How to Correctly Update Local Variables with Python's `exec` Function?

How to Update Local Variables Using exec in Python

The exec function in Python allows code to be dynamically executed at runtime. However, when using exec to assign values to local variables, certain behaviors can be unexpected.

The Issue in Python 3

In Python 3, the following code prints 1 instead of the expected 3:

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

f()

This is because in Python 3, the exec function defaults to modifying the global namespace. To modify local variables, an explicit locals dictionary must be provided.

Solution in Python 3

The correct way to update local variables using exec in Python 3 is:

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

Python 2 vs. Python 3 Behavior

In Python 2, the original code behaved as expected because it assumed that the exec function would modify the local namespace. However, Python 3 changed this behavior for optimization purposes.

Technical Details

In Python 3, local variables are typically stored in an array, not a dictionary. This optimization prevents the overhead of maintaining a dictionary for local variables. However, it also restricts the ability of exec to modify local variables directly.

By providing an explicit locals dictionary, the exec function is instructed to execute the code in that namespace, allowing local variables to be updated.

Conclusion

To update local variables using exec in Python 3, an explicit locals dictionary must be provided. This ensures that the modification occurs in the correct namespace and prevents unexpected code behavior.

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