How to create a static local variable in python instance method?

Question

I'm coming from a C/C++ world and am having a hard time making all of the OOP analogies in python. I wrapped my script in a class and now get this error in my log() 'private' instance method:

AttributeError: 'instancemethod' object has no attribute 'counter'

def __log(self, contents):                                                  
    sys.stdout.write(contents)                                              

    # Append all writes after the first                                     
    if not hasattr(self.__log, "counter"):                                  
        self.__log.counter = 1                                              
        f = open(self.output_filepath, 'w')                                 
    else :                                                                  
        f = open(self.output_filepath, 'a')                                 

    f.write(contents)                                                       
    f.close()

I strongly recommend that you not use "private" names like "__log". They are ugly, and don't actually prevent access. Python works differently that C++. Use it for its own sake. — Ned Batchelder, Oct 05 '14 at 18:21
They're a very strong "stay off" sign, though. As a co-developer of a major Python library, I can tell you how important it is to use underscores to signal the interface/implementation boundary to users. — Fred Foo, Oct 05 '14 at 18:45
Fine, use a single underscore. Double underscores just say, "I'm used to Java or C++, and want my private keyword back!" — Ned Batchelder, Oct 05 '14 at 23:30

score 1 · Answer 1 · edited May 23 '17 at 12:11

self.__log.counter references to a counter variable which does not exist in the __log method.

if you need the counter variable to be part of the instance of the object, refer to it via self.counter

if you need the counter to be static to the class, define the variable in the class definition outside any methods. check this SO question

if you really really need the counter variable to be part of the instance method, the way to reference it is via ClassName.__log.counter

a good place to start learning OO concepts in python would be the python documentation. https://docs.python.org/2/tutorial/classes.html

Piotr Dabkowski · Answer 2 · 2014-10-05T18:20:18.287

-1

This you cant add attributes to methods, but you can modify their dict attribute:

class Test:
    def __log(self, contents):                                                  
        sys.stdout.write(contents)                                              

        # Append all writes after the first                                     
        if not "counter" in self.__log.__func__.__dict__:                                  
            self.__log.__func__.__dict__["counter"]= 1                                              
            f = open(self.output_filepath, 'w')  
            print 'w'                               
        else :                                                                  
            f = open(self.output_filepath, 'a') 
            print 'a'                               

        f.write(contents)                                                       
        f.close()  

a = Test()
a._Test__log('') #Prints w
a._Test__log('') #Prints a
b = Test()
b._Test_log('') #Prints a

edited Oct 05 '14 at 18:20

answered Oct 05 '14 at 18:06

Piotr Dabkowski

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Please, check the edit and let me know if this is what you want – Piotr Dabkowski Oct 05 '14 at 18:13
Could anybody explain what's wrong whith my answer? – Piotr Dabkowski Oct 05 '14 at 18:27
It's silly to add attributes to the function. Python doesn't have static local variables, you shouldn't try to force them in. Instead, come up with a Python solution to the problem. – Ned Batchelder Oct 05 '14 at 18:28
Well the OP wanted to add attribute to the function and I showed him how to do that. He wasnt asking for 'Pythonic' solution. – Piotr Dabkowski Oct 05 '14 at 18:32
1

I think it's silly to give him a bad answer. If you were reviewing his Python code, and found `self.__log.__func__.__dict__["counter"] = 1`, wouldn't you tell him to change it? – Ned Batchelder Oct 05 '14 at 18:37

score -1 · Accepted Answer · answered Oct 05 '14 at 18:26

Python doesn't have static local variables. The best solution to what you want is to use a class attribute:

class WithLog(object):

    opened = False

    def log(self, contents):                                                  
        sys.stdout.write(contents)                                              

        mode = "a" if self.__class__.opened else "w"
        with open(self.output_filepath, mode) as f:
            f.write(contents)       

        self.__class__.opened = True

Of course, it might be better not to constantly open and close the log file....

How to create a static local variable in python instance method?

3 Answers3