Converting a String containing text for a Python variable assignment into an actual variable

Question

I'm trying to convert a String containing a Python variable assignment into an actual variable.

The following was working for me.

    s = "VAR = [1,55]"
    exec(s)
    print(VAR)

But then when places this code in a function, VAR never gets defined.

    def myFunction():
        s = "VAR = [1,55]"
        exec(s)
        print(VAR)

    myFunction()

I'm not sure what I am missing. Thanks in advance for the help!

Responses to a few of the questions...

Error message: "NameError: name 'VAR' is not defined"

Using: Python 3

This is usually a really bad idea. Can I ask what you're using this for? — Chad S.
– Chad S., Commented Sep 10, 2015 at 22:38

Community · Accepted Answer · 2017-05-23 11:58:05Z

2

You can also pass globals to exec:

def myFunction():
    s = "VAR = [1,55]"
    exec(s, globals())
    print(VAR)

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answered Sep 10, 2015 at 22:47

Padraic Cunningham

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Comments

allencharp · Accepted Answer · 2015-09-10 22:45:44Z

2

python 2.7

def myfunc():
    v1 = 11
    exec "v1 = 22"
    print v1

myfunc() # 22

def myFunction():
    VAR = None
    s = "VAR = [1,55]"
    exec(s)
    print(VAR)

myFunction() # [1, 55]

OR

def myFunction():
    #VAR = None
    s = "VAR = [1,55]"
    exec(s)
    print(locals()["VAR"])

myFunction() # [1,55]

edited Sep 10, 2015 at 22:45

answered Sep 10, 2015 at 22:39

allencharp

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3 Comments

Warren Weckesser Over a year ago

I suspect the OP is using Python 3, where exec is a function.

jojonas Over a year ago

Is there any official documentation of this behavior? I noticed that depending whether one uses VAR later, exec decides on inserting it into locals() (perculiar: if one uses it, exec does not insert it into locals())

rbaleksandar Over a year ago

@WarrenWeckesser The OP uses exec(s) which is a function call.

rbaleksandar · Accepted Answer · 2015-09-10 22:53:03Z

1

In Python 2.7 you can simply do:

>> s = "VAR=55"
>> exec(s)
>> VAR
55

If you need custom namespace do:

>> my_ns = {}
>> exec "VAR=55" in my_ns
>> my_ns["VAR"]
55

Similar applies for Python 3 but exec there is actually a function so exec(...) is to be used. For example:

>> s = "VAR=55"
>> exec(s)
>> VAR
55

When you use functions scope comes into play. You can use locals(), globals() or use your custom namespace:

>>def foo():
    ns = {}
    s = "VAR=55"
    exec(s) in ns
    print(ns["VAR"])

>>foo()
55

edited Sep 10, 2015 at 22:53

answered Sep 10, 2015 at 22:46

rbaleksandar

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1 Comment

rbaleksandar Over a year ago

And damn it I type slow! Started typing first and finished last. LOL

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