Python : pass variable name as argument

Question

I have a function f(x) in which many local variables are created. x is a string with the same name as one of these local variables and I would like to change this local variable by changing x. What is the clean way to do this? Currently I am using a lot of if/elif statements.

Some dummy code to represent my problem:

def f(x):
         a = [1,2,3]
         b = [2,3,4]
         c = [3,4,5]
         if x == "a":
                     a[0] = 10
         elif x == "b":
                     b[0] = 10
         elif x == "c":
                     c[0] = 10
         return a,b,c

I would like for the right variable to change value but using all these if/elif statements feels a bit redundant.

Can you be more specific - e.g. post some code. Are you saying you want a variable inside `f` to take a value you pass in - but it depends on a string? Is this heading towards key word args? — doctorlove, Sep 26 '17 at 14:41
why do you need 2 variables with the same name with different purposes??? or i did not understand your question - please copy your code to the question — ddor254, Sep 26 '17 at 14:41
If you need to access a variable via a string holding a name, you don't want individual variables - you want a dictionary. — jasonharper, Sep 26 '17 at 14:44
lets wait for the code example , seems like no one understood him — ddor254, Sep 26 '17 at 14:45
Well, cross that. Your code seems correct @HolyMonk . There is usually not many options for replacing `if/else`. And since you are using only 3 such statements, the code is not messy or such . — Kaushik NP, Sep 26 '17 at 14:48
In my real example there are some more than 3 variables, but using a dictionary I still get reasonably elegant code. Thanks for responding! — HolyMonk, Sep 26 '17 at 14:52

score 5 · Answer 1 · answered Sep 26 '17 at 14:48

Use a `dict`

Simply, use a dict:

def f(x):
    values = {
        "a": [1,2,3],
        "b": [2,3,4],
        "c": [3,4,5]
    }
    values[x][0] = 10
    return values["a"], values["b"], values["c"]

If you really really want, use your original code and do locals()[x][0] = 10 but that's not really recommended because you could cause unwanted issues if the argument is the name of some other variable you don't want changed.

score 4 · Answer 2 · answered Sep 26 '17 at 14:49

4

use dictionary like this:

def f(x):
     d = {"a" :[1,2,3],"b" : [2,3,4],"c" : [3,4,5]}
     d[x][0] = 10

     return d

answered Sep 26 '17 at 14:49

ddor254

1,570
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score 1 · Answer 3 · answered Sep 26 '17 at 14:43

1

If your input is a string and you want to refer to a variable that matches that string you can use globals() like this:

globals()['x']

This way you can get the value and/or edit its contents.

answered Sep 26 '17 at 14:43

zipa

27,316
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4

Ok, so, 1) using `globals()` is a bad idea 99.999% of the time, and 2) you don't know whether x is a name at the global scope anyway. what if `x` is first created inside a `main` function, which then calls `f`? globals() won't have an 'x' entry in that case. – Kevin Sep 26 '17 at 14:46
How else is he going to access the variable, except maybe using `locals()`? Sure he can rewrite the code, but this answer is for this specific situation. – zipa Sep 26 '17 at 14:48
1

This is a classic X-Y problem, you answer their question too literally, use a `dict` instead – Chris_Rands Sep 26 '17 at 14:51
I guess it's a moot point now, since OP's edited post clarifies that it really doesn't have anything to do with differently scoped variables. – Kevin Sep 26 '17 at 14:51

score -2 · Answer 4 · answered Sep 26 '17 at 14:52

-2

You can use exec() and put your code in argument as a string

def f(x):
    a = [1,2,3]
    b = [2,3,4]
    c = [3,4,5]
    exec(x + "[0] = 10")

    return a,b,c

print f("a")
# ([10, 2, 3], [2, 3, 4], [3, 4, 5])

answered Sep 26 '17 at 14:52

B 7

670
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2

Sure you _can_, but you _shouldn't_. `exec` is both a security hazard and a performance hog. See for example https://stackoverflow.com/questions/2220699/whats-the-difference-between-eval-exec-and-compile-in-python?rq=1 – Guillaume Sep 26 '17 at 14:54

Python : pass variable name as argument

4 Answers4

Use a `dict`

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Related

Python : pass variable name as argument

4 Answers4

Use a dict

Linked

Related

Use a `dict`