python
8 minPython Collections: Lists, Tuples, Sets, and Map/Reduce Trade-offs
Choose Python collection types by invariants and access patterns, then use map, comprehensions, and reductions without obscuring intent.
Search the library
10 matching resultsArticles and interview answers
Q&A
Q&AWhen should you choose a Python list, tuple, or set?
Base the choice on ordering, mutation, duplicates, and membership operations.
Read the concise answer
A list is ordered, mutable, and permits duplicates. A tuple is an immutable sequence suited to fixed records, though named structures can improve readability. A set stores unique hashable values and is useful for membership; do not use it when output order or duplicates are part of the contract.
databases
6 minSQL keyset pagination: continue an order without promising a snapshot
Use a unique ordering tuple and a deliberate visibility policy for reliable page traversal.
Q&A
Q&AWhy does a timestamp cursor need a unique tie-breaker?
Equal timestamps otherwise make continuation skip or repeat peers.
Read the concise answer
Carry the full ordering tuple, such as created_at and ID, and match its direction in the continuation predicate. Stable continuation still does not freeze membership across requests.
Q&A
Q&AWhat does a large TCP TIME_WAIT count mean?
TIME_WAIT is part of safe TCP teardown; verify actual ephemeral-port pressure before tuning sysctls.
Read the concise answer
TIME_WAIT helps prevent delayed packets from an old connection from contaminating a new one. A high count can be normal. Diagnose port exhaustion by checking outbound tuple reuse, NAT limits, ephemeral port range, and connection pooling. tcp_tw_reuse semantics vary by kernel and are not a universal first fix.
Q&A
Q&AWhy can a Python list not be an element of a set?
Set members must be hashable, and a mutable list does not have a stable hash.
Read the concise answer
Sets and dictionary keys rely on a stable hash and equality relationship while a value is stored. A list is mutable, so changing it could invalidate the collection’s lookup structure; Python therefore makes lists unhashable. Use a tuple if its members are also hashable and the value should be immutable.
Q&A
Q&AWhy did a SQL join unexpectedly multiply my totals?
A one-to-many join changes row grain; multiple child joins can multiply each other.
Read the concise answer
Check the cardinality of each join. Joining one order to several lines produces multiple rows per order; joining another one-to-many table can multiply those rows again. Aggregate each relation to the intended grain before joining or use EXISTS when you only need to test presence. DISTINCT is not a reliable repair for incorrect join logic.
Q&A
Q&AWhat should you inspect after CREATE INDEX CONCURRENTLY fails?
Check active work and catalog validity before dropping or retrying.
Read the concise answer
Concurrent builds use multiple phases and can leave an invalid index. Inspect the server error and pg_index readiness/validity, waits, and transactions; do not assume the operation was atomic or load-free.