why can't I declare a global variable inside an "exec" string

Question

I am trying to declare and change a global variable in an exec string, like this:

ostr = "didn't work"
nstr = "worked"
def function():
    exec("global ostr; ostr = nstr")
    #global ostr; ostr = nstr
    print(ostr)
    lv='ostr' in globals()
    print(lv)
    ostr='asd'

function()

However, this errors out on the print statement with:

UnboundLocalError: local variable 'ostr' referenced before assignment

But, if I comment out the "exec" line and uncomment line after the exec statement, the code works fine.

How can I fix this error using "exec"? I want to declare variables global and modify those global variables inside an exec string and have those modifications visible in "function" on subsequent lines.

Answer 1

You have to declare that you are using global ostr in the function to be able to print it. This piece of code outputs

def function():
   global ostr
   exec("global ostr; ostr = nstr")
   #global ostr; ostr = nstr
   print(ostr)
   lv='ostr' in globals()
   print(lv)
   ostr='asd'

worked

True

Edit: Just realised the exec actually works with global variables, if you re-run your code and print(ostr) in the global main you will see it was changed.

ostr = "didn't work"
nstr = "worked"
def function():
    #global ostr
    exec("global ostr; ostr = nstr")

function()
print(ostr)

worked

Edit#2: Either declare ostr as a global variable before modifying it, or assign it to another local variable.

ostr = "didn't work"
nstr = "worked"
def function():
    exec("global ostr; ostr = nstr")
    #global ostr; ostr = nstr
    print(ostr)
    lv='ostr' in globals()
    print(lv)
        
function()

worked

True