[FIXED] Weird IPython ipdb behaviour

Issue

I have some really weird behaviour that I just don't understand and therefore cannot explain, so I hope someone here can help me out. First thing I noticed was ipdb not letting me define variables any more:

ipdb> what=5
ipdb> what
*** NameError: name 'what' is not defined
whatelse=6

And a bit later I found ipdb returning this on my input (after running the code again):

dir()
ipdb> ['args', 'content_type', 'function', 'ipdb', 'item_code', 'kwargs', 'object_id', 'request', 'ud_dict', 'update_querydict', 'what', 'whatelse']
what=5
ipdb> what
5
ipdb> whatelse=7
ipdb> whatelse
ipdb> 6
whatelse
ipdb> 7
whatelse
ipdb> 6
whatelse
ipdb> 7

To me this looks as I have two interleaving debug sessions, which I have access to in some strange alternating pattern. How can I get rid of that?

edit: Killing all python processes and re-running the code did help. Everything is back to normal now. But as I just don't understand what was going on, I would be very interested in an answer to what happened, and how to reproduce the behaviour.

Solution

If you have two processes each consuming stdin, they can cause symptoms very similar to what you've described.

>>> import subprocess
>>> subprocess.Popen('python', shell=True)
<subprocess.Popen object at 0x0000000001DEFEB8>
>>> Python 2.6.5 (r265:79096, Mar 19 2010, 18:02:59) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> a = 5
>>> a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>> a
5
>>> a = 6
>>> a
5
>>> a
6
>>> a
5
>>> a
6
>>> a
5
>>> a
6

The first time I called a, it was undefined, even though I'd defined it in the line above. I call it again and it's 5. Then I set it to 6, and each subsequent call alternates between the two. This seems to be exactly what you're experiencing.

stdin is line-buffering, causing alternating lines of text to pass to alternating processes, in this case two instances of Python.

Answered By - mhlester