python decorator modify variable

Question

I am trying to use decorator to modify variable value in python with the following code

def config(func):
    def new_function():
        func()
        print(lr)
    return new_function

import ex
@ex.config
def config():
    lr = 0.1
    globals().update(locals())
config()

I would like to print out 0.1, but error with variable not defined was shown. Is there anyway to achieve this without changing func_a?

https://stackoverflow.com/questions/1360721/how-to-get-set-local-variables-of-a-function-from-outside-in-python — emirc, Jun 19 '20 at 01:05
The decorator makes almost no difference here (aside from ignoring the return value). — alani, Jun 19 '20 at 01:22

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

1

In short, no.
What is happening here is func_a() is declaring and assigning lr as a local variable. You can fix this by adding the global keyword:

lr = 0
def config(func):
    def new_function(*args,**kwargs):
        func(*args,**kwargs)
    return new_function
@config
def func_a():
    global lr
    lr = 0.1
func_a()
print(lr)

But using the global keyword is annoying and can cause bugs if you forget it, so you might be better off returning:

lr = 0
def config(func):
    def new_function(*args,**kwargs):
        return func(*args,**kwargs)
    return new_function
@config
def func_a():
    return 0.1
lr = func_a()
print(lr)

Edit

You could put it in a class somehow like this:

# Make an empty class:
class Foo: pass
# Or if you want to initalize the variables:
class Foo:
    def __init__(self):
        lr = 0
f = Foo()

# The decorator doesn't make much of a difference
def func_a():
    f.lr = 0.1

func_a()
print(f.lr)

Now, if you want to keep absolutely all the local variables that func_a defines, you could do this:

def func_a():
    lr = 0.1
    f.__dict__.update(locals())

edited Jun 20 '20 at 09:12

Community

1
1

answered Jun 19 '20 at 01:13

cajomar

428
5
9

1

Thanks for the comment, however the point is to keep ```func_a``` as it is. I am trying to replicate someone else's work and make adjustment based on it, the only part of the code I have is similar to ```func_a``` but with hundreds more variables. For more complete information, ```@config``` used to be ```@obj.config``` in their original code, but I don't think obj is a class. – Joe Liu Jun 19 '20 at 01:26
@JoeLiu I'm assuming that `lr` must be a file-wide global variable? Because ideally you could put that (and he other hundreds) in a class or something. Here's another idea, with only slight modification to `func_a()`: https://stackoverflow.com/a/11543718/12465821 – cajomar Jun 19 '20 at 02:11
The link you shared is very interesting. but I am not sure if the method would work if I were to separate the two functions into two files, like shown in the newly edited question above. I am also really interested in your ideal of putting the variables into a class? I tried it before but failed, Could you please share your idea of the implementation? – Joe Liu Jun 19 '20 at 18:22
@JoeLiu I just edited my answer to include two ways of doing that. – cajomar Jun 19 '20 at 21:43
This answer is really interesting, I think I can modify this to what I need. Thank you for your answer. – Joe Liu Jun 19 '20 at 22:04

score 0 · Answer 2 · answered Jun 19 '20 at 01:07

0

'lr' is an local variable and you didn't save or return its value.So you just can get the value of global variable 'lr' in the first line and its value is 0.

answered Jun 19 '20 at 01:07

霖同学

33
4

python decorator modify variable

2 Answers2

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