CODEWITHSUNDEEP
python
FlowofSoul
FlowofSoul

Reputation: 510

Alternative to Passing Global Variables Around to Classes and Functions

I'm new to python, and I've been reading that using global to pass variables to other functions is considered noobie, as well as a bad practice. I would like to move away from using global variables, but I'm not sure what to do instead.

Right now I have a UI I've created in wxPython as its own separate class, and I have another class that loads settings from a .ini file. Since the settings in the UI should match those in the .ini, how do I pass around those values? I could using something like: Settings = Settings() and then define the variables as something like self.settings1, but then I would have to make Settings a global variable to pass it to my UI class (which it wouldn't be if I assign in it main()).

So what is the correct and pythonic way to pass around these variables?

Edit: Here is the code that I'm working with, and I'm trying to get it to work like Alex Martelli's example. The following code is saved in Settings.py:

import ConfigParser

class _Settings():
    @property
    def enableautodownload(self): return self._enableautodownload
    def __init__(self):
        self.config = ConfigParser.ConfigParser()

        self.config.readfp(open('settings.ini'))
        self._enableautodownload=self.config.getboolean('DLSettings', 'enableautodownload')

settings = _Settings()

Whenever I try to refer to Settings.settings.enableautodownload from another file I get: AttributeError: 'module' object has no attribute 'settings'. What am I doing wrong?

Edit 2: Never mind about the issue, I retyped the code and it works now, so it must have been a simple spelling or syntax error.

Upvotes: 2

Views: 4056

Answers (1)

Alex Martelli
Alex Martelli

Reputation: 881635

The alternatives to global variables are many -- mostly:

  • explicit arguments to functions, classes called to create one of their instance, etc (this is usually the clearest, since it makes the dependency most explicit, when feasible and not too repetitious);
  • instance variables of an object, when the functions that need access to those values are methods on that same object (that's OK too, and a reasonable way to use OOP);
  • "accessor functions" that provide the values (or an object which has attributes or properties for the values).

Each of these (esp. the first and third ones) is particularly useful for values whose names must not be re-bound by all and sundry, but only accessed. The really big problem with global is that it provides a "covert communication channel" (not in the cryptographic sense, but in the literal one: apparently separate functions can actually be depending on each other, influencing each other, via global values that are not "obvious" from the functions' signatures -- this makes the code hard to test, debug, maintain, and understand).

For your specific problem, if you never use the global statement, but rather access the settings in a "read-only" way from everywhere (and you can ensure that more fully by making said object's attributes be read-only properties!), then having the "read-only" accesses be performed on a single, made-once-then-not-changed, module-level instance, is not too bad. I.e., in some module foo.py:

class _Settings(object):
    @property
    def one(self): return self._one
    @property
    def two(self): return self._two
    def __init__(self, one, two):
       self._one, self._two = one, two
settings = _Settings(23, 45)

and from everywhere else, import foo then just access foo.settings.one and foo.settings.two as needed. Note that I've named the class with a single leading underscore (just like the two instance attributes that underlie the read-only properties) to suggest that it's not meant to be used from "outside" the module -- only the settings object is supposed to be (there's no enforcement -- but any user violating such requested privacy is most obviously the only party responsible for whatever mayhem may ensue;-).

Upvotes: 5

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