Python Cheat Sheet 💻🐍

type(1)   # int 
type(-10) # int
type(0)   # int
type(0.0) # float
type(2.2) # float
type(4E2) # float - 4*10 to the power of 2

# Arithmetic
10 + 3  # 13
10 - 3  # 7
10 * 3  # 30
10 ** 3 # 1000
10 / 3  # 3.3333333333333335
10 // 3 # 3 --> floor division - no decimals and returns an int
10 % 3  # 1 --> modulo operator - return the remainder. Good for deciding if number is even or odd

# Basic Functions
pow(5, 2)      # 25 --> like doing 5**2
abs(-50)       # 50
round(5.46)    # 5
round(5.468, 2)# 5.47 --> round to nth digit
bin(512)       # '0b1000000000' -->  binary format
hex(512)       # '0x200' --> hexadecimal format

# Converting Strings to Numbers
age = input("How old are you?")
age = int(age)
pi = input("What is the value of pi?")
pi = float(pi)

type('Hellloooooo') # str

'I\'m thirsty'
"I'm thirsty"
"\n" # new line
"\t" # adds a tab

'Hey you!'[4] # y
name = 'Andrei Neagoie'
name[4]     # e
name[:]     # Andrei Neagoie
name[1:]    # ndrei Neagoie
name[:1]    # A
name[-1]    # e
name[::1]   # Andrei Neagoie
name[::-1]  # eiogaeN ierdnA
name[0:10:2]# Ade e
# : is called slicing and has the format [ start : end : step ]

'Hi there ' + 'Timmy' # 'Hi there Timmy' --> This is called string concatenation
'*'*10 # **********

# Basic Functions
len('turtle') # 6

# Basic Methods
'  I am alone '.strip()               # 'I am alone' --> Strips all whitespace characters from both ends.
'On an island'.strip('d')             # 'On an islan' --> # Strips all passed characters from both ends.
'but life is good!'.split()           # ['but', 'life', 'is', 'good!']
'Help me'.replace('me', 'you')        # 'Help you' --> Replaces first with second param
'Need to make fire'.startswith('Need')# True
'and cook rice'.endswith('rice')      # True
'bye bye'.index('e')                  # 2
'still there?'.upper()                # STILL THERE?
'HELLO?!'.lower()                     # hello?!
'ok, I am done.'.capitalize()         # 'Ok, I am done.'
'oh hi there'.find('i')               # 4 --> returns the starting index position of the first occurrence
'oh hi there'.count('e')              # 2

# String Formatting
name1 = 'Andrei'
name2 = 'Sunny'
print(f'Hello there {name1} and {name2}')       # Hello there Andrei and Sunny - Newer way to do things as of python 3.6
print('Hello there {} and {}'.format(name1, name2))# Hello there Andrei and Sunny
print('Hello there %s and %s' %(name1, name2))  # Hello there Andrei and Sunny --> you can also use %d, %f, %r for integers, floats, string representations of objects respectively

#Palindrome check
word = 'reviver'
p = bool(word.find(word[::-1]) + 1)
print(p) # True

bool(True)
bool(False)

# all of the below evaluate to False. Everything else will evaluate to True in Python.
print(bool(None))
print(bool(False))
print(bool(0))
print(bool(0.0))
print(bool([]))
print(bool({}))
print(bool(()))
print(bool(''))
print(bool(range(0)))
print(bool(set()))

# See Logical Operators and Comparison Operators section for more on booleans.

my_list = [1, 2, '3', True]# We assume this list won't mutate for each example below
len(my_list)               # 4
my_list.index('3')         # 2
my_list.count(2)           # 1 --> count how many times 2 appears

my_list[3]                 # True
my_list[1:]                # [2, '3', True]
my_list[:1]                # [1]
my_list[-1]                # True
my_list[::1]               # [1, 2, '3', True]
my_list[::-1]              # [True, '3', 2, 1]
my_list[0:3:2]             # [1, '3']

# : is called slicing and has the format [ start : end : step ]

# Add to List
my_list * 2                # [1, 2, '3', True, 1, 2, '3', True]
my_list + [100]            # [1, 2, '3', True, 100] --> doesn't mutate original list, creates new one
my_list.append(100)        # None --> Mutates original list to [1, 2, '3', True, 100]          # Or: <list> += [<el>]
my_list.extend([100, 200]) # None --> Mutates original list to [1, 2, '3', True, 100, 200]
my_list.insert(2, '!!!')   # None -->  [1, 2, '!!!', '3', True] - Inserts item at index and moves the rest to the right.

