have been fixed since 3.4.2.
I verified with 3.7.0a2 on Win10. The failing example can be simplified to the
first two lines. Step over the import and into the Fraction call to get to
fractions.Fraction.__new__, line 115
self = super(Fraction, cls).__new__(cls)
(Step 8 in the report above has the wrong code.) Step 2 gives this simplified
traceback:
Traceback (most recent call last):
File "F:\Python\a\tem2.py", line 3, in <module>
U=Fraction(1)
File "C:\Programs\Python37\lib\fractions.py", line 117, in __new__
if denominator is None:
File "C:\Programs\Python37\lib\fractions.py", line 117, in __new__
if denominator is None:
File "C:\Programs\Python37\lib\bdb.py", line 86, in trace_dispatch
return self.dispatch_line(frame)
File "C:\Programs\Python37\lib\bdb.py", line 110, in dispatch_line
self.user_line(frame)
File "C:\Programs\Python37\lib\idlelib\debugger.py", line 24, in user_line
self.gui.interaction(message, frame)
AttributeError: _numerator
If I replace the line above with
new = super(Fraction, cls) # resolves to object.__new__
self = new(cls)
stepping the first works. For the new call, hitting 'step' or 'over' gives
essentially the same traceback as above. Hitting 'go' runs the code to
completion.
If I add 'print('xxxx') after the new call, the print call shows up in the
traceback, twice, but is not executed.
If I simplify the code to
class C():
def __new__(cls):
self = object.__new__(cls)
return self
c = C()
and step into the C() code the the __new__ code, there is no problem. Hence
the revised title.
If I add slots make C, as with Fraction, there is still no problem.
class C():
__slots__ = ('a',)
def __new__(cls, a):
self = object.__new__(cls)
self.a = a
return self
c = C(1)
Puzzles:
1. Why does the next line, 'if denominator...' show up in the traceback? It
does not have a function call. Why is it printed twice?
2. Where does the _numerator error happen? It is only set, never accessed.
Setting should work because _numerator is one of the two slots.
3. What extra feature of Fraction results in the failure.
If I leave IDLE's debugger off and invoke pdb by adding
import pdb; pbd.set_trace()
as line 2, before the Fraction call, stepping with s works on the line where
Debugger failed. Until a fix is released, use this (and/or print) as a
workaround for code that makes debugger croak.
I do not now know how to write a fully automated unittest for debugger. But it
does not now even have a human-driven htest, and it should.
----------
stage: -> test needed
title: Probable bug in all python3 / idle3 debugger -> IDLE debugger fails with
non-trivial __new__ super call
type: crash -> behavior
versions: +Python 3.7
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Python tracker <[email protected]>
<https://bugs.python.org/issue32140>
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