Help replacing os.system call with subprocess call

[Matt Nordhoff]

David Pratt wrote:
> Hi. I am trying to replace a system call with a subprocess call. I have 
> tried subprocess.Popen and subprocess.call with but have not been 
> successful. The command line would be:
> 
> svnadmin dump /my/repository > svndump.db
> 
> This is what I am using currently:
> 
> os.system('svnadmin dump %s > %s' % (svn_dir,
>      os.path.join(backup_dir, 'svndump.db')))
> 
> Many thanks.

Try this:

import os.path
import subprocess

p = subprocess.Popen(
    ['svnadmin', 'dump', svndir],
    stdout=subprocess.PIPE,
)

fh = open(os.path.join(backup_dir, 'svndump.db'), 'wb')

while True:
    chunk = p.stdout.read(2**20) # 1 MB
    if not chunk:
        break
    fh.write(chunk)

fh.close()

It reads svnadmin's stdout in 1 MB chunks, in case it's large enough
that reading the whole thing into RAM at once would be a bad idea.

No error handling. For one, you might want to add a try...finally to
ensure that fh will get closed. (Or if you have Python 2.5, use a with
statement! :-) ) Also, Popen will raise an OSError if svnadmin can't be
found or something. And this isn't even considering svnadmin erroring out...

svnadmin's stderr will go to your stderr.

I didn't test it, but I'm pretty sure it will work. (I spotted a syntax
error while writing that though.) I don't have much experience with
Popen's stdio objects, so it's possible you'd need to do something like
call p.wait() to wait for it to exit before being able to read its stdout.

It could be slower than the os.system version, since now Python is doing
all of the I/O, instead of your shell, but I doubt that'll be a big problem.

(Also, insert suggestion about using a good VCS. ;-) )
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