' '.join(['Hello','There'])# 'Hello There' --> Joins elements using string as separator.

# Copy a List
basket = ['apples', 'pears', 'oranges']
new_basket = basket.copy()
new_basket2 = basket[:]

# Remove from List
[1,2,3].pop()    # 3 --> mutates original list, default index in the pop method is -1 (the last item)
[1,2,3].pop(1)   # 2 --> mutates original list
[1,2,3].remove(2)# None --> [1,3] Removes first occurrence of item or raises ValueError.
[1,2,3].clear()  # None --> mutates original list and removes all items: []
del [1,2,3][0] # 

# Ordering
[1,2,5,3].sort()         # None --> Mutates list to [1, 2, 3, 5]
[1,2,5,3].sort(reverse=True) # None --> Mutates list to [5, 3, 2, 1]
[1,2,5,3].reverse()      # None --> Mutates list to [3, 5, 2, 1]
sorted([1,2,5,3])        # [1, 2, 3, 5] --> new list created
list(reversed([1,2,5,3]))# [3, 5, 2, 1] --> reversed() returns an iterator

# Useful operations
1 in [1,2,5,3]  # True
min([1,2,3,4,5])# 1
max([1,2,3,4,5])# 5
sum([1,2,3,4,5])# 15

# Get First and Last element of a list
mList = [63, 21, 30, 14, 35, 26, 77, 18, 49, 10]
first, *x, last = mList
print(first) #63
print(last) #10

# Matrix
matrix = [[1,2,3], [4,5,6], [7,8,9]]
matrix[2][0] # 7 --> Grab first first of the third item in the matrix object

# Looping through a matrix by rows:
mx = [[1,2,3],[4,5,6]]
for row in range(len(mx)):
	for col in range(len(mx[0])):
		print(mx[row][col]) # 1 2 3 4 5 6

# Transform into a list:
[mx[row][col] for row in range(len(mx)) for col in range(len(mx[0]))] # [1,2,3,4,5,6]

# Combine columns with zip and *:
[x for x in zip(*mx)] # [(1, 3), (2, 4)]

# List Comprehensions
# new_list[<action> for <item> in <iterator> if <some condition>]
a = [i for i in 'hello']                  # ['h', 'e', 'l', 'l', '0']
b = [i*2 for i in [1,2,3]]                # [2, 4, 6]
c = [i for i in range(0,10) if i % 2 == 0]# [0, 2, 4, 6, 8]

# Advanced Functions
list_of_chars = list('Helloooo')                                   # ['H', 'e', 'l', 'l', 'o', 'o', 'o', 'o']
sum_of_elements = sum([1,2,3,4,5])                                 # 15
element_sum = [sum(pair) for pair in zip([1,2,3],[4,5,6])]         # [5, 7, 9]
sorted_by_second = sorted(['hi','you','man'], key=lambda el: el[1])# ['man', 'hi', 'you']
sorted_by_key = sorted([
                       {'name': 'Bina', 'age': 30},
                       {'name':'Andy', 'age': 18},
                       {'name': 'Zoey', 'age': 55}],
                       key=lambda el: (el['name']))# [{'name': 'Andy', 'age': 18}, {'name': 'Bina', 'age': 30}, {'name': 'Zoey', 'age': 55}]

