How to Efficiently Test Membership of Multiple Values in a Python List: Iterating, Sets, or all()?

Testing Membership of Multiple Values in a List Through Iteration

Python's behavior when testing multiple values for membership in a list can be confusing. Using 'a','b' in ['b', 'a', 'foo', 'bar'] returns ('a', True), indicating that the result is a tuple where the first element is the first value checked and the second element is the membership of that value. To test for the membership of multiple values in a list, the following approach is recommended:

all(x in ['b', 'a', 'foo', 'bar'] for x in ['a', 'b'])

This statement uses a generator expression within the all() function to test each value in the second list for membership in the first.

Alternative Approaches

While the iteration-based method is generally reliable, alternative approaches exist. Using sets provides a subset test option:

set(['a', 'b']).issubset(set(['a', 'b', 'foo', 'bar']))

However, sets can only contain hashable elements, limiting their application.

Speed Considerations

The subset test is generally faster, but the difference is only significant when both the container and items are small. In most cases, using the all() function is still efficient.

If the items are already in a list, converting them to a set before using the subset test can provide a slight speedup. Converting a container that is not a set to a set is not recommended, as the speedup is minimal and the additional storage overhead can be problematic.

Exceptional Cases

When testing for membership of a large number of values, especially if some are not in the container, all() can provide a significant speed advantage over the subset test. This is due to its short-circuiting behavior, which allows it to bypass testing elements that are not present in the container.

Summary

For general use, converting the container to a set is recommended if its elements are hashable. The subset test is advantageous only if the test items are already stored in a set. In certain situations, such as testing membership of a large number of values, all() provides superior performance.

The above is the detailed content of How to Efficiently Test Membership of Multiple Values in a Python List: Iterating, Sets, or all()?. For more information, please follow other related articles on the PHP Chinese website!