Python function global variables

Question

I know I should avoid using global variables in the first place due to confusion like this  but if I were to use them  is the following a valid way to go about using them   I am trying to call the global copy of a variable created in a separate function    x    somevalue   def func A        global x      Do things to x    return x  def func B       x   func A        Do things    return x  func A   func B     Does the x that the second function uses have the same value of the global copy of x that func a uses and modifies  When calling the functions after definition  does order matter

User · Answer

As others have noted  you need to declare a variable global in a function when you want that function to be able to modify the global variable  If you only want to access it  then you don t need global   To go into a bit more detail on that  what  modify  means is this  if you want to re-bind the global name so it points to a different object  the name must be declared global in the function   Many operations that modify  mutate  an object do not re-bind the global name to point to a different object  and so they are all valid without declaring the name global in the function   d      l      o   type  object    object           def valid          these are all valid without declaring any names global     d 0    1        changes what s in d  but d still points to the same object    d 0     1       ditto    d clear         ditto  d is now empty but it s still the same object     l append 0      l is still the same list but has an additional member    o test   1      creating new attribute on o  but o is still the same object

User · Answer

You must use the global declaration when you wish to alter the value assigned to a global variable    You do not need it to read from a global variable  Note that calling a method on an object  even if it alters the data within that object  does not alter the value of the variable holding that object  absent reflective magic

User · Answer

Within a Python scope  any assignment to a variable not already declared within that scope creates a new local variable unless that variable is declared earlier in the function as referring to a globally scoped variable with the keyword global   Let s look at a modified version of your pseudocode to see what happens     Here  we re creating a variable  x   in the   main   scope  x    None    def func A        The below declaration lets the function know that we      mean the global  x  when we refer to that variable  not      any local one    global x   x    A    return x  def func B        Here  we are somewhat mislead   We re actually involving two different      variables named  x    One is local to func B  the other is global       By calling func A    we do two things  we re reassigning the value      of the GLOBAL x as part of func A  and then taking that same value      since it s returned by func A  and assigning it to a LOCAL variable      named  x          x   func A     look at this as  x local   func A        Here  we re assigning the value of  B  to the LOCAL x    x    B    look at this as  x local    B     return x   look at this as  return x local   In fact  you could rewrite all of func B with the variable named x local and it would work identically   The order matters only as far as the order in which your functions do operations that change the value of the global x   Thus in our example  order doesn t matter  since func B calls func A   In this example  order does matter   def a      global foo   foo    A   def b      global foo   foo    B   b   a   print foo   prints  A  because a   was the last function to modify  foo     Note that global is only required to modify global objects   You can still access them from within a function without declaring global  Thus  we have   x   5  def access only      return x     This returns whatever the global value of  x  is  def modify      global x   x    modified    return x     This function makes the global  x  equal to  modified   and then returns that value  def create locally      x    local     return x     This function creates a new local variable named  x   and sets it as  local        and returns that   The global  x  is untouched    Note the difference between create locally and access only -- access only is accessing the global x despite not calling global  and even though create locally doesn t use global either  it creates a local copy since it s assigning a value   The confusion here is why you shouldn t use global variables

User · Answer

If you want to simply access a global variable you just use its name  However to change its value you need to use the global keyword   E g   global someVar someVar   55   This would change the value of the global variable to 55  Otherwise it would just assign 55 to a local variable   The order of function definition listings doesn t matter  assuming they don t refer to each other in some way   the order they are called does

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Here is one case that caught me out  using a global as a default value of a parameter   globVar   None      initialize value of global variable  def func param   globVar       use globVar as default value for param     print  param     param   globVar     globVar    display values  def test        global globVar     globVar   42    change value of global     func    test             output  param   None  globVar   42   I had expected param to have a value of 42   Surprise   Python 2 7 evaluated the value of globVar when it first parsed the function func   Changing the value of globVar did not affect the default value assigned to param   Delaying the evaluation  as in the following  worked as I needed it to   def func param   eval  globVar             this seems to work     print  param     param   globVar     globVar    display values   Or  if you want to be safe   def func param   None        if param    None          param   globVar     print  param     param   globVar     globVar    display values

User · Answer

You can directly access a global variable inside a function  If you want to change the value of that global variable  use  global variable name   See the following example   var   1 def global var change          global var       var    value changed  global var change    call the function for changes print var   Generally speaking  this is not a good programming practice  By breaking namespace logic  code can become difficult to understand and debug

[python] Python function global variables?

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