Filter Out Strings from a List

1. The Challenge: Data Cleaning

Create a function filter_list(lst) that takes a list of non-negative integers and strings and returns a new list with the strings filtered out (retaining only integers).

2. Why It Matters

Data cleaning pipelines frequently receive mixed raw inputs that must be filtered down to specific numerical data types before running analytical models.

3. The Python Solution

Use a List Comprehension combined with isinstance(item, int) or type(item) == int:

def filter_list(lst):
    return [x for x in lst if isinstance(x, int)]

# Testing our function
print(filter_list([1, 2, "a", "b"]))                 # Output: [1, 2]
print(filter_list([1, "a", "b", 0, 15]))             # Output: [1, 0, 15]
print(filter_list([1, 2, "aasf", "1", "123", 123]))  # Output: [1, 2, 123]

4. List Comprehension Syntax Breakdown

[expression for item in iterable if condition]:

1. for x in lst: Iterates through every item.
2. if isinstance(x, int): Filters items keeping only integers (excluding strings and booleans if strictly typed).
3. x: Collects matched elements into a new list.

5. Common Mistakes

• Checking isinstance(True, int): In Python, bool is a subclass of int! isinstance(True, int) returns True. To strictly exclude booleans, check type(x) == int.

6. Next Steps

Use list comprehensions for concise filtering. Next, let's convert text into H4ck3r Sp34k!