Use of global keyword in Python

Question

What I understand from reading the documentation is that Python has a separate namespace for functions  and if I want to use a global variable in that function  I need to use global   I m using Python 2 7 and I tried this little test   gt  gt  gt  sub     0    0    0    0    gt  gt  gt  def getJoin            return     join sub       gt  gt  gt  getJoin    0 0 0 0    It seems things are working fine even without global  I was able to access global variable without any problem   Am I missing anything  Also  following is from Python documentation      Names listed in a global statement   must not be defined as formal   parameters or in a for loop control   target  class definition  function   definition  or import statement    While formal parameters and class definition make sense to me  I m not able to understand the restriction on for loop control target and function definition

User · Answer

Accessing a name and assigning a name are different. In your case, you are just accessing a name.

If you assign to a variable within a function, that variable is assumed to be local unless you declare it global. In the absence of that, it is assumed to be global.

>>> x = 1         # global 
>>> def foo():
        print x       # accessing it, it is global

>>> foo()
1
>>> def foo():   
        x = 2        # local x
        print x 

>>> x            # global x
1
>>> foo()        # prints local x
2

User · Answer

Global makes the variable  Global   def out        global x     x   1     print x      return   out    print  x    This makes  x  act like a normal variable outside the function  If you took the global out then it would give an error since it cannot print a variable inside a function   def out           Taking out the global will give you an error since the variable x is no longer  global  or in other words  accessible for other commands     x   1     print x      return   out    print  x

User · Answer

It means that you should not do the following   x   1  def myfunc      global x      formal parameter   def localfunction x       return x 1      import statement   import os path as x      for loop control target   for x in range 10       print x      class definition   class x object       def   init   self         pass     function definition   def x        print  I m bad

User · Answer

Any variable declared outside of a function is assumed to be global  it s only when declaring them from inside of functions  except constructors  that you must specify that the variable be global

User · Answer

This is the difference between accessing the name and binding it within a scope   If you re just looking up a variable to read its value  you ve got access to global as well as local scope   However if you assign to a variable who s name isn t in local scope  you are binding that name into this scope  and if that name also exists as a global  you ll hide that     If you want to be able to assign to the global name  you need to tell the parser to use the global name rather than bind a new local name - which is what the  global  keyword does   Binding anywhere within a block causes the name everywhere in that block to become bound  which can cause some rather odd looking consequences  e g  UnboundLocalError suddenly appearing in previously working code     gt  gt  gt  a   1  gt  gt  gt  def p        print a    accessing global scope  no binding going on  gt  gt  gt  def q        a   3   binding a name in local scope - hiding global     print a   gt  gt  gt  def r        print a    fail - a is bound to local scope  but not assigned yet     a   4  gt  gt  gt  p   1  gt  gt  gt  q   3  gt  gt  gt  r   Traceback  most recent call last     File   lt pyshell 35 gt    line 1  in  lt module gt      r     File   lt pyshell 32 gt    line 2  in r     print a    fail - a is bound to local scope  but not assigned yet UnboundLocalError  local variable  a  referenced before assignment  gt  gt  gt

User · Answer

The keyword global is only useful to change or create global variables in a local context  although creating global variables is seldom considered a good solution   def bob        me    locally defined       Defined only in local context     print me   bob   print me        Asking for a global variable   The above will give you   locally defined Traceback  most recent call last     File  file py   line 9  in  lt module gt      print me  NameError  name  me  is not defined   While if you use the global statement  the variable will become available  outside  the scope of the function  effectively becoming a global variable   def bob        global me     me    locally defined      Defined locally but declared as global     print me   bob   print me        Asking for a global variable   So the above code will give you   locally defined locally defined   In addition  due to the nature of python  you could also use global to declare functions  classes or other objects in a local context  Although I would advise against it since it causes nightmares if something goes wrong or needs debugging

User · Answer

The other answers answer your question  Another important thing to know about names in Python is that they are either local or global on a per-scope basis   Consider this  for example   value   42  def doit        print value     value   0  doit   print value   You can probably guess that the value   0 statement will be assigning to a local variable and not affect the value of the same variable declared outside the doit   function  You may be more surprised to discover that the code above won t run  The statement print value inside the function produces an UnboundLocalError   The reason is that Python has noticed that  elsewhere in the function  you assign the name value  and also value is nowhere declared global  That makes it a local variable  But when you try to print it  the local name hasn t been defined yet  Python in this case does not fall back to looking for the name as a global variable  as some other languages do  Essentially  you cannot access a global variable if you have defined a local variable of the same name anywhere in the function

User · Answer

While you can access global variables without the global keyword  if you want to modify them you have to use the global keyword  For example   foo   1 def test        foo   2   new local foo  def blub        global foo     foo   3   changes the value of the global foo   In your case  you re just accessing the list sub

User · Answer

This is explained well in the Python FAQ  What are the rules for local and global variables in Python  In Python  variables that are only referenced inside a function are implicitly global  If a variable is assigned a value anywhere within the function   s body  it   s assumed to be a local unless explicitly declared as global  Though a bit surprising at first  a moment   s consideration explains this  On one hand  requiring global for assigned variables provides a bar against unintended side-effects  On the other hand  if global was required for all global references  you   d be using global all the time  You   d have to declare as global every reference to a built-in function or to a component of an imported module  This clutter would defeat the usefulness of the global declaration for identifying side-effects   https   docs python org 3 faq programming html what-are-the-rules-for-local-and-global-variables-in-python

User · Answer

You can access global keywords without keyword global To be able to modify them you need to explicitly state that the keyword is global  Otherwise  the keyword will be declared in local scope    Example   words           def contains  word        global words                lt - not really needed     return  word in words    def add  word        global words               must specify that we re working with a global keyword     if word not in words           words     word

[python] Use of "global" keyword in Python

Examples related to python

Examples related to global-variables