(Python) Blocking sub-process

Question

I have a script class that queries from a database and displays the result. the problem is when i add a sub process below the script, the script hangs (or waits, and will continue if terminate with ctr-c)

eg. Group A will run if Group B is deleted. Group B will run if Group A is deleted

#Group A
queryStrings = ['SELECT top 100 * FROM myDb', 
'SELECT top 10 * FROM anotherDb']

## class that connects to db and output the content ## 
db = Database
conn = db.connectToDb()

for query in queryStrings:
     db.runPreQueries(conn, query)

conn.close

##Group B 

if os.path.exists("DoSomething.vbs"):
    p = subprocess.Popen("cscript DoSomething.vbs", stdout=subprocess.PIPE, stdin=subprocess.PIPE, shell=True)
    stdout, stderr = p.communicate()

print("vbs completed")

I also tried using subprocess.call, and then terminating it. This wont hang but it doesn't execute the script

p = subprocess.call("cscript DoSomething.vbs")
p.terminate()

Answer 1

when running conn.close you're not really closing the database. It does nothing because you're not calling the function.

So next call stays blocked waiting for database access.

Fix:

conn.close()

note that the proper way of running your process afterwards is (since you don't care about input, output, ...):

subprocess.check_call(["cscript","DoSomething.vbs"])

this will just fail if cscript returns a non-zero return code, which is safe enough.

Note that your database interface probably supports context manager, in that case, it would be better to write:

with db.connectToDb() as conn:    
    for query in queryStrings:
         db.runPreQueries(conn, query)

in that case, connection is closed automatically when exiting the with block.