24

Suppose I have a global variable a. And within a function definition, we also have a local variable named a. Is there any way to assign the value of the global variable to that of the local variable?

a = 'foo'

def my_func(a = 'bar'):
    # how to set global a to value of the local a?
tskuzzy
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  • use 'global a' statement within the function definition? – newtover Apr 19 '12 at 20:13
  • @newtover: But then I can't access the value of the local `a` in order to assign it to the global one. – tskuzzy Apr 19 '12 at 20:15
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    `WHY???!!1!11one|11!1ELEVE|\|1!?`. To start, using globals is a bad practice, add a parameter to your func or make a class. Second, why would you want to use the same variable name in different contexts and relate their content. Can you try harder in making your code more ugly and confusing? – KurzedMetal Apr 19 '12 at 20:19
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    This is exactly why using global variables is discouraged. Even if there is a way to do this, you shouldn't do it. Change the local variable name, or -- better yet -- don't use a global variable. – senderle Apr 19 '12 at 20:20
  • @KurzedMetal: I'm asking this to better understand the language (all aspects of it, not just "best practices"). – tskuzzy Apr 19 '12 at 20:22
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    A bit late to the party, but whenever I hit the local/global collision issue, I use an empty class as a namespace container (`class Container(): pass`, `settings = Container()`, `settings.a = 'foo'`) and store my global variables in there. It's both mutable and distinguishable: `if var1 is None: var1 = settings.var1`, `else: settings.var1 = var1` and so on. – Nisan.H May 09 '12 at 20:34

3 Answers3

38

Use built-in function globals().

globals()

Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called).

a = 'foo'

def my_func(a = 'bar'):
    globals()['a'] = a

BTW, it's worth mentioning that a global is only "global" within the scope of a module.

Rob Stewart
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vartec
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  • Can you explain more on the note you provided that "global is only "global" within the scope of a module" by detailing what is a module here? – Saurabh Saha Aug 29 '19 at 06:44
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    @SaurabhSaha typically single file. Module, as in something that can be `import`-ed https://docs.python.org/3/tutorial/modules.html – vartec Sep 06 '19 at 22:24
  • Thanks @vartec. It would mean we can have global variables with the same name in different modules. – Saurabh Saha Oct 31 '19 at 08:02
2

Don't muddle global and local namespaces to begin with. Always try and use a local variable versus a global one when possible. If you must share variables between scopes you can still pass the variables without need for a global placeholder. Local variables are also referenced much more efficiently accessed than globals.

A few links:

Sharing Variables in Python

Variable performance

PenguinCoder
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1
>>> a = 'foo'
>>> def my_func(a='bar'):
...     return globals()['a']
...
>>> my_func()
'foo'
KurzedMetal
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