# Read line of a file into a list
with open("myfile.txt") as f:
  lines = [line.strip() for line in f]

my_dict = {'name': 'Andrei Neagoie', 'age': 30, 'magic_power': False}
my_dict['name']                      # Andrei Neagoie
len(my_dict)                         # 3
list(my_dict.keys())                 # ['name', 'age', 'magic_power']
list(my_dict.values())               # ['Andrei Neagoie', 30, False]
list(my_dict.items())                # [('name', 'Andrei Neagoie'), ('age', 30), ('magic_power', False)]
my_dict['favourite_snack'] = 'Grapes'# {'name': 'Andrei Neagoie', 'age': 30, 'magic_power': False, 'favourite_snack': 'Grapes'}
my_dict.get('age')                   # 30 --> Returns None if key does not exist.
my_dict.get('ages', 0 )              # 0 --> Returns default (2nd param) if key is not found

#Remove key
del my_dict['name']
my_dict.pop('name', None)

my_dict.update({'cool': True})                                         # {'name': 'Andrei Neagoie', 'age': 30, 'magic_power': False, 'favourite_snack': 'Grapes', 'cool': True}
{**my_dict, **{'cool': True} }                                         # {'name': 'Andrei Neagoie', 'age': 30, 'magic_power': False, 'favourite_snack': 'Grapes', 'cool': True}
new_dict = dict([['name','Andrei'],['age',32],['magic_power',False]])  # Creates a dict from collection of key-value pairs.
new_dict = dict(zip(['name','age','magic_power'],['Andrei',32, False]))# Creates a dict from two collections.
new_dict = my_dict.pop('favourite_snack')                              # Removes item from dictionary.

# Dictionary Comprehension
{key: value for key, value in new_dict.items() if key == 'age' or key == 'name'} # {'name': 'Andrei', 'age': 32} --> Filter dict by keys

my_tuple = ('apple','grapes','mango', 'grapes')
apple, grapes, mango, grapes = my_tuple# Tuple unpacking
len(my_tuple)                          # 4
my_tuple[2]                            # mango
my_tuple[-1]                           # 'grapes'

# Immutability
my_tuple[1] = 'donuts'  # TypeError
my_tuple.append('candy')# AttributeError

# Methods
my_tuple.index('grapes') # 1
my_tuple.count('grapes') # 2

# Zip
list(zip([1,2,3], [4,5,6])) # [(1, 4), (2, 5), (3, 6)]

# unzip
z = [(1, 2), (3, 4), (5, 6), (7, 8)] # Some output of zip() function
unzip = lambda z: list(zip(*z))
unzip(z)

my_set = set()
my_set.add(1)  # {1}
my_set.add(100)# {1, 100}
my_set.add(100)# {1, 100} --> no duplicates!

new_list = [1,2,3,3,3,4,4,5,6,1]
set(new_list)           # {1, 2, 3, 4, 5, 6}

my_set.remove(100)      # {1} --> Raises KeyError if element not found
my_set.discard(100)     # {1} --> Doesn't raise an error if element not found
my_set.clear()          # {}
new_set = {1,2,3}.copy()# {1,2,3}

set1 = {1,2,3}
set2 = {3,4,5}
set3 = set1.union(set2)               # {1,2,3,4,5}
set4 = set1.intersection(set2)        # {3}
set5 = set1.difference(set2)          # {1, 2}
set6 = set1.symmetric_difference(set2)# {1, 2, 4, 5}
set1.issubset(set2)                   # False
set1.issuperset(set2)                 # False
set1.isdisjoint(set2)                 # False --> return True if two sets have a null intersection.

