strange behavior with os.system

[MRAB]

kmw wrote:
> Hi,
> 
> I wanted to write a simple script (in 5 minutes or so) which replaces
> the option '+1' given to the command 'sort' by '-k 2' and than runs
> 'sort' with the modified argument list. After two hours I am giving up
> and ask you for help. This is what I tried (please excuse the verbose
> code, it is due to my various efforts to understand the error):
> 
> #!/usr/bin/python
> import sys, os, re
> arguments = sys.argv[0]
> for i in sys.argv[1:]:
> 	arguments += " " + i

Shorter:

     arguments = " ".join(sys.argv)

> p = re.compile ( "(\+(\d+))" )

This looks for a "+" followed by digits.

> m = p.search ( arguments )
> print type ( m )
> m_list = list ( m.groups () )
> print type ( m_list )
> from1 = str ( m_list[0] )
> to1 = "-k " + str ( int ( m_list[1] ) + 1 )
> cmd1 = str ( arguments.replace ( from1, to1 ) )
> print cmd1
> os.system ( cmd1 )
> 
> Now, this is what I get (on three different machines with different
> versions of python):
> <type '_sre.SRE_Match'>
> <type 'list'>
> ./sort -F -k 2 -e

No "+"s, so the p.search(arguments) returns None.

> <type 'NoneType'>
> Traceback (most recent call last):
>   File "./sort", line 9, in <module>
>     m_list = list ( m.groups () )
> AttributeError: 'NoneType' object has no attribute 'groups'
> 
> Please note the unrequested output of '<type 'NoneType'>'. The strange
> thing about this all is the fact that  the whole thing works as
> expected when typed  into the interpreter. I would be glad if anyone
> could help.
>