Python: Change variable suffix with for loop

Question

I know this was asked a lot but I can not work with/understand the answers so far.

I want to change the suffix of variables in a for loop.

I tried all answers the stackoverflow search provides. But it is difficult to understand specific codes the questioner often presents.

So for clarification I use an easy example. This is not meant as application-oriented. I just want to understand how I can change the suffix.

var_1 = 10
var_2 = 100
var_3 = 1000

for i in range(1,4):
    test_i = var_i + 1
    print(test_i)

Expected result: creating and printing variables:

test_1 = 11 
test_2 = 101 
test_3 = 1001

Expected Output

11
101
1001

Error: var_i is read as a variable name without the changes for i.

If there were 100 values, do you create 100 variables? Use data structures to store values (ex: list). — Austin, Aug 18 '19 at 16:18
There is a way to do this, but it's almost certainly not what you want. Have you heard of a `list`? — Matt Messersmith, Aug 18 '19 at 16:18
Use a dictionary or a list. As mentioned, this is basically always a Bad Idea™ — beroe, Aug 18 '19 at 16:24
This puts into this conundrum. https://nedbatchelder.com/blog/201207/bad_answers_on_stack_overflow.html — beroe, Aug 18 '19 at 16:37

score 3 · Answer 1 · answered Aug 18 '19 at 16:37

3

I would advise against using eval in 99.99% of all cases. What you could do is use the built-in getattr function:

import sys
var_1 = 10
var_2 = 100
var_3 = 1000

for i in range(1,4):
    test_i = getattr(sys.modules[__name__], f"var_{i}") + 1
    print(test_i)

answered Aug 18 '19 at 16:37

Alexander Ejbekov

5,594
1
26
26

This looks painful but it works.
Just to make sure ```getattr(sys.modules[__name__], f"var_{i}")``` is my variable var_i right? – Martin Flower Aug 18 '19 at 16:43
That's correct. `sys.modules[__name__]` is the current module, whereas `f"var_{i}" is var 1 to 3. And [getattr](https://docs.python.org/3/library/functions.html#getattr) is simply there to access the variables – Alexander Ejbekov Aug 18 '19 at 16:54
the `test_i` line gives me a syntax error in python 2 and 3, and doesn't assign the variables `test_1`, `test_2` etc, just re-uses `test_i` – beroe Aug 19 '19 at 08:11
@beroe the f-string syntax is python 3.6+. 2.7 is almost out the door (thankfully) and [almost all major libraries](https://python3statement.org/) do not support it anymore and I'd advise you to ditch it as well. I'd put 3.5 on the same boat as well given [there's almost no movement there](https://github.com/python/cpython/commits/3.5) and it's original release was 5 years ago. I suggest you upgrade. – Alexander Ejbekov Aug 19 '19 at 13:55
Thanks Alexander. The f-strings do look fun, but I forgot they were not in 3.5. – beroe Aug 19 '19 at 23:22
I also solved the problem. Among the answers to similar questions, it is the easiest to follow and can be used in a variety of ways. – anfwkdrn Jun 15 '22 at 18:10

beroe · Accepted Answer · 2019-08-19T08:10:19.677

Instead of doing a convoluted naming convention, try to conceive of your problem using a data structure like dictionaries, for example.

var={}
var[1] = 10
var[2] = 100
var[3] = 1000

test={}
for i in range(1,4):
    test[i] = var[i] +1

print(test)

If somehow you are given var_1 etc as input, maybe use .split("_") to retrieve the index number and use that as the dictionary keys (they can be strings or values).

Small explanation about using indexing variable names. If you are starting out learning to program, there are many reasons not to use the eval, exec, or getattr methods. Most simply, it is inefficient, not scalable, and is extremely hard to use anywhere else in the script.

I am not one to insist on "best practices" if there is an easier way to do something, but this is something you will want to learn to avoid. We write programs to avoid having to type things like this.

If you are given that var_2 text as a starting point, then I would use string parsing tools to split and convert the string to values and variable names.

By using a dictionary, you can have 1000 non-consecutive variables and simply loop through them or assign new associations. If you are doing an experiment, for example, and call your values tree_1, tree_10 etc, then you will always be stuck typing out the full variable names in your code rather than simply looping through all the entries in a container called tree.

This is a little related to using a bunch of if:else statements to assign values:

# inefficient way -- avoid
if name == 'var_1' then:
    test_1=11
elif name == 'var_2' then:
    test_2=101

It is so much easier just to say:

test[i]= var[i]+1

and that one line will work for any number of values.

With this it just adds 10,100 and 100 to the list. It does not compute 10+1,100+1 and 1000+1. — Martin Flower, Aug 18 '19 at 16:47
Thanks. I fixed that but the point was just to show a possible alternative to all the `eval` answers. — beroe, Aug 19 '19 at 05:48
Thank you for your explanation. Now with the correct code this seems the most readable, easiest and low-risk solution. — Martin Flower, Aug 19 '19 at 19:26

amq · Answer 3 · 2019-08-18T18:14:16.280

0

for i in range(1, 4):
   print(eval('var_' + str(i)))

Step by step:

1) Make your variables strings:

stringified_number = str(i)

2) evaluate your expression during runtime:

evaluated_variable = eval('var_' + stringified_number)

edited Aug 18 '19 at 18:14

answered Aug 18 '19 at 16:34

amq

160
4
17

This produces the error: "NameError: name 'var_10' is not defined" – Martin Flower Aug 18 '19 at 16:52
@MartinFlower just removed the part with the list – amq Aug 18 '19 at 18:15
1

`var_1 = 10 var_2 = 100 var_3 = 1000 for i in range(1, 4): test_i = eval('var_' + str(i)) + 1 print(test_i)` Right. This works fine, Is understandable and good readable. – Martin Flower Aug 18 '19 at 18:32

Python: Change variable suffix with for loop

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