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1. Camion commented on Nov 26, 2017

   

   Camionmannequin

   on Nov 26, 2017

   MannequinAuthor

   More actions

   Hello all,

   I have been investigating a bug with a generator, which crashed, reporting that "TypeError: 'Fraction' object is not iterable".

   Then I have tried to find it with the debugger and/or to reproduce it with a simpler program, but, I have not managed to reproduce the bug in my test program and I have been stopped by the fact that the debugger crashes with another error:

   I have reproduced it on version 3.4.2 ans 3.6.3, BUT NOT on version 2.7.9. So I suspect it has been introduced with version 3.

   Here is how to reproduce it :

   1/ Open Idle
   2/ create and save this text program (or open if you already did - or don't do anything if it's already opened X-D)

   from fractions import Fraction
   U=Fraction(1)
   
   def test(x):
       for i in range(1, x.denominator):
           x*=x
           yield i, i*x
   
   for m, n in test(U*3/5):
       print (m, n)

   3/ Run/check module (F5) in Idle editors window : it works perfectly
   4/ start the debugger from Idle menus
   5/ activate the "source" checkbox
   6/ Run/check module (F5) in Idle editors window
   7/ Click 3 times on [over] to get to the "for m, n in test(U*3/5):" line
   8/ Click 6 times on [step], to get to the line 116 ("for m, n in test(U*3/5):") in the fractions.py files
   9/ Click a seventh time on [step] and your program will crash with the folowing error (here with 3.6.3)

   Traceback (most recent call last):
     File "/raid/ArcFolder/Mes documents/Mes Textes/Mes programmes/Machin/Test_itérateur_et_fractions.py", line 12, in <module>
       for m, n in test(U*3/5):
     File "/usr/local/lib/python3.6/fractions.py", line 376, in forward
       return monomorphic_operator(a, b)
     File "/usr/local/lib/python3.6/fractions.py", line 419, in _mul
       return Fraction(a.numerator * b.numerator, a.denominator * b.denominator)
     File "/usr/local/lib/python3.6/fractions.py", line 117, in __new__
       if denominator is None:
     File "/usr/local/lib/python3.6/fractions.py", line 117, in __new__
       if denominator is None:
     File "/usr/local/lib/python3.6/bdb.py", line 48, in trace_dispatch
       return self.dispatch_line(frame)
     File "/usr/local/lib/python3.6/bdb.py", line 66, in dispatch_line
       self.user_line(frame)
     File "/usr/local/lib/python3.6/idlelib/debugger.py", line 24, in user_line
       self.gui.interaction(message, frame)
   AttributeError: _numerator5/ Run/check module (F5) in Idle editors window

   I have observed once 3.4.2 that the same operation without activating the source checkbox, lead to a full idle freeze, but I have not been able to reproduce it.
2. Camion

   added

   type-crashA hard crash of the interpreter, possibly with a core dumpA hard crash of the interpreter, possibly with a core dump

   on Nov 26, 2017
3. Camion

   assigned

   terryjreedy

   on Nov 26, 2017
4. Camion

   added

   topic-IDLE

   on Nov 26, 2017
5. terryjreedy commented on Nov 26, 2017

   

   terryjreedy

   on Nov 26, 2017

   Member

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   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.
6. terryjreedy

   added

   3.7 (EOL)end of lifeend of life

   on Nov 26, 2017
7. terryjreedy

   changed the title [-]Probable bug in all python3 / idle3 debugger[/-] [+]IDLE debugger fails with non-trivial __new__ super call[/+] on Nov 26, 2017
8. terryjreedy

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   and removed

   type-crashA hard crash of the interpreter, possibly with a core dumpA hard crash of the interpreter, possibly with a core dump

   on Nov 26, 2017
9. pppery commented on Dec 4, 2017

   

   ppperymannequin

   on Dec 4, 2017

   Mannequin

   More actions

   Simplified reproducer for same bug without any imports:

   class BadRepr:
   	def __repr__(self):
   		1/0
   def broken():
       x=BadRepr()
       x=x #filler line for debugger

   In this case, the problematic BadRepr object in the "broken" function explictly has a broken __repr__, but the uninitialized Fraction object in the "__new__" function of the Fraction class has a similarly broken __repr__
10. ezio-melotti

    transferred this issue fromon Apr 10, 2022
11. terryjreedy

    added this to IDLE Issueson Jul 23, 2022
12. terryjreedy

    removed

    3.7 (EOL)end of lifeend of life

    on Aug 6, 2024
13. serhiy-storchaka commented on Sep 15, 2026

    

    serhiy-storchaka

    on Sep 15, 2026

    Member

    More actions

    Fixed by GH-24183 in 3.10 (bpo-33065): the debugger gets the values through reprlib.repr(), which handles exceptions in __repr__().
14. serhiy-storchaka

    closed this as completedon Sep 15, 2026
15. github-project-automation

    moved this to Done in IDLE Issueson Sep 15, 2026