Is It Safe to Modify a Python Dictionary While Iterating Over It?

Exploring the Safety of Item Manipulation During Dictionary Iteration

The act of iterating over a Python dictionary (dict) while modifying its contents can be a tricky business. Some developers may wonder if this practice is well-defined and safe.

Safe and Unsafe Operations

According to experts like Alex Martelli, it is generally safe to modify the value at an existing index of the dict while iterating. However, inserting new items into the dict may not be.

The Problem with Item Deletion

Specifically, deleting items from the dict during iteration can be problematic. The reason lies in the underlying implementation of dict iteration in Python.

Python's dict iteration methods (such as iteritems() and items()) maintain a reference to the dict itself. This means that any modifications made to the dict during iteration will impact the iterator's behavior.

Example: Deleting an Item

Consider the following code:

for k, v in d.iteritems():
    del d[f(k)]

When the del statement is executed, it removes the item corresponding to f(k) from the dict. However, since the iterator still holds a reference to the modified dict, it is possible for it to attempt to visit the deleted item later in the loop. This can lead to a RuntimeError.

Safeguarded Iteration

To avoid the risk of modifying the underlying dict while iterating, it is recommended to use d.copy() to create an independent copy of the dict before iterating. The following code snippet demonstrates this:

for k, v in d.copy().items():
    del d[f(k)]

By iterating over the copy, the underlying dict remains untouched, eliminating the potential for iteration errors.

Conclusion

Modifying a dict while iterating over it is not inherently safe, particularly when it involves item deletion. By understanding the underlying mechanism and employing safe practices like d.copy(), developers can avoid potential pitfalls and ensure the reliability of their Python code.

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