User_expression issues

[Carreau]

discoverd after question on list :

in notebook, dooin this in js :

IPython.notebook.kernel.execute('', 
{'execute_reply':function(data){console.log('data:',data)}}, 

{'user_expressions':
    {'foo':'a=1'}   # interesting part
}) 

result in

...
foo: "[ERROR] SyntaxError: invalid syntax (<string>, line 1)" 
...

in response.

1+1 does work, but more interesting things like !ls, get_ipython().system(u'ls') ... etc does not.

[minrk]

In Python expressions and statements are not the same thing. It's the difference between exec and eval.
user_expressions only allows expressions, i.e. things passed to eval. That does not include assignment.

In this way, get_ipython().system(u'ls') actually does work, in that it should give None, the result of evaluating that expression.

Now, it can be argued that we should enable more elaborate things via a mechanism like this,
such as capturing output, but that is the basic reason it behaves as it does. The keyword is 'expression'.