Python code cheatsheet

Random

import random random_idx = random.randint(low, high)

Default Min and Max

• Use float('inf') for positive infinity.

• Use float('-inf') for negative infinity.xfcha

dict

nodeInfo = dict()
nodeInfo[ng] = nodeInfo.get(ng, 0) + 1 #有则返回vaL 无则返回0 这样可以无需初始化

如何统计字典中某个值的出现次数？

用 list(dict.values()).count(x)

🔍 parent.get(key) vs parent[key]

• parent[key]：如果字典中没有这个 key，会直接抛出 KeyError

• parent.get(key)：如果 key 不存在，不会报错，而是返回 None

deque

一般 qfs里使用， 用popleft取代list.pop(0), time complexity: o(n) => o(1)

q = deque()
cur = q.popleft()

数组slicing

reverse ： s[::-1] start:stop:step

s = "hello"
reversed_s = s[::-1]
print(reversed_s)  # Output: "olleh"

presums[-1] will return last element of the list

二维数组

dp = [[0]*(m+1) for _ in range(n+1)]

n//2 vs n/2
n//2 floor, return int
n/2 true division, return float

Ord(): return unicode code point of a charactor

print(ord('a'))  # Output: 97

sort 自定义函数

pairs.sort(key = lambda x: (x[1],x[0]))#记得要加括号！！！！

*list will unpack the list

workers = [1, 2, 3, 4]
# Using *workers in a print statement
print(*workers)    # Output: 1 2 3 4

default: Functions: max() and min() support the default parameter.

min/max([], default=0)  # Returns 0

遍历dictionary:

```python
for k,v in d.items():
    v.reserse() #v is a list
```

number char coversion

char.isdigit()

char.isalpha()

Convert a Character to its ASCII/Unicode Number

char = 'A' number = ord(char) print(number) # Output: 65

Convert a Numeric Character to its Integer Value

char = '7' number = int(char) print(number) # Output: 7

Counter

Counter(list) returnr dict.

most_common(k) returns a list of (element, count) tuples sorted by counts.

return [i for i,_ in collections.Counter(nums).most_common(k)]

Sort

Define a lambda function to sort various data structures like lists, dictionaries, and tuples.

In-place vs. Copy:

• sorted() returns a new sorted list.

• .sort() sorts the list in place and returns None.

Sorting a List

# Example list of tuples
data = [(1, 3), (4, 1), (5, 2)]

# Sorting the list based on the second element of each tuple
sorted_data = sorted(data, key=lambda x: x[1])

# Output: [(4, 1), (5, 2), (1, 3)]
print(sorted_data)

Sorting a Dictionary by Value

# Example dictionary
data = {'a': 3, 'b': 1, 'c': 2}

# Sorting the dictionary by its values
sorted_data = sorted(data.items(), key=lambda x: x[1])

# Output: [('b', 1), ('c', 2), ('a', 3)]
print(sorted_data)

Sorting a List of Dictionaries

# Example list of dictionaries
data = [{'name': 'John', 'age': 25}, {'name': 'Jane', 'age': 22}, {'name': 'Doe', 'age': 30}]

# Sorting by the 'age' key
sorted_data = sorted(data, key=lambda x: x['age'])

# Output: [{'name': 'Jane', 'age': 22}, {'name': 'John', 'age': 25}, {'name': 'Doe', 'age': 30}]
print(sorted_data)

deque

整个list 加入deque, extend()

d.extend(lst)

use cache in DP

from functools import lru_cache  # Import lru_cache
@lru_cache(None)

filter

参数是一个函数和iterator，函数作用在每个iterator上

#滤掉所有非(),因为count返回返回1或0
s = "a(b)c)d"
result = ''.join(filter('()'.count, s))
print(result)  # Output: "()()"

Concise code

1️⃣ Multiple Appends in One Line

Instead of:

pythonCopyEditqueue.append(item1)
queue.append(item2)

Use:

pythonCopyEditqueue.extend([item1, item2])

Or:

pythonCopyEditqueue += item1, item2  # No need for extra brackets!

---

2️⃣ Inline If-Else (or Trick)

Instead of:

pythonCopyEditif name:
    greeting = name
else:
    greeting = "Guest"

Use:

pythonCopyEditgreeting = name or "Guest"

• If name is None or "", it defaults to "Guest".

---

3️⃣ Swapping Variables Without Temp

Instead of:

pythonCopyEdittemp = a
a = b
b = temp

Use:

pythonCopyEdita, b = b, a

---

4️⃣ List Comprehension Instead of Loops

Instead of:

pythonCopyEditsquares = []
for i in range(10):
    squares.append(i * i)

Use:

pythonCopyEditsquares = [i * i for i in range(10)]

---

5️⃣ Using defaultdict to Avoid Key Errors

Instead of:

pythonCopyEditword_count = {}
for word in words:
    if word in word_count:
        word_count[word] += 1
    else:
        word_count[word] = 1

Use:

pythonCopyEditfrom collections import defaultdict
word_count = defaultdict(int)
for word in words:
    word_count[word] += 1

---

6️⃣ Merging Two Dictionaries

Instead of:

pythonCopyEditdict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
merged = dict1.copy()
merged.update(dict2)

Use (Python 3.9+):

pythonCopyEditmerged = dict1 | dict2

---

7️⃣ Unpacking Iterables

Instead of:

pythonCopyEdita = my_list[0]
b = my_list[1]
c = my_list[2:]

Use:

pythonCopyEdita, b, *c = my_list

---

BIT

bin(mask)	转二进制字符串（带 0b 前缀）	'0b101'

mask.bit_count()	直接统计 1 的个数（Python 3.10+）	2（若 mask = 0b1010）

pos = available & -available 的作用：取最低位的 1

核心原理

• available：一个二进制数，表示当前可选的位置（1 表示可选，0 表示不可选）。

  • 例如 available = 0b1010 表示第 2 列和第 4 列可选（从右往左数，最低位是第 1 列）。

• -available：计算机中负数以补码表示，-x = ~x + 1。

  • 例如 available = 0b1010，则 -available = ~0b1010 + 1 = 0b0101 + 1 = 0b0110。

• available & -available：

  • 保留 available 最低位的 1，其余位全置 0。

  • 例如 0b1010 & 0b0110 = 0b0010（保留了最低位的 1）。

Count

数据类型	是否支持 .count()	统计对象	示例
字符串（str）	✅	子字符串	"hello".count("l")
列表（list）	✅	元素	[1, 2, 2].count(2)
元组（tuple）	✅	元素	(1, 2, 2).count(2)
字节（bytes）	✅	子字节序列	b"hello".count(b"l")
集合（set）	❌	无	需手动遍历
字典（dict）	❌	无	list(d.values()).count(1)
数字（int）	❌	无	不适用

Optional Optional[Type] 表示变量可以是 Type 类型或 None

Optional[Type] = Union[Type, None] = Type|None