Python · Tier H

The 12 coding-interview patterns in Python.

TL;DR

Python is the default interview language for most candidates and for good reason: expressive syntax, a strong standard library, and zero boilerplate between you and the algorithm.

Python strengths

  • `collections.deque`, `heapq`, and `bisect` cover most data-structure needs without imports beyond stdlib.
  • List comprehensions and tuple unpacking keep code density high — the interviewer can read your intent in one line.
  • `@cache` and `functools.lru_cache` turn a recursive DP into a memoized DP in a single decorator.

Watch-outs

  • `heapq` is a min-heap — negate keys for max-heap behavior.
  • Default mutable arguments (`def f(x=[])`) are a classic trap when your solution has nested calls.
  • Python's recursion limit is ~1,000 — iterative DFS is safer on large grids.

Reference idiom

from collections import deque
def bfs(grid, start):
    q = deque([start]); seen = {start}
    while q:
        r, c = q.popleft()
        for dr, dc in ((1,0),(-1,0),(0,1),(0,-1)):
            nr, nc = r+dr, c+dc
            if (nr, nc) not in seen and 0 <= nr < len(grid) and 0 <= nc < len(grid[0]):
                seen.add((nr, nc)); q.append((nr, nc))

Signature patterns in Python

Sliding WindowMaintain a contiguous range and slide its boundaries to avoid recomputation.Dynamic ProgrammingBreak an overlapping-subproblem problem into a recurrence and cache results.Graph BFS / DFSBreadth-first for shortest unweighted paths; depth-first for exhaustive traversal.Heap & Priority QueueA data structure that returns the min or max in O(log n) per operation.

Frequently asked questions

Is Python a good interview language?
Python is the default interview language for most candidates and for good reason: expressive syntax, a strong standard library, and zero boilerplate between you and the algorithm.
What Python patterns come up most in coding interviews?
The signature patterns in Python interviews: Sliding Window, Dynamic Programming, Graph BFS / DFS, Heap & Priority Queue.
What are common Python interview pitfalls?
`heapq` is a min-heap — negate keys for max-heap behavior. Default mutable arguments (`def f(x=[])`) are a classic trap when your solution has nested calls. Python's recursion limit is ~1,000 — iterative DFS is safer on large grids.
Can I switch from Python to Python mid-loop?
Most companies let you pick any language you're fluent in. Switching mid-loop usually hurts — pick the language you're fastest in and stick with it unless the company mandates otherwise.

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