Python function hangs when called from within sql

Colleauge of mine wrote for me python function to import data from files on server to postgresql db.

His "primary" python function is writen in python script on disk, and is using psycopg2.
This "primary" function is called from wrapper plpythonu function, defined with python 2 on the pg server.

Basically it uses "COPY" to load data into my existing table.

When I call the wrapper plpythonu function from my query window (pgAdmin), it finishes almost instantly (small test data), and loads the data correctly. But when I try to call it from within my other function (plpgsql), it just hangs.

When looking at pg_stat_activity, the COPY command is displayed there with state = 'active' and waiting = 't'. Inside the python function, we tried set_isolation_level to autocommit, and we tried explicit commit as well. Also we tried a simple hardcoded INSERT instead of COPY, but it behaves the same way.

Is there something we are not aware of, or forgetting ? I tried searching for similar topics here and on google, to no avail so far.

Edit - added code, some parts are left out for obvious reasons. First is the "primary" python script:

import os
import subprocess
import psycopg2
import psycopg2.extensions

_DB_HOST = # left out
_DB_NAME = # left out
_DB_USER = # left out
_DB_PWD  = # left out

#-----------------------------------------------------------------------------------------------------------------------
def _copyFile(cur, filePath):
    # it was tried with "COPY ... , for testing I use a simpler hardcoded insert
    cmd = "insert into etl.tmp_import_file(file_line) values('test insert from python ' || clock_timestamp());"
    # table etl.tmp_import_file exists, column file_line is varchar(100), and it works when the plpythonu function is called by itself
    cur.execute(cmd)

#-----------------------------------------------------------------------------------------------------------------------
def save_file(fileId):
    dbc = psycopg2.connect(database=_DB_NAME, user=_DB_USER, host=_DB_HOST, password=_DB_PWD)
    # dbc.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT) # didn't work either
    cur = dbc.cursor()

    _copyFile(cur, filePath)
    dbc.commit()
    dbc.close()

#-----------------------------------------------------------------------------------------------------------------------
if __name__ == "__main__":
    import sys
    if len(sys.argv) < 2:
        print 'Usage: save_file.py <file_id>'
        sys.exit(1)
    else:
        save_file(sys.argv[1])
        sys.exit(0)

And this is the "wrapper" plpythonu function in pg server:

CREATE OR REPLACE FUNCTION etl.save_file(file_id integer)
  RETURNS integer AS
$BODY$

import sys
sys.path.append("/path_to_the_python_script")

import test_save_file as c

try:
    c.save_file(file_id)
    return 0
except:
    return -1

$BODY$
  LANGUAGE plpythonu VOLATILE
  COST 100;
ALTER FUNCTION etl.save_file(integer)
  OWNER TO postgres;