Defining lists as global variables in Python

Question

I am using a list on which some functions works in my program  This is a shared list actually and all of my functions can edit it  Is it really necessary to define it as  global  in all the functions   I mean putting the global keyword behind it in each function that uses it  or defining it outside of all the functions is enough without using the global word behind its definition

User · Answer

Yes  you need to use global foo if you are going to write to it   foo       def bar        global foo             foo    1

User · Answer

When you assign a variable  x         you are creating a variable in the current scope  e g  local to the current function   If it happens to shadow a variable fron an outer  e g  global  scope  well too bad - Python doesn t care  and that s a good thing   So you can t do this   x   0 def f        x   1 f   print x    gt 0   and expect 1  Instead  you need do declare that you intend to use the global x   x   0 def f        global x     x   1 f   print x    gt 1   But note that assignment of a variable is very different from method calls  You can always call methods on anything in scope - e g  on variables that come from an outer  e g  the global  scope because nothing local shadows them   Also very important  Member assignment  x name         item assignment  collection key          slice assignment  sliceable start end         and propably more are all method calls as well  And therefore you don t need global to change a global s members or call it methods  even when they mutate the object

User · Answer

No  you can specify the list as a keyword argument to your function   alist      def fn alist alist       alist append 1  fn   print alist       1    I d say it s bad practice though  Kind of too hackish  If you really need to use a globally available singleton-like data structure  I d use the module level variable approach  i e  put  alist  in a module and then in your other modules import that variable   In file foomodule py   alist        In file barmodule py   import foomodule def fn        foomodule alist append 1  print foomodule alist       1

[python] Defining lists as global variables in Python

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