Global variable funkiness

[tonivr]

Hi,

I'm trying to use code that I have found online at https://sourceforge.net/projects/pynetvars/
In short: it allows to use CoDeSys PLC network variables in Python.

I'm using their example code to capture network variables sent out by my PLC and to act upon a change of the content.
So far I can capture the data and parse it without any problem.

Now I'm trying to see whether there is a change in the received data compared to the previous received data.
For this I use a global variable netvars_old which will be used to compare against in the callback function.

Now the strange part is that with any run of the callback function, my global variable netvars_old is always the same as the variable netvars which is provided by the callback function as argument.
This doesn't make sense to me as I only assign the current data netvars to netvars_old on the end of the function.

Anybody an idea how this can happen?

(leaving out some predefined values, it is about netvar_old and netvar anyway)
(in this code I only print the value of net_alarm to ease debugging)

def parseReceivedValues1(netvars):
    global netvars_old
    print "OLD: net_alarm: ", netvars_old['net_alarm']
    # Compare previous with current
    for netvar_element, netvar_value in netvars.items():
        netvar_value_old = netvars_old[netvar_element]
        if netvar_value <> netvar_value_old:
            print "Changed: ", netvar_element, " - ", netvar_value
    print "NEW: net_alarm: ", netvars['net_alarm']
    print "OLD: net_alarm: ", netvars_old['net_alarm']
    netvars_old = netvars

netVars1 = NetVars( 1, 'UDP', mcastGrp, mcastPort, localIf )
netVars1.varsFromExpFile( netvarfile )
# Initialize netvars_old. Will be empty.
netvars_old = netVars1.latestValues
netVars1.subscribeNewValues(parseReceivedValues1)
netVars1.startListenerLoop()

Output example:

Output:
Initial netvars_old:  {}
establish mcast-listening on 225.10.10.10 via
receive loop...
OLD: net_alarm:  0
NEW: net_alarm:  0
OLD: net_alarm:  0

OLD: net_alarm:  1
NEW: net_alarm:  1
OLD: net_alarm:  1

OLD: net_alarm:  1
NEW: net_alarm:  1
OLD: net_alarm:  1

OLD: net_alarm:  0
NEW: net_alarm:  0
OLD: net_alarm:  0

While I expect:

Output:
Initial netvars_old:  {}
establish mcast-listening on 225.10.10.10 via
receive loop...
OLD: net_alarm:  0
NEW: net_alarm:  0
OLD: net_alarm:  0

OLD: net_alarm:  0
NEW: net_alarm:  1
OLD: net_alarm:  0

OLD: net_alarm:  1
NEW: net_alarm:  1
OLD: net_alarm:  1

OLD: net_alarm:  1
NEW: net_alarm:  0
OLD: net_alarm:  1

OLD: net_alarm:  0
NEW: net_alarm:  0
OLD: net_alarm:  0

[nilamo]

>>> True != False
True
>>> True <> False
  File "<stdin>", line 1
    True <> False
          ^
SyntaxError: invalid syntax

How does it run at all?

[micseydel]

How does it run at all?

It's an old version of != that doesn't work in more modern Python versions.

[metulburr]

still works on the latest 2.x

Output:
metulburr@ubuntu:~$ python
Python 2.7.12 (default, Jul  1 2016, 15:12:24) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 1 <> 1
False

[benkfra]

Hi tonivr,
your problem is simply a misunderstanding of dictionary references vs. dictionary copies.
If you assign a dictionary to another one it only duplicates the reference, not the whole dictionary. So modifying one will also modify the other one because it's the same memory. You only created a new name for the same storage.
You have to do a "netvars_old = netvars.copy()". That's all.

cu
Frank

[nilamo]

still works on the latest 2.x

Output:
metulburr@ubuntu:~$ python
Python 2.7.12 (default, Jul  1 2016, 15:12:24) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 1 <> 1
False

Gross.  It's a good thing 3.x got rid of it, similarities to VisualBasic send chills down my spine.

[tonivr]

Hi Frank,

This is my first code in Python, so I'm afraid I will have more misunderstandings of Python as my main scripting language is Perl.

Anyway, thanks for your clear and fast explanation.
It works with the .copy()!

And I also replaced <> with !=.

Thanks all for the help!

Regards,
Toni