How do I create variable variables

Question

How do I accomplish variable variables in Python  Here is an elaborative manual entry  for instance  Variable variables I hear this is a bad idea in general though  and it is a security hole in PHP  Is that true

User · Answer

Any set of variables can also be wrapped up in a class. "Variable" variables may be added to the class instance during runtime by directly accessing the built-in dictionary through __dict__ attribute.

The following code defines Variables class, which adds variables (in this case attributes) to its instance during the construction. Variable names are taken from a specified list (which, for example, could have been generated by program code):

# some list of variable names
L = ['a', 'b', 'c']

class Variables:
    def __init__(self, L):
        for item in L:
            self.__dict__[item] = 100

v = Variables(L)
print(v.a, v.b, v.c)
#will produce 100 100 100

User · Answer

Python 2 7 16  default  Jul 13 2019  16 01 51     GCC 8 3 0  on linux2  Creating variables and unpacking tuple  g   globals   listB      for i in range 10       g  quot num s quot    i    i    10     listB append  quot num 0  quot  format i    def printNum        print  quot Printing num0 to num9  quot      for i in range 10           print  quot num s    quot    i           print g  quot num s quot    i   printNum    listA      for i in range 10       listA append i   listA   tuple listA  print listA    quot Tuple to unpack quot    listB   str str listB  strip  quot    quot   replace  quot   quot    quot  quot      quot    listA quot    print listB  exec listB  printNum    Output  Printing num0 to num9  num0    0 num1    1 num2    1024 num3    59049 num4    1048576 num5    9765625 num6    60466176 num7    282475249 num8    1073741824 num9    3486784401  0  1  2  3  4  5  6  7  8  9   quot Tuple to unpack quot  num0  num1  num2  num3  num4  num5  num6  num7  num8  num9   listA Printing num0 to num9  num0    0 num1    1 num2    2 num3    3 num4    4 num5    5 num6    6 num7    7 num8    8 num9    9

User · Answer

Whenever you want to use variable variables  it s probably better to use a dictionary  So instead of writing   foo    bar    foo    baz    you write   mydict      foo    bar  mydict foo     baz    This way you won t accidentally overwrite previously existing variables  which is the security aspect  and you can have different  namespaces

User · Answer

I have tried both in python 3 7 3  you can use either globals   or vars     gt  gt  gt  food  Error  gt  gt  gt  milkshake  Error  gt  gt  gt  food  bread   gt  gt  gt  drink  milkshake   gt  gt  gt  globals   food     strawberry flavor   gt  gt  gt  vars   drink     chocolate flavor   gt  gt  gt  bread  strawberry flavor   gt  gt  gt  milkshake  chocolate flavor   gt  gt  gt  globals   drink   chocolate flavor   gt  gt  gt  vars   food   strawberry flavor      Reference  https   www daniweb com programming software-development threads 111526 setting-a-string-as-a-variable-name post548936

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The SimpleNamespace class could be used to create new attributes with setattr  or subclass SimpleNamespace and create your own function to add new attribute names  variables     from types import SimpleNamespace  variables     b   B   c   C   a   SimpleNamespace   variables  setattr a  g   G   a g    G   something   a a

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I m am answering the question  How to get the value of a variable given its name in a string  which is closed as a duplicate with a link to this question    If the variables in question are part of an object  part of a class for example  then some useful functions to achieve exactly that are hasattr  getattr  and setattr    So for example you can have   class Variables object       def   init   self           self foo    initial variable      def create new var self name value           setattr self name value      def get var self name           if hasattr self name               return getattr self name          else              raise  Class does not have a variable named    name    Then you can do   v   Variables   v get var  foo         initial variable    v create new var v foo  is actually not initial   v initial variable       is actually not initial

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You have to use globals   built in method  to achieve that behaviour  def var of var k  v       globals   k    v  print variable name   NameError  name  variable name  is not defined some name    variable name  globals   some name    123 print variable name    123  some name    variable name2  var of var some name  456  print variable name2    456

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Instead of a dictionary you can also use namedtuple from the collections module  which makes access easier   For example     using dictionary variables      variables  first     34 variables  second     45 print variables  first    variables  second       using namedtuple Variables   namedtuple  Variables     first    second    vars   Variables 34  45  print vars first  vars second

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Use the built-in getattr function to get an attribute on an object by name   Modify the name as needed   obj spam    eggs  name    spam  getattr obj  name     returns  eggs

