Execute script from string with accessing the variables?

Question

I have a python script as a string, as example:

exec("sent = {'test': 1}")
global sent
print(sent)

I executed it using exec function, then I accessed the variable using global python command. This way works without problem without using classes, but when I have the same code in a class, like:

class example:
    def fun1(self):
        exec("sent = {'test': 1}")
        global sent
        print(sent)

v = example()
print(v.fun1())

I get the following error:

NameError: name 'sent' is not defined

Try: exec("global sent;sent = {}") . But avoid using global variables if you can. — kantal
– kantal, Commented Nov 19, 2018 at 19:52
Ghanem: What do you mean it's too long? Strings can be almost any length in Python. — martineau
– martineau, Commented Nov 20, 2018 at 0:40
It's a dataset, someone build it in a code format .. "dummy way" — Minions
– Minions, Commented Nov 20, 2018 at 9:26

John M I Davis · Accepted Answer · 2018-11-19 20:03:00Z

2

You are not passing the global dictionary to modify. Try:

 exec("sent = {}",globals())

answered Nov 19, 2018 at 20:03

John M I Davis

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2 Comments

Minions Over a year ago

to make the question clear, I append a value to the "sent" variable

John M I Davis Over a year ago

The original question was trying to initialize 'sent' to an empty array. This does that. you may also set 'send' to a given dictionary, as in the modified question by: exec("sent = {'test': 1}",globals()) or modify a dictionary that was previously defined as global: exec(" my_global_dictionary['new_key'] = 'new_value'", global()) but adding the second argument, a reference to the global dictionary is crucial. If you do not add this exec creates a global dictionary = {} which is discarded after the function is called.

martineau · Accepted Answer · 2018-11-20 00:32:36Z

1

You really should avoid using global variables. Regardless, here's how to do it:

class example:
    def fun1(self):
#        globals sent  # Not needed in this special case.
        exec("sent = {}", globals())
        print('in fun1, "sent" is now', sent )


v = example()
print(v.fun1())  # Result will be None because fun1() doesn't return anything.
print('after call to fun1(), global "sent" is', sent)

Output:

in fun1, "sent" is now {}
None
after call to fun1(), global "sent" is {}

A global declaration only does something inside a function or class method and even then is only needed when the global variable's value is going to be set to something. However, as a special case, one isn't really needed here in the fun1() method because it explicitly passes globals() (but not a separate locals dict) when it calls exec(). It might be a good idea to put one in anyway to make it more clear what's going on.

Using exec() this way is explained in its documentation which says:

If only globals is provided, it must be a dictionary, which will be used for both the global and the local variables.

(emphasis mine)

Here's a way to avoid referencing a global variable at all in the method:

class example:
    def fun1(self):
        namespace = {}
        exec("sent = {}", namespace)
        sent = namespace['sent']  # Retrieve result.
        print('in fun1, "sent" is now', sent )
        return sent

v = example()
sent = v.fun1()
print('after calling fun1(), "sent" got set to', sent)

Output:

in fun1, "sent" is now {}
after calling fun1(), "sent" got set to {}

edited Nov 20, 2018 at 0:32

answered Nov 19, 2018 at 19:54

martineau

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2 Comments

Minions Over a year ago

it didn't work, it prints "None .. regarding to: "You really should avoid using global variables", what is the alternative in such a case?

martineau Over a year ago

Ghanem: I think it works now, sorry about that. Alternatives to using globals depend on exactly what you're doing. The most common way is by passing them to the function/method as arguments (if they have mutable values). The other is by returning them from the function/method. i.e. sent = v1.fun1() (assuming you also put a return sent statement at the end of the method).

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