A dictionary comprehension builds a dictionary in one line. Good for transforming, filtering and building lookups. Bad for nested logic, side effects and anything that needs error handling. Shorter code is not always better code.
A decorator is just a function that wraps another function. Build a real timing decorator step by step: functions as values, the wrapper, the @ syntax, and functools.wraps. After that, the syntax stops looking like magic.
Reading a big file into a list can kill your program. A generator handles one line at a time. See the memory difference measured on a real file, learn what yield does, and chain small generators into a pipeline.
A closure remembers variables from the function that created it. Build separate counters with no globals, fix the UnboundLocalError with nonlocal, avoid the lambda-in-a-loop trap, and know when a class is clearer.
A for loop calls iter() once and next() until StopIteration. Write __iter__ and __next__ yourself, avoid the iterator that only works once, and know when a generator in __iter__ does the same job in fewer lines.
Importing a file runs it. if __name__ == "__main__" keeps a file's script part from running when another file imports it. Use it with a main() function, and know the cases where you don't need it.
Write a context manager when something must be undone no matter how the block ends. Build one as a class and with @contextmanager, keep yield inside try/finally, and don't swallow errors by accident.
Keep a counter's value in a closure so nothing outside can set it to -5 or "abc". The pattern, how it compares with a class with
Trace a wrong callback receiver, bind one stable listener, and verify start, stop and restart without duplicate or stale subscriptions.
Six short snippets with setTimeout, promises, await and queueMicrotask. Predict the order, run it, then read why. The one rule: sync code first, then all microtasks, then the next task.
Send many requests, keep the successes, and report the failures instead of losing everything to one rejected promise. Plus limiting how many run at once.
Hide page numbers and "next" links inside an async generator, then loop over every record with for await. Stop early and the remaining pages are never fetched.