Passing variable from one funtion to another

Question

One thing I keep having trouble with in Python is passing information from one function to another, changing it and then passing it back to the original function to usefully use. Take a look at this very basic example, trying to create a ping function that can be called by various other functions.

import subprocess
pingchk = 0

def pinger(pcName, pingchk):
    ping_out = subprocess.Popen(["ping", "-n", "1", pcName],stdout = subprocess.PIPE).communicate()[0]
    if ('unreachable' in ping_out):
        print "Warning", "PC not responding 1."
        pingchk += 1
        return pingchk
    elif ('Request timed out.' in ping_out):
        print "Warning", "PC not responding 2."
        pingchk += 1
        print pingchk
        return pingchk
    elif ('Ping request could not find host' in ping_out):
        print "Warning", "PC not responding 3."
        pingchk += 2
        print pingchk
        return pingchk
    else:
        print "Machine available!"

def someFunction():
    pcName = raw_input("Please enter a pc name: ")
    pinger(pcName, pingchk)
    print pingchk
    if pingchk == 1:
        print "machine not switched on."
    elif pingchk == 2:
        print "machine name not recognized."
    else:
        print "success - machine on."

someFunction()

It's the pingchk bit that I'm struggling with. In this example someFunction is passing the machineName up to pinger to do the work, then I want pinger to pass the result back to someFunction to use. However, someFunction never re-reads the variable pingchk so always reads it as 0.

What do I need to change to pass the results of pinger back to the originating function for use?

@jonrsharpe - thanks for the reply but if I try that it claims that it is being referenced before assignment... — user3514446, Jun 10 '16 at 13:37
@jonrsharpe maybe you missed it but `pinger(pcName,` **`pinkchk)`** uses the value of the global variable, by introducing assignment you make it a local variable that is references (as the argument to `pinger`) before it is assigned. — Tadhg McDonald-Jensen, Jun 10 '16 at 14:03

score 1 · Accepted Answer · answered Jun 10 '16 at 13:50

As jonrsharpe suggested in a comment the way to go is simply to affect the returned value to the variable:

pingchk = pinger(pcName, pingchk)

There are only 2 caveats here:

you have a branch that does not return anything (when you print available) so you will end with a None which may not be what you expect: consistently return 0 in that case or better move the return out of the if:

def pinger(pcName, pingchk):
    ping_out = subprocess.Popen(["ping", "-n", "1", pcName],stdout = subprocess.PIPE).communicate()[0]
    if ('unreachable' in ping_out):
        print "Warning", "PC not responding 1."
        pingchk += 1
    elif ...
    else:
        print "Machine available!"
    return pingcheck

as you affect the variable pingcheck in somefunction before is has been used, Python consideres that it is a local variable hiding the global one. You must declare it as global - if you really need it to be global:
```
def someFunction():
    global pingchk
    pcName = raw_input("Please enter a pc name: ")
    pingchk = pinger(pcName, pingchk)
```

But if you have no real reason for pingcheck to be global, just let it be local:

    def someFunction():
        pcName = raw_input("Please enter a pc name: ")
        pingchk = pinger(pcName, 0)

That's brilliant. Just adding "pingchk = pinger(pcName, 0)" was all I needed. Creating the variable inside the function rather than outside. Thank-you so much! Chris. — user3514446, Jun 10 '16 at 14:47

score 0 · Answer 2 · answered Jun 10 '16 at 13:38

0

To modify variables inside a function, pass them as list.

def ping(counter):
    counter[0] += 1  # lists are passed by reference, hence they can be changed in place

mycounter = [0]
ping(mycounter)
print mycounter[0]  # this will print 1
ping(mycounter)
print mycounter[0]  # this will print 2

answered Jun 10 '16 at 13:38

user1308345

1,032
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Don't use a nuclear weapon to kill a fly... OP's code already return (almost allways...) the new value. jonrsharpe's suggestion is the way to go here – Serge Ballesta Jun 10 '16 at 13:40

score 0 · Answer 3 · answered Jun 10 '16 at 13:40

When you are calling a function in python, it returns a variable just like you are doing a basic math operation. For instance, if you do:

3*2

Your shell will return 6, but doesn't save this result. If you want to save it, just do:

x = 3*2

The same goes for your function pinger, when you call it from someFunction, you have to do:

pingchk = pinger(pcName, pingchk)

As pingchk otherwise is a local variable.

Another remark on your code: in someFunction you first call pinger with pingchk as an argument but it doesn't exist beforehand, therefore, your code won't turn.

Bill M · Answer 4 · 2016-06-10T13:50:33.523

0

Two ways:

Use a Global variable - Google will give you plenty of info and examples
My favorite, Put your variable in another module and import it. e.g.

from commonstuff import pingcheck

edited Jun 10 '16 at 13:50

answered Jun 10 '16 at 13:40

Bill M

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how does putting it in a different module help? – Tadhg McDonald-Jensen Jun 10 '16 at 14:04

Passing variable from one funtion to another

4 Answers4