Heisenberg strikes again!
google at elesi.org
Thu Sep 11 02:51:17 CEST 2003
Thank you! That answers my questions precisely. :) You can see my
c/c++/java biases showing. Expecting global variables to carry into
functions and not expecting silent exceptions to alter my execution
paths. Valuable lessons!
"David C. Fox" <davidcfox at post.harvard.edu> wrote in message news:<nRM7b.311656$Oz4.100417 at rwcrnsc54>...
> neeson wrote:
> > Alright, I'm fairly new to Python, and this one has got me stumped.
> > I've just started to write a cli program that'll use readline, and
> > I've attached the relevant bits.
> > Here's the mystery: If I so much as /look/ at the list 'matches',
> > readline stops working. Run the program, hit tab at the prompt,
> > you'll get 'one', which is incorrect, as there are in fact four
> > possible completions. Now comment out the line that I've marked
> > (which, incidentally, does nothing). /Now/ it works.
> > Is there some very strange side effect to accessing a list element
> > that I'm unaware of? I've tried it in two different versions of
> > Python.
> No, but there is a side effect of accessing the first element of an
> empty list: namely, Python raises an IndexError exception (i.e. index
> out of range).
> > Any elightenment would be appreciated...
> > Heath
> > import readline
> > import sys
> > commands = ["one", "two", "three", "four"]
> > matches = 
> > def comp(text, state):
> > if state == 0:
> > matches = 
> Because you haven't used
> global matches
> the next statement creates a new list called matches as a variable local
> to comp. All your matching commands are added to this variable, which
> disappears when comp returns.
> > n = len(text)
> > for cmd in commands:
> > if cmd[:n] == text:
> > matches.append(cmd)
> > throwaway = matches # <--- Comment out this line
> When comp is called with state == 0, matches is still referring to the
> local list which is not empty (unless there were no matches to the
> text), so this line does indeed do nothing. However, when comp is
> called with state != 0, this line refers to the global matches list,
> which is empty, so it raises an IndexError, and the following line is
> not reached.
> Apparently, readline treats an exception in the completer as equivalent
> to returning None. In fact, you seem to be relying on this behavior
> implicitly in the line below, because you are not checking whether state
> < len(commands)
> > return commands[state]
> > readline.set_completer(comp)
> > readline.parse_and_bind("tab: complete")
More information about the Python-list