# Frozenset
# hashable --> it can be used as a key in a dictionary or as an element in a set.
<frozenset> = frozenset(<collection>)

type(None) # NoneType
a = None

==                   # equal values
!=                   # not equal
>                    # left operand is greater than right operand
<                    # left operand is less than right operand
>=                   # left operand is greater than or equal to right operand
<=                   # left operand is less than or equal to right operand
<element> is <element> # check if two operands refer to same object in memory

1 < 2 and 4 > 1 # True
1 > 3 or 4 > 1  # True
1 is not 4      # True
not True        # False
1 not in [2,3,4]# True

if <condition that evaluates to boolean>:
  # perform action1
elif <condition that evaluates to boolean>:
  # perform action2
else:
  # perform action3

my_list = [1,2,3]
my_tuple = (1,2,3)
my_list2 = [(1,2), (3,4), (5,6)]
my_dict = {'a': 1, 'b': 2. 'c': 3}

for num in my_list:
    print(num) # 1, 2, 3

for num in my_tuple:
    print(num) # 1, 2, 3

for num in my_list2:
    print(num) # (1,2), (3,4), (5,6)

for num in '123':
    print(num) # 1, 2, 3

for k,v in my_dict.items(): # Dictionary Unpacking
    print(k) # 'a', 'b', 'c'
    print(v) # 1, 2, 3

while <condition that evaluates to boolean>:
  # action
  if <condition that evaluates to boolean>:
    break # break out of while loop
  if <condition that evaluates to boolean>:
    continue # continue to the next line in the block

# waiting until user quits
msg = ''
while msg != 'quit':
    msg = input("What should I do?")
    print(msg)

range(10)          # range(0, 10) --> 0 to 9
range(1,10)        # range(1, 10)
list(range(0,10,2))# [0, 2, 4, 6, 8]

for i, el in enumerate('helloo'):
  print(f'{i}, {el}')
# 0, h
# 1, e
# 2, l
# 3, l
# 4, o
# 5, o

from collections import Counter
colors = ['red', 'blue', 'yellow', 'blue', 'red', 'blue']
counter = Counter(colors)# Counter({'blue': 3, 'red': 2, 'yellow': 1})
counter.most_common()[0] # ('blue', 3)

from collections import namedtuple
Point = namedtuple('Point', 'x y')
p = Point(1, y=2)# Point(x=1, y=2)
p[0]             # 1
p.x              # 1
getattr(p, 'y')  # 2
p._fields        # Or: Point._fields #('x', 'y')

from collections import namedtuple
Person = namedtuple('Person', 'name height')
person = Person('Jean-Luc', 187)
f'{person.height}'           # '187'
'{p.height}'.format(p=person)# '187'

from collections import OrderedDict
# Store each person's languages, keeping # track of who responded first. 
programmers = OrderedDict()
programmers['Tim'] = ['python', 'javascript']
programmers['Sarah'] = ['C++']
programmers['Bia'] = ['Ruby', 'Python', 'Go']

for name, langs in programmers.items():
    print(name + '-->')
    for lang in langs:
      print('\t' + lang)

args   = (1, 2)
kwargs = {'x': 3, 'y': 4, 'z': 5}
some_func(*args, **kwargs) # same as some_func(1, 2, x=3, y=4, z=5)

def add(*a):
    return sum(a)

add(1, 2, 3) # 6

def f(*args):                  # f(1, 2, 3)
def f(x, *args):               # f(1, 2, 3)
def f(*args, z):               # f(1, 2, z=3)
def f(x, *args, z):            # f(1, 2, z=3)

def f(**kwargs):               # f(x=1, y=2, z=3)
def f(x, **kwargs):            # f(x=1, y=2, z=3) | f(1, y=2, z=3)

def f(*args, **kwargs):        # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)
def f(x, *args, **kwargs):     # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)
def f(*args, y, **kwargs):     # f(x=1, y=2, z=3) | f(1, y=2, z=3)
def f(x, *args, z, **kwargs):  # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3)

[*[1,2,3], *[4]]                # [1, 2, 3, 4]
{*[1,2,3], *[4]}                # {1, 2, 3, 4}
(*[1,2,3], *[4])                # (1, 2, 3, 4)
{**{'a': 1, 'b': 2}, **{'c': 3}}# {'a': 1, 'b': 2, 'c': 3}

head, *body, tail = [1,2,3,4,5]

