Passing an integer by reference in Python

Question

How can I pass an integer by reference in Python   I want to modify the value of a variable that I am passing to the function  I have read that everything in Python is pass by value  but there has to be an easy trick  For example  in Java you could pass the reference types of Integer  Long  etc    How can I pass an integer into a function by reference  What are the best practices

User · Answer

The correct answer  is to use a class and put the value inside the class  this lets you pass by reference exactly as you desire  class Thing    def   init   self a       self a   a def dosomething ref    ref a    1  t   Thing 3  dosomething t  print  quot T is now quot  t a

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Maybe it s not pythonic way  but you can do this  import ctypes  def incr a       a    1  x   ctypes c int 1    create c-var incr ctypes ctypes byref x     passing by ref

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class PassByReference      def Change self  var           self a   var         print self a  s PassByReference   s Change 5

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Most cases where you would need to pass by reference are where you need to return more than one value back to the caller  A  best practice  is to use multiple return values  which is much easier to do in Python than in languages like Java   Here s a simple example   def RectToPolar x  y       r    x    2   y    2     0 5     theta   math atan2 y  x      return r  theta   return 2 things at once  r  theta   RectToPolar 3  4    assign 2 things at once

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It doesn t quite work that way in Python  Python passes references to objects  Inside your function you have an object -- You re free to mutate that object  if possible   However  integers are immutable  One workaround is to pass the integer in a container which can be mutated   def change x       x 0    3  x    1  change x  print x   This is ugly clumsy at best  but you re not going to do any better in Python  The reason is because in Python  assignment     takes whatever object is the result of the right hand side and binds it to whatever is on the left hand side   or passes it to the appropriate function    Understanding this  we can see why there is no way to change the value of an immutable object inside a function -- you can t change any of its attributes because it s immutable  and you can t just assign the  variable  a new value because then you re actually creating a new object  which is distinct from the old one  and giving it the name that the old object had in the local namespace   Usually the workaround is to simply return the object that you want   def multiply by 2 x       return 2 x  x   1 x   multiply by 2 x       In the first example case above  3 actually gets passed to x   setitem

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class Obj    def   init   self a       self value   a   def sum self  a       self value    a      a   Obj 1  b   a a sum 1  print a value  b value    2 2

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A numpy single-element array is mutable and yet for most purposes  it can be evaluated as if it was a numerical python variable  Therefore  it s a more convenient by-reference number container than a single-element list       import numpy as np     def triple var by ref x           x 0  x 0  3     a np array  2       triple var by ref a      print a 1    output   3

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Not exactly passing a value directly  but using it as if it was passed   x   7 def my method        nonlocal x     x    1 my method   print x    8   Caveats    nonlocal was introduced in python 3 If the enclosing scope is the global one  use global instead of nonlocal

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Really  the best practice is to step back and ask whether you really need to do this  Why do you want to modify the value of a variable that you re passing in to the function   If you need to do it for a quick hack  the quickest way is to pass a list holding the integer  and stick a  0  around every use of it  as mgilson s answer demonstrates   If you need to do it for something more significant  write a class that has an int as an attribute  so you can just set it  Of course this forces you to come up with a good name for the class  and for the attribute   if you can t think of anything  go back and read the sentence again a few times  and then use the list   More generally  if you re trying to port some Java idiom directly to Python  you re doing it wrong  Even when there is something directly corresponding  as with static  staticmethod   you still don t want to use it in most Python programs just because you d use it in Java

User · Answer

In Python  everything is passed by value  but if you want to modify some state  you can change the value of an integer inside a list or object that s passed to a method

User · Answer

In Python  every value is a reference  a pointer to an object   just like non-primitives in Java  Also  like Java  Python only has pass by value  So  semantically  they are pretty much the same   Since you mention Java in your question  I would like to see how you achieve what you want in Java  If you can show it in Java  I can show you how to do it exactly equivalently in Python

User · Answer

Maybe slightly more self-documenting than the list-of-length-1 trick is the old empty type trick   def inc i v       v i    1  x   type               x i   7 inc i x  print x i

[python] Passing an integer by reference in Python

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