Search and write to .txt file

[Simon Forman]

On Aug 11, 7:22 am, Helvin <helvin... at gmail.com> wrote:
> Hi everyone,
>
> I am writing some python script that should find a line which contains
> '1' in the data.txt file, then be able to move a certain number of
> lines down, before replacing a line. At the moment, I am able to find
> the line '1', but when I use f.seek to move, and then rewrite, what I
> write goes to the end of the .txt file, instead of being adjusted by
> my f.seek.
>
> Do you know what way I should take?
>
> Data.txt is a file of 3 lines:
>    line1
>    line2
>    line3
>
> Code:
>
>    with open('data.txt', 'r+') as f:
>        firstread = f.readlines()   # Take a snapshot of initial file
>
>        f.seek(0,0)    # Go back to beginning and search
>        for line in f:
>            print line
>            if line.find('1'):
>                print 'line matched'
>                f.seek(1,1)       # Move one space along
>                f.write('house\n')     # f.write overwrites the exact
> number of bytes.
>                break                    # leave loop once '1' is found
>
>        f.seek(0,0)              # Go back to beginning, and read
> data.txt again
>        lastread = f.readlines()
>
>        print 'firstread is', firstread
>        print 'lastread is', lastread
>
> This shouldn't be too difficult, but I don't know how. > <
> Help appreciated!

There's a bug in this line:

if line.find('1'):

the string find() method returns an integer, which will be -1 if the
substring is not found. In python -1 is treated as True (only 0 is
False) so your if statement will always succeed unless '1' is the
first character on the line.

HTH,
~Simon


P.S. you can use the help() command in the python interpreter to get
docs on most things:

>>> help(str.find)

Help on method_descriptor:

find(...)
    S.find(sub [,start [,end]]) -> int

    Return the lowest index in S where substring sub is found,
    such that sub is contained within s[start:end].  Optional
    arguments start and end are interpreted as in slice notation.

    Return -1 on failure.