Python cheat sheet

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Background:

I have been using Python intensively for about 1 year. From now and then I look up some of the common operations and I decided to record all the code snippets in a handbook.

Table:

No. Goal Solution Note
1 Reverse a string or a list
def reverse(s):
return s[::-1]
view raw Python cheat sheet 1.py hosted with ❤ by GitHub
 
2 Join a list of strings to a single string
def join(t):
return ''.join(t)
view raw Python cheat sheet 2.py hosted with ❤ by GitHub
 
3 Flatten list from list of list
def flatten(t):
return [i for sub in t for i in sub]
# another implementation
def flatten(t):
return sum(t, [])
view raw python cheat sheet 3.py hosted with ❤ by GitHub
 
4 Trim a string
def trim(s):
return s.strip()
view raw python cheat sheet 4.py hosted with ❤ by GitHub
 
5 String replace a substring pattern with another substring
def replace(s, old, new):
return s.replace(old, new)
view raw python cheat sheet 5.py hosted with ❤ by GitHub
 
6 Cast float or string to int
def str2int(s):
return int(s)
view raw python cheat sheet 6.py hosted with ❤ by GitHub
auto floor to the nearest integer
7 Check string or char is in uppercase
def upper(s):
return s.isupper()
view raw python cheat sheet 7.py hosted with ❤ by GitHub
 
8 If statement in list comprehension
def neq0(t):
return [v for v in t if v != 0]
view raw python cheat sheet 8.py hosted with ❤ by GitHub
 
9 If else statement in list comprehension
def maskNone(t):
return [x if x else '' for x in t]
view raw python cheat sheet 9.py hosted with ❤ by GitHub
 
10 Transpose matrix (2-d array)
def transpose(m):
return [list(i) for i in zip(*m)]
view raw python cheat sheet 10.py hosted with ❤ by GitHub
 
11 Rotate matrix (2-d array) 90° clockwise
def rotate90(m):
return [list(i) for i in zip(*m[::-1])]
view raw python cheat sheet 11.py hosted with ❤ by GitHub
if you want to do it inplace, do:
matrix[:] = [list(i) for i in zip(*matrix[::-1])]
12 Check whether str1 is substring of str2
def substring(s, t):
return s in t
view raw python cheat sheet 12.py hosted with ❤ by GitHub
 
13 Get Current work directory
import os
print(os.getcwd())
view raw python cheat sheet 13.py hosted with ❤ by GitHub
 
14 Iterate dictionary
def iterate(d):
for k, v in d.items():
yield k, v
view raw python cheat sheet 14.py hosted with ❤ by GitHub
 
15 Split string with multiple delimiters
import re
re.split(',| |;', 'a; b d c,d, e, s')
view raw python cheat sheet 15.py hosted with ❤ by GitHub
 
16 Merge 2 dictionaries
from collections import Counter
a = Counter({'menu': 20, 'good': 15, 'happy': 10, 'bar': 5})
b = Counter({'menu': 1, 'good': 1, 'bar': 3})
a.update(b)
view raw python cheat sheet 16.py hosted with ❤ by GitHub
new value is the sum of 2 values from 2 dictionaries
17 Initialize list with same primitive values
my_list = [True] * 400
view raw python cheat sheet 17.py hosted with ❤ by GitHub
don’t use this with 2-d array
18 Python supports tertiary operator
if 3 < 5 < 8:
print(5)
view raw python cheat sheet 18.py hosted with ❤ by GitHub
 
19 Sort a list of strings by length
my_list = ['cbc', 'a', 'bc', 'pbdc']
my_list.sort(key=len)
view raw python cheat sheet 19.py hosted with ❤ by GitHub
 
20 String split by keep the delimiters
import re
re.split('(;|,|\n)', 'He,llo * worl;d')
view raw python cheat sheet 20.py hosted with ❤ by GitHub
just add outer ( ) to the delimiters
21 Sort dictionary by values
import operator
x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}
sorted_x = sorted(x.items(), key=operator.itemgetter(1))
view raw python cheat sheet 21.py hosted with ❤ by GitHub
if want to sort by keys, change itemgetter(1) to itemgetter(0)
22 Print 2-d array in a better format
import numpy as np
m = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
np.matrix(m)
view raw python cheat sheet 22.py hosted with ❤ by GitHub
 
23 Find index and last index of char in string
my_str = 'abcdefgb'
print(my_str.index('b'), my_str.rindex('b'))
view raw python cheat sheet 23.py hosted with ❤ by GitHub
 
24 Find index and last index of item in list
m = [1, 2, 3, 1, 4, 5]
print(m.index(1))
print(len(m) - 1 - m[::-1].index(1))
view raw python cheat sheet 24.py hosted with ❤ by GitHub
 
25 Automatically initialize for new key in dictionary
from collections import defaultdict
my_dict = defaultdict(int)
print(my_dict[4])
view raw python cheat sheet 25.py hosted with ❤ by GitHub
input of defaultdict should be callable, and takes no arguments
26 Get arbitrary one element from set
s = {8, 1, 2, 3}
next(iter(s))
view raw python cheat sheet 26.py hosted with ❤ by GitHub
 
