[IPython-user] Using yaml with ipython is goofy

[Robert Kern]

On 2009-02-14 16:19, Matthew Wilson wrote:
> I tried playing with yaml in an ipython session and got weird results.
> yaml.load returned a string!
>
> First, here's what happens in a not-ipython session:
>
>      $ python
>      Python 2.6b2+ (trunk:65208, Jul 23 2008, 17:02:49)
>      [GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
>      Type "help", "copyright", "credits" or "license" for more information.
>      >>>  import yaml
>      >>>  class Y:
>      ...     pass
>      ...
>      >>>  yaml.load("""
>      ... !!python/object:__main__.Y
>      ... name: Yahoo
>      ... """)
>      <__main__.Y instance at 0x82ec5ec>
>      >>>  type(yaml.load("""
>      ... !!python/object:__main__.Y
>      ... name: Yahoo
>      ... """))
>      <type 'instance'>
>
>
> Now this is what happens in my beloved ipython:
>
>      $ ipython
>      /home/matt/virtualenvs/pitz-2.6/lib/python2.6/site-packages/ipython-0.9.1-py2.6.egg/IPython/Magic.py:38:
>      DeprecationWarning: the sets module is deprecated
>        from sets import Set
>      Python 2.6b2+ (trunk:65208, Jul 23 2008, 17:02:49)
>      Type "copyright", "credits" or "license" for more information.
>
>      IPython 0.9.1 -- An enhanced Interactive Python.
>      ?         ->  Introduction and overview of IPython's features.
>      %quickref ->  Quick reference.
>      help      ->  Python's own help system.
>      object?   ->  Details about 'object'. ?object also works, ?? prints more.
>
>      In [1]: import yaml
>
>      In [2]: class Y:
>         ...:     pass
>         ...:
>
>      In [3]: y = yaml.load("""
>         ...: !!python/object:__main__.Y
>         ...: name: Yahoo
>         ...: """)
>
>
> And then I got a big traceback, ending with:
>
>      ScannerError: mapping values are not allowed here
>        in "<string>", line 3, column 5:
>          name: Yahoo
>          ^
>
> So, what is going on here?  What am I doing wrong?

IPython's __main__ is kind of weird and suboptimal in a few respects. If you 
import __main__, you can see that it doesn't have a Y in it. yaml is probably 
trying to import __main__.Y and failing. For example, instead of __main__ try 
another module that doesn't have a Y; you'll get the same error message. For the 
suboptimal error message, you can blame yaml.  :-)

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco