How to Filter a Dictionary by Predicate on Values in Python

This code defines a reusable function that builds a new dictionary containing only the items whose values satisfy a given predicate function.

Easy Python 3.9+ Aug 9, 2026 Dictionaries & sets 13 views 0 copies

dictionary filtering lambda comprehension utility

Python code

15 lines

Python 3.9+

def filter_dict_by_predicate(d, predicate):
    """Return a new dict with only items whose value passes the predicate."""
    return {k: v for k, v in d.items() if predicate(v)}


if __name__ == "__main__":
    scores = {"Alice": 85, "Bob": 42, "Charlie": 91, "Diana": 60}
    # Keep only values greater than or equal to 70
    passed = filter_dict_by_predicate(scores, lambda x: x >= 70)
    print(passed)

    # Keep only even-length strings
    words = {"a": "hi", "b": "hello", "c": "world"}
    even_len = filter_dict_by_predicate(words, lambda s: len(s) % 2 == 0)
    print(even_len)

Output

stdout

{'Alice': 85, 'Charlie': 91}
{'a': 'hi'}

How it works

The function filter_dict_by_predicate uses a dictionary comprehension to iterate over key‑value pairs from the original dict. For each pair, it checks whether predicate(v) returns True; if so, the key and value are included in the new dictionary. Because dictionaries are unordered in older Python versions, but insertion order is preserved in Python 3.7+, the output key order matches the original insertion order. Passing a lambda keeps the function generic — you can filter on any property of the value, not just numbers. The function returns a new dict and leaves the original untouched, making it a pure, side‑effect‑free utility.

Common mistakes

Using `filter()` directly returns an iterable of keys, not a dict, unless you wrap it in another comprehension.
Forgetting that any falsy value (like `0`, `""`, or `None`) fails a truthiness predicate, which may surprise you.
Checking the key instead of the value when the predicate is meant for values.
Mutating the original dictionary inside the loop instead of building a new one.

Variations

Use `d.items()` and a simple loop with an if statement to build the result step by step.
For filtering by key, create a variation that applies the predicate to the key instead of the value.

Real-world use cases

Removing spurious entries from a data source, such as discarding readings below a confidence threshold.
Preparing a configuration dictionary for a service, keeping only settings that pass validation criteria.
Building a filtered feature map from a model's output, retaining only high‑confidence predictions.

How to Filter a Dictionary by Predicate on Values in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Real-world use cases

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