How to Use List Comprehensions and Generators to Format Data in Python
A beginner-friendly helper that formats dictionaries into strings using a list comprehension and generates squared numbers lazily with a generator.
Python code
32 linesdef format_data(items):
"""Format a list of dictionaries into readable strings."""
formatted = [
f"{item.get('name', 'Unknown')}: {item.get('value', 0)} units"
for item in items
if item.get('value', 0) > 0
]
return formatted if formatted else ["No positive values found"]
def generate_numbers(start=1, end=10):
"""Generate squared numbers as a generator."""
for num in range(start, end + 1):
yield num * num
if __name__ == "__main__":
sample_data = [
{"name": "Apples", "value": 5},
{"name": "Oranges", "value": 0},
{"name": "Bananas", "value": 3},
{"name": "Grapes", "value": -2},
{"name": "Cherries", "value": 8},
]
print("Formatted data:")
for line in format_data(sample_data):
print(f" {line}")
print("\nSquared numbers (1 to 5):")
squared_gen = generate_numbers(1, 5)
print(f" {list(squared_gen)}")Output
Formatted data:
Apples: 5 units
Bananas: 3 units
Cherries: 8 units
Squared numbers (1 to 5):
[1, 4, 9, 16, 25]How it works
The format_data function uses a list comprehension to iterate over dictionaries, safely access values with .get(), and filter out non-positive entries in a single readable expression. The generator generate_numbers uses yield to produce squared values one at a time, saving memory for large ranges. A generator becomes exhausted after iteration, which is why converting to a list inside the print statement works cleanly here. Both patterns keep the code concise while remaining easy to read for beginners.
Common mistakes
- Trying to reuse a generator after it's been consumed—it yields nothing on the second loop.
- Forgetting that `yield` makes a function a generator; calling it does not run the body immediately.
- Using `item['name']` directly instead of `.get()` causes a KeyError when a key is missing.
Variations
- Write the comprehension as a generator expression like `(f"{...}" for item in items)` to avoid building a full list.
- Combine the output into one string with `'\n'.join(format_data(items))` for direct printing.
Real-world use cases
- Formatting API-driven inventory reports where lines with zero or negative stock are skipped.
- Generating lazy sequences of computed metrics (like squares, cubes, or rates) for large datasets.
- Transforming config entries into human-readable log lines while filtering invalid records.
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