rspec/rules/S6417/python/rule.adoc

This rule raises an issue when a method modifies the size of a collection, while the same collection is iterated.

== Why is this an issue?

Iterating over a collection using a `for` loop in Python relies on iterators.

An iterator is an object that allows you to traverse a collection of elements, such as a list or a dictionary. Iterators are used in `for` loops to iterate over the elements of a collection one at a time.

When you create an iterator, it keeps track of the current position in the collection and provides a way to access the next element. The `next()` function is used to retrieve the next element from the iterator. When there are no more elements to iterate over, the `next()` function raises a StopIteration exception and the iteration stops.

It is important to note that iterators are designed to be read-only. Modifying a collection while iterating over it can cause unexpected behavior, as the iterator may skip over or repeat elements. A `RuntimeError` may also be raised in this situation, with the message `changed size during iteration`. Therefore, it is important to avoid modifying a collection while iterating over it to ensure that your code behaves as expected.

If you still want to modify the collection, it is best to use a second collection or to iterate over a copy of the original collection instead.


=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
def my_fun():
    my_dict = {'a': 1, 'b': 2, 'c': 3}
    for key in my_dict:
        if my_dict[key] == 'foo':
            my_dict.pop(key) # Noncompliant: this will make the iteration unreliable
----

=== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
def my_fun():
    my_dict = {'a': 1, 'b': 2, 'c': 3}
    for key in list(my_dict.keys()):
        if my_dict[key] == 'foo':
            my_dict.pop(key)
----


== Resources

=== Documentation

* Python Documentation - https://docs.python.org/3/tutorial/controlflow.html#for-statements[`for` Statements]