# lambda: <return_value>
# lambda <argument1>, <argument2>: <return_value>

# Factorial
from functools import reduce
n = 3
factorial = reduce(lambda x, y: x*y, range(1, n+1))
print(factorial) #6

# Fibonacci
fib = lambda n : n if n <= 1 else fib(n-1) + fib(n-2)
result = fib(10)
print(result) #55

<list> = [i+1 for i in range(10)]         # [1, 2, ..., 10]
<set>  = {i for i in range(10) if i > 5}  # {6, 7, 8, 9}
<iter> = (i+5 for i in range(10))         # (5, 6, ..., 14)
<dict> = {i: i*2 for i in range(10)}      # {0: 0, 1: 2, ..., 9: 18}

output = [i+j for i in range(3) for j in range(3)] # [0, 1, 2, 1, 2, 3, 2, 3, 4]

# Is the same as:
output = []
for i in range(3):
  for j in range(3):
    output.append(i+j)

# <expression_if_true> if <condition> else <expression_if_false>

[a if a else 'zero' for a in [0, 1, 0, 3]] # ['zero', 1, 'zero', 3]

from functools import reduce
list(map(lambda x: x + 1, range(10)))            # [1, 2, 3, 4, 5, 6, 7, 8, 9,10]
list(filter(lambda x: x > 5, range(10)))         # (6, 7, 8, 9)
reduce(lambda acc, x: acc + x, range(10))        # 45

any([False, True, False])# True if at least one item in collection is truthy, False if empty.
all([True,1,3,True])     # True if all items in collection are true

def get_multiplier(a):
    def out(b):
        return a * b
    return out

>>> multiply_by_3 = get_multiplier(3)
>>> multiply_by_3(10)
30

def get_counter():
    i = 0
    def out():
        nonlocal i
        i += 1
        return i
    return out

>>> counter = get_counter()
>>> counter(), counter(), counter()
(1, 2, 3)

if __name__ == '__main__': # Runs main() if file wasn't imported.
    main()

import <module_name>
from <module_name> import <function_name>
import <module_name> as m
from <module_name> import <function_name> as m_function
from <module_name> import *

<iter> = iter(<collection>)
<iter> = iter(<function>, to_exclusive)     # Sequence of return values until 'to_exclusive'.
<el>   = next(<iter> [, default])           # Raises StopIteration or returns 'default' on end.

def count(start, step):
    while True:
        yield start
        start += step

>>> counter = count(10, 2)
>>> next(counter), next(counter), next(counter)
(10, 12, 14)

@decorator_name
def function_that_gets_passed_to_decorator():
    ...

from functools import wraps

def debug(func):
    @wraps(func)
    def out(*args, **kwargs):
        print(func.__name__)
        return func(*args, **kwargs)
    return out

@debug
def add(x, y):
    return x + y

class <name>:
    age = 80 # Class Object Attribute
    def __init__(self, a):
        self.a = a # Object Attribute

    @classmethod
    def get_class_name(cls):
        return cls.__name__

<h2 id="multiple-inheritance">Multiple Inheritance</h2>
```python
class A: pass
class B: pass
class C(A, B): pass

>>> C.mro()
[<class 'C'>, <class 'A'>, <class 'B'>, <class 'object'>]

try:
  5/0
except ZeroDivisionError:
  print("No division by zero!")

while True:
  try:
    x = int(input('Enter your age: '))
  except ValueError:
    print('Oops!  That was no valid number.  Try again...')
  else: # code that depends on the try block running successfully should be placed in the else block.
    print('Carry on!')
    break

<h2 id="command-line-arguments">Command Line Arguments</h2>

```python
import sys
script_name = sys.argv[0]
arguments   = sys.argv[1:]

<file> = open('<path>', mode='r', encoding=None)

<file>.seek(0)                      # Moves to the start of the file.

<str/bytes> = <file>.readline()     # Returns a line.
<list>      = <file>.readlines()    # Returns a list of lines.

