How Can I Update Local Variables Within Python's `exec` Function?

Overcoming Variable Update Limitations with Python Exec

In Python, the exec function allows for the dynamic execution of Python code at runtime. However, when used with local variables, it can behave unexpectedly.

The Problem:

Consider the following code:

def f():
    a = 1
    exec("a = 3")
    print(a)

f()

In Python 2, this code would print 3, indicating that the local variable a has been updated within the exec call. However, in Python 3, it prints 1, raising the question of how to get local variables updated during exec calls.

The Solution:

To address this issue, you need to explicitly pass a locals dictionary to the exec function:

def foo():
    ldict = {}
    exec("a = 3", globals(), ldict)
    a = ldict['a']
    print(a)

By using locals(), you create a new local variable namespace for the exec call. Modifications to this namespace will be reflected in the foo function's local scope.

Key Points:

• Python 3 optimizes local variable storage, affecting the behavior of exec.
• The default locals dictionary for exec cannot be modified in Python 3.
• Using an explicit locals dictionary allows for local variable updates within exec calls.

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