python + postgres psql + os.popen

[damacy]

hi, there. thanks for the help.

now i have a different problem now. i decided to use 'subprocess' and
'Popen' objects instead of 'os.popen()' function, which i believe do
not make much difference.

my code is like the following...

[1] link = subprocess.Popen(command, stdin = subprocess.PIPE, stdout =
subprocess.PIPE, stderr = subprocess.PIPE, shell = True)
[2] link.communicate(password)
[3] link.wait()
[4] err = link.communicate()[1]
[5] if err != None: print str(err)

i have read several threads about 'subprocess' posted on this group and
still i have way too much confusion regarding the above section of
code.

1. i'm currently using MS Windows.
i remember some have said that communicate() function is not usable on
this OS.
could anyone confirm this?

2. i'm expecting an error message, as i am trying to create a table
which does already exist in the database.
but if i try to print out the error message as [5], it is just an EMPTY
string.
and, if i try the SAME THING using command-line, i get a correct error
message this time ('psql:createstudent.sql:12: ERROR:  relation
"student" already exists').

HOWEVER, if i comment out [2] link.communicate(password), meaning i do
not supply a password, it shows an error message, 'psql: fe_sendauth:
no password supplied', which is correct as expected.

my question is...
why does it work (i.e. showing a correct error message) when no
password supplied but NOT when creating a table which already exists in
the database? it should work for both cases.

thank you very much.


Simon Forman wrote:
> damacy wrote:
> > hello, everyone.
> ...
> > this works well. however, it does not show me any warning nor error
> > messages if there is one. for example, i am trying to create a table
> > which already exists in the database, it should show me a warning/error
> > message saying there already is one present in the database, or
> > something like that.
> >
> > can anyone help me?
>
> I recently needed to use psql from python on a computer that I couldn't
> install psycopg on and I used something similar to this to do it (I
> edited the code slightly to make it clearer):
>
> from subprocess import Popen, PIPE
>
> # Pass the password through an environment
> # variable to prevent psql asking for it.
> psql_env = dict(PGPASSWORD='********')
>
> # Create the subprocess.
> proc = Popen(cmd, shell=True, env=psql_env, stdout=PIPE, stderr=PIPE)
>
> # Try reading it's data.
> data = proc.stdout.read()
>
> # Check for errors.
> err = proc.stderr.read()
> if err: raise Exception(err)
>
> 
> It worked nicely for me, YMMV.
> 
> 
> Hope that helps,
> 
> ~Simon