how to distinguish a python expression from a statement?

[James_Althoff at i2.com]

Thanks for your response, Peter.  Actually, that is the approach I had been
experimenting with.  But I was worried that catching the SyntaxError in
eval as a way of "discovering" a statement (as opposed to an expression)
could somehow mask an actual syntax error in an expression.  But in
thinking about it more, I guess there is no problem with that since the
exec works on both expressions and statements.  And so a bad expression
passed to exec will result in exec throwing a SyntaxError a second time,
which I suppose is fine for my purposes.

I will also look into the code module more.  But I'm not really doing an
exact replica of the interactive Python interpreter.  I'm doing a GUI-based
variant that is more like a Smalltalk workspace -- but with the twist that
the output always goes into a separate (from the input) text pane.  Maybe
the code module will still help.

Thanks again,

Jim


Peter Hansen wrote:

=====================


James_Althoff at i2.com wrote:
>
> I have a string, inputText, which is a fragment of python code plucked
from
> a text control in a GUI.  If the value of inputText  is an expression
then
> I will do "result = eval(inputText,globalsDict,localsDict)" and show
result
> in the GUI.  If it is a statement, I will do "exec inputText in
> globalsDict, localsDict" (and I know about redirecting sys.stdout while
> doing either).
>
> I have all of this working except that I don't know how to distinguish
> between expressions and statements (without making the end user push one
of
> two different buttons in order to tell me -- yuk).  I am using Jython and
> so the parser module is not available (so I can't use parser.isexpr).
>
> Is there any means to accomplish this what-I-had-thought-would-be-routine
> task short of writing a Python parser?

This might give you some ideas:

>>> def f(str):
...     try:
...         result = eval(str)
...     except SyntaxError:
...         exec str
...         result = None
...     return result
...
>>> f('5*5')
25
>>> f('print "hi"')
hi
>>>

On the other hand, as some kind soul showed me when I asked
a related question a while back, there are also the modules
'codeop' and 'code', which probably already have want you
need...

--
----------------------
Peter Hansen, P.Eng.
peter at engcorp.com
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