How to filter dictionary keys by substring in Python?

General Discussions
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1

How can I filter items in a Python dictionary where the keys contain a specific substring?

I’m a C programmer transitioning to Python, and I’m familiar with how this can be done in C. In C, I would iterate over the dictionary (or hash map) and check if the key contains a specific substring. If it does, I’d perform an operation; otherwise, I’d skip it.

For example, in C-like logic, it would look like this:

for (key in dictionary) {
    if (filter_string in key) {
        // do something
    } else {
        // do nothing, continue
    }
}

I’m wondering what the Pythonic way of doing this is.

For instance, I’m thinking that the Python equivalent might look like this:

filtered_dict = crazy_python_syntax(d, substring)
for key, value in filtered_dict.iteritems():
    # do something

I’ve seen many posts on filtering dictionaries, but none that exactly deal with filtering based on whether the keys contain a specific substring.

Note: I’m using Python 2.7.

2

I’ve worked with Python dictionaries extensively, and one of the most Pythonic ways to handle this is by using dictionary comprehension. It’s clean, efficient, and easy to read:

filtered_dict = {key: value for key, value in d.items() if substring in key}  
for key, value in filtered_dict.items():  
    # do something

This method works well when you want a quick and elegant way to filter your dictionary based on keys containing a specific substring.

3

Adding to Tim’s approach, if you prefer a functional programming style or need to combine this with other operations, you can use the filter function along with the dict constructor.

Here’s how it looks:

filtered_dict = dict(filter(lambda item: substring in item[0], d.items()))  
for key, value in filtered_dict.items():  
    # do something

This method is particularly useful if you’re already using functional programming techniques in your code, and it fits seamlessly with pipelines or other transformations.

4

Sometimes, you might want more explicit control over the filtering process. In those cases, a manual for loop works perfectly. While it’s slightly longer, it can be more flexible if you need to add extra logic.

filtered_dict = {}  
for key, value in d.items():  
    if substring in key:  
        filtered_dict[key] = value  
for key, value in filtered_dict.items():  
    # do something

This approach might feel a bit more verbose, but it’s great if you need to debug or extend your filtering logic with additional conditions.