Python Filter Function with Default Parameters for Beginners

Create a reusable filter function with default parameters to keep or exclude numbers above or below a threshold.

Easy Python 3.9+ Aug 9, 2026 Functions & basics 13 views 0 copies

functions default-parameters filter list-comprehension beginners

Python code

21 lines

Python 3.9+

def filter_numbers(numbers, threshold=0, reverse=False):
    """Return numbers that pass the threshold filter.

    Args:
        numbers: list of numbers to filter
        threshold: minimum value to keep (default 0)
        reverse: if True, keep numbers below threshold (default False)
    """
    if reverse:
        return [num for num in numbers if num < threshold]
    return [num for num in numbers if num >= threshold]


if __name__ == "__main__":
    data = [10, -5, 3, -8, 15, 0, 7]
    
    print("Original list:", data)
    print("Default filter (>= 0):", filter_numbers(data))
    print("Custom threshold (>= 5):", filter_numbers(data, threshold=5))
    print("Reverse filter (< 0):", filter_numbers(data, threshold=0, reverse=True))
    print("Custom threshold with reverse (< 3):", filter_numbers(data, threshold=3, reverse=True))

Output

stdout

Original list: [10, -5, 3, -8, 15, 0, 7]
Default filter (>= 0): [10, 3, 15, 0, 7]
Custom threshold (>= 5): [10, 15, 7]
Reverse filter (< 0): [-5, -8]
Custom threshold with reverse (< 3): [-5, -8, 0]

How it works

This function uses default parameters to make the threshold and direction optional, so callers can filter with minimal setup or customize behavior. The reverse flag flips the comparison operator from >= to <, giving you a single function for both keep-above and keep-below scenarios. List comprehensions keep the logic compact and readable compared to manual loops. The __name__ == "__main__" guard ensures the demo only runs when the script is executed directly, not when imported as a module.

Common mistakes

Forgetting to include upper-bound numbers like 0 when using default threshold, since it matches `>=`
Misunderstanding reverse: it filters *below* threshold, it doesn't reverse the list order
Passing positional arguments instead of keywords, which can make the code harder to read
Assuming the function mutates the input list—it returns a new list instead

Variations

Use a lambda with the built-in `filter()`: `list(filter(lambda n: n >= threshold, numbers))`
Add an `inclusive` parameter to switch between `<` and `<=` for boundary handling

Real-world use cases

Filtering sensor readings to flag values outside a safe operating range in IoT monitoring scripts.
Cleaning log data by keeping only entries above a severity threshold before sending to an alert system.
Selecting test scores above a passing grade from a batch of student results for report generation.

Python Filter Function with Default Parameters for Beginners

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Functions & basics

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