python + postgres psql + os.popen

[damacy]

hello, everyone.

i am trying to write a program which executes SQL commands stored in
.sql files.

i wrote a function called psql() whose contents look like the
following.

...
os.popen(command)
file = os.popen(command, 'w')
file.write(password)
file.close()
...

where command looks like
psql -h [host] -d [dbname] -U [username] -W -f "[filename]"

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?