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The consensus is to use a dictionary for this - see the other answers  This is a good idea for most cases  however  there are many aspects arising from this    you ll yourself be responsible for this dictionary  including garbage collection  of in-dict variables  etc  there s either no locality or globality for variable variables  it depends on the globality of the dictionary if you want to rename a variable name  you ll have to do it manually however  you are much more flexible  e g     you can decide to overwrite existing variables or          choose to implement const variables to raise an exception on overwriting for different types etc     That said  I ve implemented a variable variables manager-class which provides some of the above ideas  It works for python 2 and 3   You d use the class like this   from variableVariablesManager import VariableVariablesManager  myVars   VariableVariablesManager   myVars  test     25 print myVars  test       define a const variable myVars defineConstVariable  myconst   13  try      myVars  myconst     14    lt - this raises an error  since  myconst  must not be changed     print  not allowed   except AttributeError as e      pass    rename a variable myVars renameVariable  myconst    myconstOther      preserve locality def testLocalVar        myVars   VariableVariablesManager       myVars  test     13     print  inside function myVars  test      myVars  test    testLocalVar   print  outside function myVars  test      myVars  test       define a global variable myVars defineGlobalVariable  globalVar   12  def testGlobalVar        myVars   VariableVariablesManager       print  inside function myVars  globalVar      myVars  globalVar        myVars  globalVar     13     print  inside function myVars  globalVar    having been changed     myVars  globalVar    testGlobalVar   print  outside function myVars  globalVar      myVars  globalVar      If you wish to allow overwriting of variables with the same type only   myVars   VariableVariablesManager enforceSameTypeOnOverride   True  myVars  test     25 myVars  test      Cat     lt - raises Exception  different type on overwriting

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New coders sometimes write code like this   my calculator button 0   tkinter Button root  text 0  my calculator button 1   tkinter Button root  text 1  my calculator button 2   tkinter Button root  text 2        The coder is then left with a pile of named variables  with a coding effort of O m   n   where m is the number of named variables and n is the number of times that group of variables needs to be accessed  including creation   The more astute beginner observes that the only difference in each of those lines is a number that changes based on a rule  and decides to use a loop  However  they get stuck on how to dynamically create those variable names  and may try something like this   for i in range 10       my calculator   button  d    i    tkinter Button root  text i    They soon find that this does not work   If the program requires arbitrary variable  names   a dictionary is the best choice  as explained in other answers  However  if you re simply trying to create many variables and you don t mind referring to them with a sequence of integers  you re probably looking for a list  This is particularly true if your data are homogeneous  such as daily temperature readings  weekly quiz scores  or a grid of graphical widgets   This can be assembled as follows   my calculator buttons      for i in range 10       my calculator buttons append tkinter Button root  text i     This list can also be created in one line with a comprehension   my calculator buttons    tkinter Button root  text i  for i in range 10     The result in either case is a populated list  with the first element accessed with my calculator buttons 0   the next with my calculator buttons 1   and so on  The  base  variable name becomes the name of the list and the varying identifier is used to access it   Finally  don t forget other data structures  such as the set - this is similar to a dictionary  except that each  name  doesn t have a value attached to it  If you simply need a  bag  of objects  this can be a great choice  Instead of something like this   keyword 1    apple  keyword 2    banana   if query    keyword 1 or query    keyword 2      print  Match      You will have this   keywords     apple    banana   if query in keywords      print  Match      Use a list for a sequence of similar objects  a set for an arbitrarily-ordered bag of objects  or a dict for a bag of names with associated values

User · Answer

You can use dictionaries to accomplish this  Dictionaries are stores of keys and values     gt  gt  gt  dct     x   1   y   2   z   3   gt  gt  gt  dct   y   2   x   1   z   3   gt  gt  gt  dct  y   2   You can use variable key names to achieve the effect of variable variables without the security risk    gt  gt  gt  x    spam   gt  gt  gt  z    x   eggs    gt  gt  gt  z  spam    eggs    For cases where you re thinking of doing something like  var1    foo  var2    bar  var3    baz        a list may be more appropriate than a dict  A list represents an ordered sequence of objects  with integer indices   lst     foo    bar    baz   print lst 1               prints bar  because indices start at 0 lst append  potatoes      lst is now   foo    bar    baz    potatoes     For ordered sequences  lists are more convenient than dicts with integer keys  because lists support iteration in index order  slicing  append  and other operations that would require awkward key management with a dict

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If you don t want to use any object  you can still use setattr   inside your current module   import sys current module   module   sys modules   name       i e the  file  where your code is written setattr current module   variable name   15     15 is the value you assign to the var print variable name      gt  gt  gt  15  created from a string

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Use globals    You can actually assign variables to global scope dynamically  for instance  if you want 10 variables that can be accessed on a global scope i 1  i 2     i 10   for i in range 10       globals    i     format i      a    This will assign  a  to all of these 10 variables  of course you can change the value dynamically as well  All of these variables can be accessed now like other globally declared variable    gt  gt  gt  i 5  a

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It s not a good idea  If you are accessing a global variable you can use globals      gt  gt  gt  a   10  gt  gt  gt  globals    a   10   If you want to access a variable in the local scope you can use locals    but you cannot assign values to the returned dict   A better solution is to use getattr or store your variables in a dictionary and then access them by name

User · Answer

It should be extremely risky    but you can use exec    a    b 5  exec a  c   b 2 print  c   Result  10

[python] How do I create variable variables?

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