How do I accomplish variable variables in Python?

Here is an elaborative manual entry, for instance: Variable variables

I have heard this is a bad idea in general though, and it is a security hole in Python. Is that true?

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it's the maintainance and debugging aspects that cause the horror. Imagine trying to find out where variable 'foo' changed when there's no place in your code where you actually change 'foo'. Imagine further that it's someone else's code that you have to maintain... OK, you can go to your happy place now. – glenn jackman
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The need does still arise, though. I used to think I needed to do this sort of thing all the time before I met real programming languages. Great suggestions here for transitioning to a saner mindset. – Jenn D.
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You can do this in the SAS macro languge. Sometimes you have to, because the only data type in SAS macros is the string :-/ – John Fouhy
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This is an excellent question to ask, if for no other reason than to help people learn how to avoid it. :) – SunSparc
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A further pitfall that hasn't been mentioned so far is if such a dynamically-created variable has the same name as a variable used in your logic. You essentially open up your software as a hostage to the input it is given. – holdenweb
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It's useful in rare places. I need it somewhere in my code, and it wouldn't cause maintenance problems. – sudo

12 Answers 11

up vote 146 down vote accepted

You can use dictionaries to accomplish this. Dictionaries are stores of keys and values.

>>> dct = {'x': 1, 'y': 2, 'z': 3}
>>> dct
{'y': 2, 'x': 1, 'z': 3}
>>> dct["y"]
2

You can use variable key names to achieve the effect of variable variables without the security risk.

>>> x = "spam"
>>> z = {x: "eggs"}
>>> 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:

l = ['foo', 'bar', 'baz']
print(l[1])           # prints bar, because indices start at 0
l.append('potatoes')  # l 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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I'm afraid I don't understand too well. Can you explain? – Pyornide
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Edited my answer to explain dictionaries. – chuckharmston
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Yeah, I understand now. Thanks. – Pyornide

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".

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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That works great with a namedtuple – kap
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Lovely. This is exactly what I needed. I have an object and needed to access variables by a string and I didn't want to use eval. Thanks! – rayryeng

It's not a good idea. If you are accessing a global variable you can use globals().

>>> a = 10
>>> 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.

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+1 answering the question as stated (even though it's horrible) – bobince
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Don't forget to mention that you can't modify variables through locals() (docs.python.org/library/functions.html#locals). – Glenn Maynard
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You can't set variables this way. You can't do globals()['a'] = 10. – sudo
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Thx, also working with locals()['x'] = "xxx" – oxidworks
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@oxidworks No, it's not working. However, it may seem to work if you're in the global context, where locals() simply returns the globals() dict. – PM 2Ring

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 # <- 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" # <- raises Exception (different type on overwriting)

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

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

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.

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"

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

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(**v)
setattr(a,"g","G")
a.g = "G+"
something = a.a

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)  # >>> 15, created from a string
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This one sounds better to me than using 'exec'. – fralau

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