How to create module-wide variables in Python

Question

Is there a way to set up a global variable inside of a module  When I tried to do it the most obvious way as appears below  the Python interpreter said the variable   DBNAME   did not exist         DBNAME     None  def initDB name       if not   DBNAME              DBNAME     name     else          raise RuntimeError  Database name has already been set          And after importing the module in a different file      import mymodule mymodule initDB  mydb sqlite         And the traceback was            UnboundLocalError  local variable  DBNAME  referenced before assignment         Any ideas  I m trying to set up a singleton by using a module  as per this fellow s recommendation

User · Answer

Explicit access to module level variables by accessing them explicity on the module    In short  The technique described here is the same as in steveha s answer  except  that no artificial helper object is created to explicitly scope variables  Instead the module object itself is given a variable pointer  and therefore provides explicit scoping upon access from everywhere   like assignments in local function scope    Think of it like self for the current module instead of the current instance      db py import sys    this is a pointer to the module object instance itself  this   sys modules   name       we can explicitly make assignments on it  this db name   None  def initialize db name       if  this db name is None             also in local function scope  no scope specifier like global is needed         this db name   name           also the name remains free for local use         db name    Locally scoped db name variable  Doesn t do anything here       else          msg    Database is already initialized to  0            raise RuntimeError msg format this db name     As modules are cached and therefore import only once  you can import db py as often on as many clients as you want  manipulating the same  universal state     client a py import db  db initialize db  mongo       client b py import db  if  db db name     mongo        db db name   None    this is the preferred way of usage  as it updates the value for all clients  because they access the same reference from the same module object     client c py from db import db name   be careful when importing like this  as a new reference  db name  will   be created in the module namespace of client c  which points to the value    that  db db name  has at import time of  client c    if  db name     mongo       checking is fine if  db db name  doesn t change     db name   None    be careful  because this only assigns the reference client c db name to a new value  but leaves db db name pointing to its current value      As an additional bonus I find it quite pythonic overall as it nicely fits Pythons policy of Explicit is better than implicit

User · Answer

You are falling for a subtle quirk  You cannot re-assign module-level variables inside a python function  I think this is there to stop people re-assigning stuff inside a function by accident    You can access the module namespace  you just shouldn t try to re-assign  If your function assigns something  it automatically becomes a function variable - and python won t look in the module namespace   You can do     DB NAME     None  def func        if   DB NAME            connect   DB NAME        else          connect Default value    but you cannot re-assign   DB NAME   inside a function    One workaround     DB NAME      None   def func        if   DB NAME   0           connect   DB NAME   0       else            DB NAME   0    Default value   Note  I m not re-assigning   DB NAME    I m just modifying its contents

User · Answer

Steveha s answer was helpful to me  but omits an important point  one that I think wisty was getting at   The global keyword is not necessary if you only access but do not assign the variable in the function   If you assign the variable without the global keyword then Python creates a new local var -- the module variable s value will now be hidden inside the function   Use the global keyword to assign the module var inside a function   Pylint 1 3 1 under Python 2 7 enforces NOT using global if you don t assign the var   module var     dev hello   def readonly access        connect module var   def readwrite access        global module var     module var     dev hello2      connect module var

User · Answer

For this  you need to declare the variable as global  However  a global variable is also accessible from outside the module by using module name var name  Add this as the first line of your module   global   DBNAME

User · Answer

Here is what is going on   First  the only global variables Python really has are module-scoped variables   You cannot make a variable that is truly global  all you can do is make a variable in a particular scope    If you make a variable inside the Python interpreter  and then import other modules  your variable is in the outermost scope and thus global within your Python session    All you have to do to make a module-global variable is just assign to a name   Imagine a file called foo py  containing this single line   X   1   Now imagine you import it   import foo print foo X     prints 1   However  let s suppose you want to use one of your module-scope variables as a global inside a function  as in your example   Python s default is to assume that function variables are local   You simply add a global declaration in your function  before you try to use the global   def initDB name       global   DBNAME      add this line      if   DBNAME   is None    see notes below  explicit test for None           DBNAME     name     else          raise RuntimeError  Database name has already been set      By the way  for this example  the simple if not   DBNAME   test is adequate  because any string value other than an empty string will evaluate true  so any actual database name will evaluate true   But for variables that might contain a number value that might be 0  you can t just say if not variablename  in that case  you should explicitly test for None using the is operator   I modified the example to add an explicit None test   The explicit test for None is never wrong  so I default to using it   Finally  as others have noted on this page  two leading underscores signals to Python that you want the variable to be  private  to the module   If you ever do an import   from mymodule  Python will not import names with two leading underscores into your name space   But if you just do a simple import mymodule and then say dir mymodule  you will see the  private  variables in the list  and if you explicitly refer to mymodule   DBNAME   Python won t care  it will just let you refer to it   The double leading underscores are a major clue to users of your module that you don t want them rebinding that name to some value of their own   It is considered best practice in Python not to do import    but to minimize the coupling and maximize explicitness by either using mymodule something or by explicitly doing an import like from mymodule import something   EDIT  If  for some reason  you need to do something like this in a very old version of Python that doesn t have the global keyword  there is an easy workaround   Instead of setting a module global variable directly  use a mutable type at the module global level  and store your values inside it   In your functions  the global variable name will be read-only  you won t be able to rebind the actual global variable name    If you assign to that variable name inside your function it will only affect the local variable name inside the function    But you can use that local variable name to access the actual global object  and store data inside it   You can use a list but your code will be ugly     DBNAME      None    use length-1 list as a mutable    later  in code    if   DBNAME   0  is None        DBNAME   0    name   A dict is better   But the most convenient is a class instance  and you can just use a trivial class   class Box      pass    m   Box      m will contain all module-level values   m dbname   None    database name global in module    later  in code  if   m dbname is None        m dbname   name    You don t really need to capitalize the database name variable    I like the syntactic sugar of just using   m dbname rather than   m  DBNAME    it seems the most convenient solution in my opinion   But the dict solution works fine also   With a dict you can use any hashable value as a key  but when you are happy with names that are valid identifiers  you can use a trivial class like Box in the above

[python] How to create module-wide variables in Python?

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