Python NameError in a function body

Question

I have two blocks of python code, one works, and the other doesn't.

Working block:

env = {'user':'xyz'}
for key, value in env.items():
    exec("{} = value".format(key))

print(user)

output:

xyz

The block, which doesn't work:

def test():
    env = {'user':'xyz'}
    for key, value in env.items():
        exec("{} = value".format(key))

    print(user)

test()

output:

NameError: name 'user' is not defined

The only difference I see is that the first block is being called in the global scope.

Could you please explain?

Many thanks!

PS: With all due respect, I know, I should avoid using exec() but what if I want to.

The global scope is just a dictionary, and can be modified by exec or various other means. But the local scope of a function is carved in stone at the time the function was compiled (all references get turned into an index into an array), there's no way to add names at runtime. — jasonharper
– jasonharper, Commented Feb 8, 2021 at 2:35
@jasonharper thanks, if I print locals(), it confirms the presence of this variable:{'env': {'user': 'xyz'}, 'key': 'user', 'value': 'xyz', 'user': 'xyz'}. What are your thoughts? — Anil Bisht
– Anil Bisht, Commented Feb 8, 2021 at 2:40
As said above, it shows in locals() and also in the debugger, it shows up as a variable. Why does that happen? i.imgur.com/Ry1xoDX.png When I try to access it on the next line, it raises exception. — vighnesh153
– vighnesh153, Commented Feb 8, 2021 at 2:42
if you really need to use exec() then use it - but I would rather keep it as dictionary env['user'] = ... because it can be more useful then creating variables user = ... — furas
– furas, Commented Feb 8, 2021 at 3:33
@furas the whole point of discussion is that I am not able to use exec() — Anil Bisht
– Anil Bisht, Commented Feb 8, 2021 at 4:14

david190810 · Accepted Answer · 2021-02-08 05:07:40Z

1

I recommend you reading this

You have to use locals() or exec or eval to access variables defined by exec in a function in Python3.

def test():
    env = {'user': 'xyz'}
    for key, value in env.items():
        exec("{} = value".format(key))
    exec("print(user)")
    print(locals()['user'])
    print(eval("user"))
 
test()

It should be noted that, if you attempt to store value returned from eval. You will get NameError.

def test():
    env = {'user': 'xyz'}
    for key, value in env.items():
        exec("{} = value".format(key))
    user = eval("user")
    print(user)

test()

returns

Traceback (most recent call last):
  File "D:/Git/sscgc/test.py", line 8, in <module>
    test()
  File "D:/Git/sscgc/test.py", line 5, in test
    user = eval("user")
  File "<string>", line 1, in <module>
NameError: name 'user' is not defined

edited Feb 8, 2021 at 5:07

answered Feb 8, 2021 at 5:02

david190810

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1 Comment

Anil Bisht Over a year ago

I had similar findings!! Thanks, @david190810 for sharing the article. It was helpful

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