Python namespace and global variables

Question

I've read several answers about making variables global and sharing them between classes with the class.var functionality, but none of those seem to work in my example.

I only have one file: app.py which instantiates multiple classes. However, the variable that I want to make global isn't in a class, it's just in the main script. Then when I type global globvar in a function that is inside one of the classes, I get the error: `

NameError: global name 'globvar' is not defined.

How can I either pass this variable to the class by reference (I know, not pythonic language) so that the class can modify it but other classes can access it, or make it global so all the classes can access it?

EDIT bottom of file with no indentation:

app = MyApp(False)
logFile = open('C:/Python26/temp/test.txt', 'w')
globvar = 7
ser = serial.Serial('com5',115200,timeout=0.01)
app.MainLoop()

Then in my first class I have:

class MyApp(wx.App):
    def MainLoop(self):
        line = ser.readline()
        if len(line) > 2:
            global globvar
            globvar = line

And in my second class I have:

class MyFrame(wx.Frame):
    def OnMove(self, event):
        pos = event.GetPosition()
        global globvar
        self.posCtrl.SetValue("%s, %s" % (globvar, pos.y))

These are all in the same file. The error text is:

Traceback (most recent call last):
  File "C:\Program Files (x86)\wxPython2.8 Docs and Demos\samples\mainloop\mainloop.py", line 66, in OnMove
    self.posCtrl.SetValue("%s, %s" % (globvar, pos.y))
NameError: global name 'globvar' is not defined

The variable is already global because you've declared it in your main script. There isn't any code or traceback in your question so it's difficult to guess what problem you are actually having. — kindall, Nov 14 '12 at 17:06

score 2 · Accepted Answer · edited May 23 '17 at 12:30

2

All parameters are passed by reference (in the C++ sense). The Python language doesn't have pass by value (in the C++ sense) although it can be simulated by copying, and certain things are automatically copied on mutate like numbers, strings, and tuples such that they may give the appearance of having been passed by value. To be very specific python is call-by-object (aka call-by-sharing, call-by-object-reference) as pointed out here on stackoverflow previously.

To make a variable global you need to make the first assignment to it outside of any function or create in a function with the global keyword. To modify it in a function you'll also need to use the global keyword. See here for more details on the global keyword.

For example:

i = 0
print i
class A(object):
    global i
    i = 1
print i
class B(object):
    def Foo(self):
        global i
        i = 2
b = B()
b.Foo()
print i

yields output of:

0
1
2

edited May 23 '17 at 12:30

Community

1
1

answered Nov 14 '12 at 17:12

Josh Heitzman

1,816
1
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Can you give an example of how I would pass it to the class and then make it a variable that all the classes would access? – NickHalden Nov 14 '12 at 17:20
It want it to be global it isn't necessary to pass it to the class first, the class can access it just like any other class. – Josh Heitzman Nov 14 '12 at 17:38
To be fussy, it should either be "all parameters are passed by *copy* of references" or "all parameters are always passed by value", but the value is a reference to the object. See [here](http://stackoverflow.com/a/534509/510937) – Bakuriu Nov 14 '12 at 17:49
Thanks the problem was my code for main app was at the bottom of the file, including my declaration of globvar – NickHalden Nov 14 '12 at 18:02
If fussiness is desired, it is actually call-by-object. Post updated with links. – Josh Heitzman Nov 14 '12 at 18:16

Python namespace and global variables

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