Python Warmups

~70 min total

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Python Cheat Sheet

Common syntax for coding interviews

Lists

nums = [3, 1, 2]
nums.append(4)          # add one item
nums.extend([5, 6])     # add many items
nums.insert(1, 10)
nums.pop()              # remove + return last
nums.pop(0)             # remove + return by index
nums.remove(10)         # remove first match

nums[1:4]               # slice
nums[::-1]              # reversed copy
2 in nums
len(nums)
sum(nums)
min(nums), max(nums)

Dicts

user = {"name": "Alice", "age": 20}
user["team"] = "A"     # add
user["age"] = 21       # update

"name" in user
user["name"]           # KeyError if missing
user.get("salary")     # None if missing
user.get("salary", 0)  # default value

for key, value in user.items():
    print(key, value)

value = user.pop("age", None)
del user["team"]

Sets

seen = {1, 2, 3}
seen.add(4)
seen.discard(2)         # no error if missing
seen.remove(3)          # KeyError if missing

2 in seen
a | b                   # union
a & b                   # intersection
a - b                   # difference
a ^ b                   # symmetric difference

unique = set(items)

Strings

s = "  Hello, World  "
s.strip()
s.lower(), s.upper()
s.startswith("Hello")
s.endswith("World")
s.replace("World", "Python")

parts = "a,b,c".split(",")
text = "-".join(parts)

s[1:5]
s[::-1]
"ell" in s
f"Hello, {name}"

Loops & comprehensions

for index, value in enumerate(items):
    print(index, value)

for left, right in zip(a, b):
    print(left, right)

squares = [x * x for x in nums]
evens = [x for x in nums if x % 2 == 0]

by_id = {item["id"]: item for item in records}
lengths = {word: len(word) for word in words}
unique_lengths = {len(word) for word in words}

Sorting

sorted(nums)                    # new list
sorted(nums, reverse=True)
nums.sort()                     # mutate list

sorted(words, key=len)
sorted(records, key=lambda x: x["score"])
sorted(records, key=lambda x: (-x["score"], x["name"]))

min(records, key=lambda x: x["score"])
max(records, key=lambda x: x["score"])

Tuples & unpacking

point = (10, 20)
x, y = point

first, *middle, last = items
a, b = b, a             # swap

for key, value in pairs:
    print(key, value)

# Tuples can be dict keys / set values
visited = {(0, 0), (0, 1)}

Useful built-ins

range(5)                       # 0..4
range(2, 10, 2)                # 2, 4, 6, 8

any(x < 0 for x in nums)
all(x >= 0 for x in nums)

list(reversed(items))
list(zip(keys, values))
dict(zip(keys, values))

int("42"), str(42)
list("abc")
ord("a"), chr(97)

collections

from collections import Counter, defaultdict, deque

counts = Counter(items)
counts.most_common(3)

groups = defaultdict(list)
for item in records:
    groups[item["team"]].append(item)

queue = deque(items)
queue.append(value)
queue.appendleft(value)
queue.popleft()

Heap & binary search

import heapq
from bisect import bisect_left, bisect_right, insort

heap = []
heapq.heappush(heap, 3)
heapq.heappush(heap, 1)
smallest = heapq.heappop(heap)

# Max heap: store negative values
heapq.heappush(heap, -score)
largest = -heapq.heappop(heap)

index = bisect_left(sorted_nums, target)
after_duplicates = bisect_right(sorted_nums, target)
insort(sorted_nums, target)

nums = [3, 1, 2] nums.append(4) # add one item nums.extend([5, 6]) # add many items nums.insert(1, 10) nums.pop() # remove + return last nums.pop(0) # remove + return by index nums.remove(10) # remove first match nums[1:4] # slice nums[::-1] # reversed copy 2 in nums len(nums) sum(nums) min(nums), max(nums)

user = {"name": "Alice", "age": 20} user["team"] = "A" # add user["age"] = 21 # update "name" in user user["name"] # KeyError if missing user.get("salary") # None if missing user.get("salary", 0) # default value for key, value in user.items(): print(key, value) value = user.pop("age", None) del user["team"]

seen = {1, 2, 3} seen.add(4) seen.discard(2) # no error if missing seen.remove(3) # KeyError if missing 2 in seen a | b # union a & b # intersection a - b # difference a ^ b # symmetric difference unique = set(items)

s = " Hello, World " s.strip() s.lower(), s.upper() s.startswith("Hello") s.endswith("World") s.replace("World", "Python") parts = "a,b,c".split(",") text = "-".join(parts) s[1:5] s[::-1] "ell" in s f"Hello, {name}"

for index, value in enumerate(items): print(index, value) for left, right in zip(a, b): print(left, right) squares = [x * x for x in nums] evens = [x for x in nums if x % 2 == 0] by_id = {item["id"]: item for item in records} lengths = {word: len(word) for word in words} unique_lengths = {len(word) for word in words}

sorted(nums) # new list sorted(nums, reverse=True) nums.sort() # mutate list sorted(words, key=len) sorted(records, key=lambda x: x["score"]) sorted(records, key=lambda x: (-x["score"], x["name"])) min(records, key=lambda x: x["score"]) max(records, key=lambda x: x["score"])

point = (10, 20) x, y = point first, *middle, last = items a, b = b, a # swap for key, value in pairs: print(key, value) # Tuples can be dict keys / set values visited = {(0, 0), (0, 1)}

range(5) # 0..4 range(2, 10, 2) # 2, 4, 6, 8 any(x < 0 for x in nums) all(x >= 0 for x in nums) list(reversed(items)) list(zip(keys, values)) dict(zip(keys, values)) int("42"), str(42) list("abc") ord("a"), chr(97)

from collections import Counter, defaultdict, deque counts = Counter(items) counts.most_common(3) groups = defaultdict(list) for item in records: groups[item["team"]].append(item) queue = deque(items) queue.append(value) queue.appendleft(value) queue.popleft()

import heapq from bisect import bisect_left, bisect_right, insort heap = [] heapq.heappush(heap, 3) heapq.heappush(heap, 1) smallest = heapq.heappop(heap) # Max heap: store negative values heapq.heappush(heap, -score) largest = -heapq.heappop(heap) index = bisect_left(sorted_nums, target) after_duplicates = bisect_right(sorted_nums, target) insort(sorted_nums, target)