[Tutor] file open (take 2)

[Terry Carroll]

On Wed, 27 Sep 2006, Dave S wrote:

> I am trying to read in an ascii text file, do some alterations and write it 
> back.
> 
> file = open(self.config.get('pdf','cert') + '/cert.pdf' , 'r+')
> lines = file.readlines()
> 
> ... process lines ...
> 
> file.writelines(lines)
> file.close()
> 
> works but ends up appending a second modified copy to the original ... as per 
> the python ref.

I'm surprised it wors at all.  When I use writelines on a file that's 
opened for read, I get an IOError.

>>> f = open("glorp.txt","r")
>>> lines = f.readlines()
>>> lines.reverse()  # just something to show the file's been changed
>>> f.writelines(lines)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
IOError: (0, 'Error')

> Am I right in thinking that the only way is to open with a 'r', close them 
> open with a 'w' ?

Yup:

>>> f = open("glorp.txt","r")
>>> lines = f.readlines()
>>> lines.reverse()
>>> f.close()
>>> f = open("glorp.txt","w")
>>> f.writelines(lines)
>>> f.close()