How Does the Behaviour of the `exec` Function Differ in Python 2 and Python 3?

Behavior of exec Function in Python 2 vs. Python 3

In Python 2 and Python 3, the exec function exhibits notable differences in behavior.

Reason for the Difference

In Python 2, exec was a statement that explicitly disabled local scope optimizations and allowed access to variables in both local and global scopes. In contrast, Python 3's exec() is a function that always optimizes function scopes.

Impact on Variable Binding

In Python 2, when using the exec statement, variables found in locals() were copied back to the function locals. This resulted in binding the variable inside the function to the values specified in the exec statement.

In Python 3, however, exec() does not have this behavior by default. Therefore, variables inside the function are not bound to those in the exec() statement.

Accessing Local Variables

To access local variables using exec() in Python 3, a new namespace (typically a dictionary) should be created and passed as an argument to the function.

Revised Code

Below is a revised code example that demonstrates the correct use of exec() in Python 3 to achieve the behavior of Python 2:

def execute(a, st):
    namespace = {}
    exec("b = {}\nprint('b:', b)".format(st), namespace)
    print(namespace['b'])

In this code, a dictionary named namespace is created and passed to exec(). Any variables defined within the exec() statement will be accessible through the dictionary.

Notes

The exec() documentation explicitly warns against attempting modifications to the default locals dictionary, as it may not reflect changes made within the exec() call.

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