Do you use the “global” statement in Python? [closed]

问题

I was reading a question about the Python global statement ( "Python scope" ) and I was remembering about how often I used this statement when I was a Python beginner (I used global a lot) and how, nowadays, years later, I don't use it at all, ever. I even consider it a bit "un-pythonic".

Do you use this statement in Python ? Has your usage of it changed with time ?

回答1:

I use 'global' in a context such as this:

_cached_result = None
def myComputationallyExpensiveFunction():
    global _cached_result
    if _cached_result:
       return _cached_result

    # ... figure out result

    _cached_result = result
    return result

I use 'global' because it makes sense and is clear to the reader of the function what is happening. I also know there is this pattern, which is equivalent, but places more cognitive load on the reader:

def myComputationallyExpensiveFunction():
    if myComputationallyExpensiveFunction.cache:
        return myComputationallyExpensiveFunction.cache

    # ... figure out result

    myComputationallyExpensiveFunction.cache = result
    return result
myComputationallyExpensiveFunction.cache = None

回答2:

I've never had a legit use for the statement in any production code in my 3+ years of professional use of Python and over five years as a Python hobbyist. Any state I need to change resides in classes or, if there is some "global" state, it sits in some shared structure like a global cache.

回答3:

I've used it in situations where a function creates or sets variables which will be used globally. Here are some examples:

discretes = 0
def use_discretes():
    #this global statement is a message to the parser to refer 
    #to the globally defined identifier "discretes"
    global discretes
    if using_real_hardware():
        discretes = 1
...

file1.py:
    def setup():
        global DISP1, DISP2, DISP3
        DISP1 = grab_handle('display_1')
        DISP2 = grab_handle('display_2')
        DISP3 = grab_handle('display_3')
        ...

file2.py:
    import file1

    file1.setup()
    #file1.DISP1 DOES NOT EXIST until after setup() is called.
    file1.DISP1.resolution = 1024, 768

回答4:

In my view, as soon as you feel the need to use global variables in a python code, it's a great time to stop for a bit and work on refactoring of your code.
Putting the global in the code and delaying the refactoring process might sound promising if your dead-line is close, but, believe me, you're not gonna go back to this and fix unless you really have to - like your code stopped working for some odd reason, you have to debug it, you encounter some of those global variables and all they do is mess things up.

So, honestly, even it's allowed, I would as much as I can avoid using it. Even if it means a simple classes-build around your piece of code.

回答5:

Objects are the prefered way of having non-local state, so global is rarely needed. I dont think the upcoming nonlocal modifier is going to be widely used either, I think its mostly there to make lispers stop complaining :-)

回答6:

I use it for global options with command-line scripts and 'optparse':

my main() parses the arguments and passes those to whatever function does the work of the script... but writes the supplied options to a global 'opts' dictionary.

Shell script options often tweak 'leaf' behavior, and it's inconvenient (and unnecessary) to thread the 'opts' dictionary through every argument list.

回答7:

I avoid it and we even have a pylint rule that forbids it in our production code. I actually believe it shouldn't even exist at all.

回答8:

Rarely. I've yet to find a use for it at all.

回答9:

It can be useful in threads for sharing state (with locking mechanisms around it).

However, I rarely if ever use it.

回答10:

I've used it in quick & dirty, single-use scripts to automate some one-time task. Anything bigger than that, or that needs to be reused, and I'll find a more elegant way.

回答11:

Once or twice. But it was always good starting point to refactor.

回答12:

If I can avoid it, no. And, to my knowledge, there is always a way to avoid it. But I'm not stating that it's totally useless either

来源：https://stackoverflow.com/questions/146557/do-you-use-the-global-statement-in-python

标签

python

global