An alternative to global in Python

Question

I currently have code like this:

cache = 1
def foo():
    global cache
    # many
    # lines
    # of code
    cache = 2

However, this may lead to hard-to-find-bugs in the future, because the reader may not notice that global cache appears somewhere above cache = 2. Alternatively, a contributor may mistakenly add def bar(): cache = 2 and forget to add the global cache.

How can I avoid this pitfall?

What exactly is the pitfall? What are you trying to accomplish, and what is not working? — BrenBarn, Jan 03 '13 at 08:30
What if someone writes the second method? It will still be a local variable. There is no issue with your code. — ATOzTOA, Jan 03 '13 at 08:31
If there's no `global` declaration, then the variable is local - if 'the reader' has any background in python, then they'll know it's local. If you really want to, just add a comment saying it's local or something. — Volatility, Jan 03 '13 at 08:35
When you have a lot of lines of code, it's cumbersome to have to look for 'global cache' somewhere above 'cache = 2'. I posted a work-around answer. — Yariv, Oct 24 '13 at 10:33

Yariv · Accepted Answer · 2014-05-03T18:02:45.600

17

class Cache:
     myvar = 1

def foo():
    Cache.myvar = 2

This way, Cache.myvar is practically a "global". It's possible to read/write to it from anywhere.

I prefer this over the dictionary alternative, because it allows for auto-complete of the variable names.

edited May 03 '14 at 18:02

answered Jan 03 '13 at 08:45

Yariv

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Great answer, elegant and exactly what I was looking for! Thank you – Gareth Davidson Oct 22 '13 at 20:56

score 8 · Answer 2 · answered Jan 03 '13 at 08:31

8

cache = 1
def foo():
    return 2
cache = foo()

or

d = {'cache': 1}
def foo(x):
    x['cache'] = 2
foo(d)

answered Jan 03 '13 at 08:31

eumiro

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score 3 · Answer 3 · answered Jan 03 '13 at 08:33

"the reader may unintentionally think that the global variable has been updated" isn't much of a pitfall. You have to expect that people reading your code know how Python works. If you want to make it extra clear, use a comment. That's what they're for.

score 2 · Answer 4 · answered Jan 03 '13 at 08:50

2

Using global variable is not a good programming practice. Pass the variable as an argument: make function returns something and use it in another function. Function can be assignment to variable that's how Python works.

answered Jan 03 '13 at 08:50

user1881957

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An alternative to global in Python

4 Answers4

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