How to access a variable from a class of another module

Question

I am a total python beginner and I have a variable created in a class of a file commandline_reader.py that I want to access from another script. I tried to do it by making the variable global, which doesn't work.

myscript.py:

    from commandline_reader import Commandline_Reader
    reader = Commandline_Reader('--get_serial_number')
    reader.run()

    print output

commandline_reader.py:

    class Commandline_Reader:
        def __init__(self,argString=''):
            global output
            output = []

        def run(self):
            # do stuff
            a = 'somevariable'
            output.append(a)

When I run myscript.py I always get a NameError: name 'output' is not defined. I've read that this is because global variables are only defined within a module. How do I correctly access the output variable in my script?

jwillis0720 · Accepted Answer · 2014-01-15 10:39:51Z

1

ouch. The whole reason object oriented programming takes place is to avoid the use of global variables. Make them instance variables to access them anywhere in the class.

class Commandline_Reader:
    def __init__(self,argString=''):
        self.output = []

    def run(self):
        # do stuff
        a = 'somevariable'
        self.output.append(a) #output is now part of the instance Commandline reader and can be accessed anywhere inside the class. 


 clr = Commandline_Reader(argstring='--get_serial_number')
 clr.run()
 print clr.output
 >>>['somevariable']

answered Jan 15, 2014 at 10:39

jwillis0720

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2 Comments

jwillis0720 Over a year ago

Well there are lots of reasons. But I can't think of a reason why you would ever need global variables in a class.

Anvesh Yalamarthy Over a year ago

For instance, you want to share a data between few @staticmethods, you would need a global variable in that class isn't it?

jonrsharpe · Accepted Answer · 2014-01-15 10:44:27Z

1

Make output an instance attribute:

class Commandline_Reader:
    def __init__(self,argString=''):
        self.output = [] # note use of self here
    def run(self):
        # do stuff
        a = 'somevariable'
        self.output.append(a) # and here

The access it via the instance:

print reader.output

edited Jan 15, 2014 at 10:44

answered Jan 15, 2014 at 10:38

jonrsharpe

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Comments

laike9m · Accepted Answer · 2014-01-15 10:54:06Z

0

Maybe class attribute is more appropriate for you?

class Commandline_Reader:

    output = []

    def run(self):
        # do stuff
        a = 'somevariable'
        self.output.append(a)

answered Jan 15, 2014 at 10:54

laike9m

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Comments

Mailerdaimon · Accepted Answer · 2014-01-15 11:50:50Z

-1

Just return the Value from the run() Method

myscript.py:

from commandline_reader import Commandline_Reader
reader = Commandline_Reader('--get_serial_number')
output = reader.run()

print output

commandline_reader.py:

class Commandline_Reader:
    def __init__(self,argString=''):
        self.output = []

    def run(self):
        # do stuff
        a = 'somevariable'
        self.output.append(a)
        return self.output

edited Jan 15, 2014 at 11:50

answered Jan 15, 2014 at 10:38

Mailerdaimon

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4 Comments

RemcoGerlich Over a year ago

You mean self.output, the way it's now, output = [] in init does nothing. Also list.append returns None, so you shouldn't return it.

jwillis0720 Over a year ago

the scope of output in your code does not know about the init function without an instance attribute self. This will still return a name error.

user3197620 Over a year ago

Thanks, I was hoping there was such a trivial solution!!

Raj725 Over a year ago

@Mailerdaimon you are returning variable, but the thing how to access variables globally accross the modules.

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