Python 2.2 code continues running before list comprehension is completed?

Peter Otten __peter__ at
Mon Jul 19 18:42:03 CEST 2004

Chris P. wrote:

> Hi.  I've made a program that logs onto a telnet server, enters a
> command, and then creates a list of useful information out of the
> information that is dumped to the screen as a result of the command.
> Here's a generic version of the code in question:
> #####
> # Prior code opens telnet connection "tn" and logs in.
> tn.read_until('> ')
> tn.write('THE COMMAND IS HERE\n')
> dump = tn.read_very_eager()

You could try tn.read_all() instead (just a guess).

assert "string1\r" in dump # would do no harm
assert ""string2\r" in dump

> dump_lines = dump.split('\n')
> dump_info = [x for x in dump_lines if (VARIOUS CONDITIONS ON x)]
> # string1 and string2 are strings that are known to
> # always appear in the dump.

Bold claim :-)

> A = dump_info.index('string1\r')
> B = dump_info.index('string2\r')
> C = A - B - 1
> #####
> The thing is, I can step through this code in the debugger and it
> works fine.  When run from the command line, however, I get the
> following error in response to my dump_info.index lines:
> ValueError: list.index(x): x not in list
> So I tried putting (for x in dump_info: print x) right after I make
> the list comprehension and, when I run that, it shows that the list is
> incomplete... so it SEEMS like Python begins to create the list
> comprehension and then continues to execute commands before the list
> comprehension is finished!

I'm pretty sure that the list comprehension is finished. Look out for
something else.


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