27 Remove element at index 2 from the list
my_list = [1, 2, 3]
my_list.pop(2)
view raw python cheat sheet 27.py hosted with ❤ by GitHub
if no argument, last one is removed
28 Find locations (start, end) of matches with regular expression
import re
t = re.finditer('[A-Z][a-z]*', 'This Is My House')
print([i.span() for i in t])
view raw python cheat sheet 28.py hosted with ❤ by GitHub
 
29 Update each value in a dictionary with an operation
d = {1: 5, 3: 2, 0: 1}
d.update((k, d[k]**2) for k in d)
view raw python cheat sheet 29.py hosted with ❤ by GitHub
 
30 Find nth occurrence of b in string a
from functools import reduce
def findNth(a, b, n):
return reduce(lambda x, y: -1 if y > x + 1 else a.find(b, x + 1),
range(n), -1)
view raw python cheat sheet 30.py hosted with ❤ by GitHub
 
31 Find all indices of an element in a list
t = [1, 4, 2, 3, 4, 5]
print([i for i, x in enumerate(t) if x == 4])
view raw python cheat sheet 31.py hosted with ❤ by GitHub
 
32 Return a random element from a list
import random
t = [1, 4, 5]
print(random.choice(t))
view raw python cheat sheet 32.py hosted with ❤ by GitHub
 
33 Product of all elements of a list
from operator import mul
from functools import reduce
reduce(mul, [1, 2, 3, 4, 5, 10], 1)
view raw python cheat sheet 33.py hosted with ❤ by GitHub
 
34 Lexicographical  compare of strings
a, b = 'abc', 'bc'
print(a > b)
view raw python cheat sheet 34.py hosted with ❤ by GitHub
“abcd” > “ab” and “ad” > “ab”
35 Integer division
def int_division(a, b):
return a // b
view raw python cheat sheet 35.py hosted with ❤ by GitHub
round down
36 Two’s complement
print(~1345)
view raw python cheat sheet 36.py hosted with ❤ by GitHub
equals to -1315
37 Padding spaces to a string to a certain length
s = 'ab'
a = s.center(4)
b = s.ljust(5)
c = s.rjust(3)
print(a, b, c)
view raw python cheat sheet 37.py hosted with ❤ by GitHub
 
38 Get a random float
import random
random.random()
view raw python cheat sheet 38.py hosted with ❤ by GitHub
 
39 Get a random integer from range
import random
random.randrange(6)
view raw python cheat sheet 39.py hosted with ❤ by GitHub
 
40 Get profile of program, print out most time consuming function calls
import cProfile
cProfile.run('int(13)')
view raw python cheat sheet 40.py hosted with ❤ by GitHub
 
41 Get line by line time execution in jupyter notebook
import time
def sleep(n):
return time.sleep(n)
%load_ext line_profiler
%lprun -f sleep sleep(3)
view raw python cheat sheet 41.py hosted with ❤ by GitHub
need to specify the function name (e.g. totalNQueens) and the real execution call (e.g. totalNQueens(13))should use “” when passing strings as arguments
42 Usage of doctest
def average(values):
"""
>>> average([20, 30, 70])
40.0
"""
return sum(values) / len(values)
import doctest
doctest.testmod()
view raw python cheat sheet 42.py hosted with ❤ by GitHub
there must be at least 1 space on the right side of »>
43 Double-ended queue
from collections import deque
print(deque('cndota'))
view raw python cheat sheet 43.py hosted with ❤ by GitHub
can be think of a queue + stack
44 Printout nicely
from pprint import pprint
pprint({5: 3, 1: 4, 3: 0, 0: 8})
view raw python cheat sheet 44.py hosted with ❤ by GitHub
print out in sorted order
45 Permutation and combination
from itertools import combinations, permutations
list(combinations('123', 2)), list(permutations('123'))
view raw python cheat sheet 45.py hosted with ❤ by GitHub
output: [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]
46 Get all attributes and their values of an object
class duck:
def __init__(self):
self.age = 3
def quak(self):
print('ga')
d = duck()
vars(d)
view raw python cheat sheet 46.py hosted with ❤ by GitHub
 
47 Get the representation string of the object
repr(d)
view raw python cheat sheet 47.py hosted with ❤ by GitHub
if not specified, it will be like:’<main.Duck object at 0x00000133BDE7A6D8>’
48 Save memory by using generator instead of list
sum(x * x for x in range(7))
view raw python cheat sheet 48.py hosted with ❤ by GitHub
instead of sum([x*x for x in range(10)])
49 Variables created inside a loop are accessible outside the loop scope
for j in 3, 5:
print(j)
print(j)
view raw python cheat sheet 49.py hosted with ❤ by GitHub
thej actually is assigned the last element of the list mylist
50 Check whether a list iterate till end normally
for i in 3, 5:
if i == 4:
break
else:
print('no 4 in the list')
view raw python cheat sheet 50.py hosted with ❤ by GitHub
 
51 To make class objects comparable, just add function __lt__
class duck:
def __init__(self, n):
self.age = n
def __lt__(self, other):
return self.age < other.age
d = duck(3)
e = duck(5)
d < e
view raw python cheat sheet 51.py hosted with ❤ by GitHub
 
52 Sort a list of list by multiple values
import operator
s = [[1, 2], [1, 0], [2, 0]]
s.sort(key=lambda x: (x[0], x[1]))
view raw python cheat sheet 52.py hosted with ❤ by GitHub
 
Written on May 24, 2018