Dumb Q #1

Alex Martelli aleax at aleax.it
Tue Jan 28 11:11:33 CET 2003


Norm wrote:

> I would like to open this unix passwd file and chang the GID of those who
> have a GID of 200 to 199
> 
> This does everything except change fields[3] to 199
> 
> Thoughts?

>     file = open ("passwd.txt" , "r+")

Don't use built-in names of types, such as "file", "list",
"tuple", etc, to name your own variables: get into the habit
of naming them in significant ways, or, worst case, at least
use "aFile", "someList", etc.  One day you'll get strange
bugs if you abuse built-in type names in this tempting way.

> records = file.readlines()
> 
> for record in records:
>     fields = record.split(':')
>     if fields[3] == "200":
>         print fields[0], " has a GID of 200"
>         print "now changing it to 199"
>         fields[3] = 199

Better to use "199" of course -- a string, NOT a number.  And
then of course you need to rebuild the record -- variable
"fields" is just holding copies of the record's data.  It's
hard to do this when you iterate directly on the list: rebinding
variable 'record' would not affect list 'records', which is the
one you NEED to affect.  So, you may want to loop on index,
instead, changing this loop into:

for i in range(len(records)):
    fields = records[i].split(':')
    if fields[3] == "200":
        print fields[0], " has a GID of 200"
        print "now changing it to 199"
        fields[3] = "199"
        records[i] = ':'.join(fields)

and then of course you need to actually write the contents
of variable records back into the file...:

aFile.seek(0)
aFile.writelines(records)
aFile.close()

(using a variable name different from 'file', as I suggested).


Alex





More information about the Python-list mailing list