Write a file - beginner's question

[Bruno Desthuilliers]

Ben Keshet a écrit :
> I have a probably simple beginner's question -
> 
> I have a script that I am currently able to print its output.  instead, 
> i want to write it into a file - I tried different versions of write() 
> but might have gotten the syntax wrong.

The syntax is:

    fileobj.write(something)

Now since you don't say exactly what you tried *and what you got*, we 
can't help much more here.

>  the variable I want to write is 
> a line from a file I am reading:
> 
> "...
> f = open('receptor.mol2', 'r')
> line = f.readline()[:-1]

If you want to strip the newline characters, you'd better use 
line.strip() or line.lstrip()

> while '@<TRIPOS>ATOM' not in line:
>    line = f.readline()[:-1]
>    #print line


Also, there are simpler ways to iterate over a file:

f = open('somefile.txt')
for line in f:
     if '@<TRIPOS>ATOM' in line:
         break
     print line.lstrip()

f.close()

>    print random_mol2
> ..."
> 
> e.g. I want to write to a file all the lines in 'receptor.mol2' up to 
> the string "@<TRIPOS>ATOM" (one after the other).  Can anyone please 
> advice?

The simplest solution for a unix-like command-line program would be to 
redirect the standard out to this file, ie:

# python myprog.py > destfile.txt

But this may not fit your needs !-)


The other simplest solution is to open the destination file in write (or 
append) mode and write to it:

source = open('somefile.txt')
dest = open('otherfile.txt', 'w')
for line in source:
     if '@<TRIPOS>ATOM' in line:
         break
     # we don't strip here, since write doesn't append a newline
     dest.write(line)

source.close()
dest.close()


> On a related note - how do I read and write to a file that is not in the 
> same directory?

Give the full absolute or relative path.

> e.g how do I provide a pathway to an open command?


source = open('/full/absolute/path/to/my/file.txt')

Note that the exact syntax for a path depends on your os (but the 
os.path module can take care of most specificities). The above example 
is for a posix system. On Windows, you'll probably have something like:

source = open('C:/full/absolute/path/to/my/file.txt')

HTH