5 Python One-Liners That Will Make You Look Like a Pro

Python is famous for being readable, but it also has a secret superpower: expressiveness. Some tasks that take 10 lines in other languages can be done in a single line of Python.

Here are 5 one-liners that aren't just clever tricks — they're genuinely useful in production code.

1. Flatten a Nested List

flat = [x for sub in [[1,2],[3,4],[5,6]] for x in sub]
# [1, 2, 3, 4, 5, 6]

This uses a nested list comprehension. The order of for clauses reads left-to-right, just like nested loops.

Real use case: Flattening API responses that return arrays of arrays.

2. Reverse a Dictionary (Swap Keys and Values)

swapped = {v: k for k, v in {"a": 1, "b": 2, "c": 3}.items()}
# {1: 'a', 2: 'b', 3: 'c'}

Real use case: Mapping status codes back to human-readable names, or building lookup tables from config dicts.

3. Find Files by Extension

import glob; files = glob.glob("/path/**/*.py", recursive=True)

This recursively finds all .py files under a directory. The recursive=True flag enables ** pattern matching.

Real use case: Finding all test files, all config files, or all images in a project.

4. Deduplicate a List While Preserving Order

deduped = list(dict.fromkeys([3, 1, 4, 1, 5, 9, 2, 6, 5, 3]))
# [3, 1, 4, 5, 9, 2, 6]

This works because dict preserves insertion order (Python 3.7+). Creating a dict from the list keeps only the first occurrence of each value, then converting back to a list gives us order-preserving deduplication.

Why not set()? Because set doesn't preserve order:

list(set([3, 1, 4, 1, 5, 9, 2, 6, 5, 3]))
# [1, 2, 3, 4, 5, 6, 9] — order lost!

Real use case: Removing duplicate user IDs from a log while keeping chronological order.

5. Merge Multiple Dictionaries

merged = {"a": 1} | {"b": 2} | {"c": 3}
# {'a': 1, 'b': 2, 'c': 3}

The | merge operator was introduced in Python 3.9. For older versions, use {**d1, **d2, **d3}.

If there are conflicting keys, the rightmost dictionary wins:

{"a": 1, "b": 2} | {"b": 99, "c": 3}
# {'a': 1, 'b': 99, 'c': 3}

Real use case: Merging default config with user config with CLI overrides:

config = DEFAULTS | user_config | cli_args

Bonus: Read an Entire File into a String

content = open("data.txt").read()

Simple, but powerful. For large files, use the pathlib version:

from pathlib import Path
content = Path("data.txt").read_text()

When to Use One-Liners (and When Not To)

✅ Use them when:

• The logic is straightforward and well-known
• It improves readability over the multi-line alternative
• Your team is familiar with Python idioms

❌ Avoid them when:

• You need to squint to understand what's happening
• Error handling is important (one-liners can't easily catch exceptions)
• A junior developer would have no idea what the code does

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian Kernighan

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What's your favorite Python one-liner? Share it in the comments! 👇