catching syntax errors via excepthook?

[Alex Martelli]

Hari Sekhon <sekhon.hari at googlemail.com> wrote:

> I've written an except hook into a script as shown below which works 
> well for the most part and catches exceptions.
> 
> import sys
> def myexcepthook(type,value,tb):
>     do something
> 
> sys.excepthook=myexcepthook
> rest of script.... (now protected by catchall exception hook)
> 
> 
> I've been intentionally introducing errors into the code to try to test
> it and while it catches import errors and other things, it doesn't catch
> syntax errors.
> 
> Is there a way to get it to catch syntax errors?

Python, of course, parses (and in fact compiles) a whole module before
executing any of it, so if you're talking about syntax error in the
source of the very module which (when executed) will install an
excepthook, no way.  Apart from this, no problem, e.g.:

>>> def myexcepthook(type,value,tb):
...     print 'error', value
... 
>>> sys.excepthook=myexcepthook
>>> 2+
error invalid syntax (<stdin>, line 1)
>>> m=open('za.py', 'w')
>>> m.writelines('''print "hello"
... print 2+ 
... ''')
>>> m.close()
>>> import za
error invalid syntax (za.py, line 2)

etc, etc, i.e., the hook does catch all syntax errors due to parsing
which occurs *after* the hook is installed (in interactive sessions or
other modules).  Clearly the hook cannot catch any error which occur
*before* the hook is installed.

You can delay the detection of syntax errors, for example, by explicitly
calling the compile built-in function on string literals, rather than
relying on implicit compilation of sources; and/or you can strive to
have your excepthook installed ASAP, e.g. by playing with
sitecustomize.py and/or $PYTHONSTARTUP .


Alex