Overwrite global var in one line in Python?

Question

I know that I can write:

foo = 'bar'
def update_foo():
  global foo
  foo = 'baz'

But do I really need two lines of code there? Python, alas, won't allow me to say

global foo = 'baz'

I could also mash the two lines together with the unfortunately repetitive

global foo; foo = 'baz'

Any other shortcuts? I'm on Python 2.6.5, but I'd be curious to hear responses for Python 3 as well.

What is the intent of such exercise? When using `global` you declare you intend to meddle with variable that does not belong to you (presumably s/o else made) - but why would you override it right away, even before checking what's in it? Does not seem to make practical sense... — Nas Banov, Jun 21 '10 at 23:46
I agree, I've always wanted to do this. I also want smarter tuple unpacking, but you can't win them all :) — Matt Joiner, Jun 22 '10 at 01:24
@NasBanov: I almost never check what is in it before overwriting a global variable? Most of the time, this is some kind of setting like speaker sound or mouse speed, why would I need to check what the current one is before setting it to the new one the user requested? If I want to _read_ a global variable, thanks to Python's scoping, I don't even need the global keyword most of the time! — Thomas, Jun 06 '19 at 09:15

score 21 · Accepted Answer · answered Jun 22 '10 at 00:00

You could use my favorite alternative to global (a pretty idiosyncratic taste...):

import sys
thismodule = sys.modules[__name__]
thismodule.foo = 'bar'

def update_foo():
  thismodule.foo = 'baz'

Once you've made the thismodule reference, you don't need to use global in this module, because you're always working with qualified names rather than bare names (a much better idea IMHO... but maybe in MHO only, I've not been able to convince Guido to supply thismodule [[or some other identifier with this functionality]] back when Python 3 was gestating).

Note that the first assignment to foo, at global level, can be done either with this explicit syntax, or by assigning to barename foo as you do in your code (I guess it's not surprising that my preference goes to the explicit form, though, in this case, just barely).

Giving the module a 'this' keyword? For some reason that just gives me the heebiejeebies. If I were Guido, I'd make global declarations include a giant red flashing warning sign. — Evan Plaice, Jun 22 '10 at 00:07

score 7 · Answer 2 · answered Jun 21 '10 at 23:37

7

It's two statements, there aren't any other forms.

answered Jun 21 '10 at 23:37

Ian Bicking

9,762
6
33
32

...only if you've sworn to only ever use barenames...;-). – Alex Martelli Jun 22 '10 at 00:00

score 6 · Answer 3 · answered Jun 21 '10 at 23:39

6

You could write it like this using the globals() dictionary:

def update_foo():
  globals()['foo'] = 'baz'

but I would just stick with the 2 lines or the separating with a ; approach.

answered Jun 21 '10 at 23:39

mikej

65,295
17
152
131

True, although generally messing with `globals()` (and even more so `locals()`) is not recommended. – David Z Jun 21 '10 at 23:41
1

From http://www.python.org/dev/peps/pep-0020/, "Explicit is better than implicit". – Evan Plaice Jun 21 '10 at 23:47
@DavidZ can you expound upon why it is not recommended? – Grant Humphries Jun 28 '19 at 23:08

score 5 · Answer 4 · answered Jun 22 '10 at 00:40

5

def update_foo():
    globals().update(foo='baz')

answered Jun 22 '10 at 00:40

John La Rooy

295,403
53
369
502

score 2 · Answer 5 · answered Jun 21 '10 at 23:48

2

If it makes you feel better to put it all on one line...

global foo; foo = 'baz'

answered Jun 21 '10 at 23:48

Evan Plaice

13,944
6
76
94

Overwrite global var in one line in Python?

5 Answers5

Linked