Python 3 Guide

A structured path from language basics to LeetCode-ready Python, with notes on modern syntax and what is safe to use in interviews.

This guide is split into section posts:

Why this guide exists

This blog solves LeetCode problems in Python 3. The templates assume you already know:

  • built-in containers (list, dict, set, deque, heapq),
  • common idioms (comprehensions, unpacking, defaultdict, Counter),
  • and a few modern conveniences (list[int], walrus operator, match).

This guide fills the gap between “I know some Python” and “I can implement templates quickly.”

What Python version should you target?

Context Practical target
LeetCode Usually 3.9+; many accounts see 3.10+
Interviews Stick to widely known syntax unless interviewer confirms version
Personal projects Use latest stable (3.12 / 3.13)

Rule of thumb for contests: prefer features from 3.8–3.10 (walrus, list[int], match, dict merge). See Modern Features for a version-by-version table.

Learning path

Stage 1 — Language basics (1–2 days)

Goal: read and write small programs without looking up syntax constantly.

  • values, variables, int / float / bool / str
  • if / for / while, break / continue
  • functions, default args, *args / **kwargs
  • lists, dicts, sets, tuples — mutability matters

→ Basics & Idioms

Stage 2 — Interview containers & stdlib (1 day)

Goal: use the “LeetCode toolbox” fluently.

  • collections: deque, Counter, defaultdict
  • heapq, bisect, functools.lru_cache
  • sorting with key=, binary search templates

→ Python Quick Reference for LeetCode

Stage 3 — Patterns & templates (ongoing)

Goal: recognize problem types and paste/adapt templates.

  • sliding window, two pointers, prefix sum
  • BFS/DFS, DP, greedy, heap

→ LeetCode Categories & Templates and Template hub

Stage 4 — Modern syntax (optional polish)

Goal: write cleaner code where the environment supports it.

  • walrus :=, structural pattern matching, zip(..., strict=True)
  • type hints for clarity (not required on LeetCode)

→ Modern Features (3.8–3.13)

How this relates to templates

Resource Role
This guide Language + learning order
Quick reference Copy-paste snippets during practice
LeetCode templates Algorithm patterns by category
Problem posts Full walkthroughs with trade-offs

Start with basics → quick reference → one template category (e.g. Arrays & Strings) → solve easy/medium problems in that category.

Contents

Mindset for algorithm practice in Python

  1. Prefer clarity over cleverness — interviewers read Python quickly when it looks idiomatic.
  2. Know your containers — most bugs are wrong DS choice, not wrong algorithm.
  3. Avoid heavy dependencies — LeetCode provides the stdlib; sortedcontainers is the rare exception some users install locally.
  4. Match template style — use list[int], helper functions, and early returns like the rest of this blog.

More resources