<file>.write(<str/bytes>)           # Writes a string or bytes object.
<file>.writelines(<list>)           # Writes a list of strings or bytes objects.

def read_file(filename):
    with open(filename, encoding='utf-8') as file:
        return file.readlines() # or read()

for line in read_file(filename):
  print(line)

def write_to_file(filename, text):
    with open(filename, 'w', encoding='utf-8') as file:
        file.write(text)

def append_to_file(filename, text):
    with open(filename, 'a', encoding='utf-8') as file:
        file.write(text)

import csv

def read_csv_file(filename):
    with open(filename, encoding='utf-8') as file:
        return csv.reader(file, delimiter=';')

def write_to_csv_file(filename, rows):
    with open(filename, 'w', encoding='utf-8') as file:
        writer = csv.writer(file, delimiter=';')
        writer.writerows(rows)

import json
<str>    = json.dumps(<object>, ensure_ascii=True, indent=None)
<object> = json.loads(<str>)

def read_json_file(filename):
    with open(filename, encoding='utf-8') as file:
        return json.load(file)

def write_to_json_file(filename, an_object):
    with open(filename, 'w', encoding='utf-8') as file:
        json.dump(an_object, file, ensure_ascii=False, indent=2)

import pickle
<bytes>  = pickle.dumps(<object>)
<object> = pickle.loads(<bytes>)

def read_pickle_file(filename):
    with open(filename, 'rb') as file:
        return pickle.load(file)

def write_to_pickle_file(filename, an_object):
    with open(filename, 'wb') as file:
        pickle.dump(an_object, file)

from time import time
start_time = time()  # Seconds since
...
duration = time() - start_time

from math import e, pi
from math import cos, acos, sin, asin, tan, atan, degrees, radians
from math import log, log10, log2
from math import inf, nan, isinf, isnan

from statistics import mean, median, variance, pvariance, pstdev

from random import random, randint, choice, shuffle
random() # random float between 0 and 1
randint(0, 100) # random integer between 0 and 100
random_el = choice([1,2,3,4]) # select a random element from list
shuffle([1,2,3,4]) # shuffles a list

from datetime import date, time, datetime, timedelta
from dateutil.tz import UTC, tzlocal, gettz

<D>  = date(year, month, day)
<T>  = time(hour=0, minute=0, second=0, microsecond=0, tzinfo=None, fold=0)
<DT> = datetime(year, month, day, hour=0, minute=0, second=0, ...)
<TD> = timedelta(days=0, seconds=0, microseconds=0, milliseconds=0,
                 minutes=0, hours=0, weeks=0)

<D/DTn>  = D/DT.today()                     # Current local date or naive datetime.
<DTn>    = DT.utcnow()                      # Naive datetime from current UTC time.
<DTa>    = DT.now(<tz>)                     # Aware datetime from current tz time.

<tz>     = UTC                              # UTC timezone.
<tz>     = tzlocal()                        # Local timezone.
<tz>     = gettz('<Cont.>/<City>')          # Timezone from 'Continent/City_Name' str.

<DTa>    = <DT>.astimezone(<tz>)            # Datetime, converted to passed timezone.
<Ta/DTa> = <T/DT>.replace(tzinfo=<tz>)      # Unconverted object with new timezone.

import re
<str>   = re.sub(<regex>, new, text, count=0)  # Substitutes all occurrences.
<list>  = re.findall(<regex>, text)            # Returns all occurrences.
<list>  = re.split(<regex>, text, maxsplit=0)  # Use brackets in regex to keep the matches.
<Match> = re.search(<regex>, text)             # Searches for first occurrence of pattern.
<Match> = re.match(<regex>, text)              # Searches only at the beginning of the text.

<str>   = <Match>.group()   # Whole match.
<str>   = <Match>.group(1)  # Part in first bracket.
<tuple> = <Match>.groups()  # All bracketed parts.
<int>   = <Match>.start()   # Start index of a match.
<int>   = <Match>.end()     # Exclusive end index of a match.

'\d' == '[0-9]'          # Digit
'\s' == '[ \t\n\r\f\v]'  # Whitespace
'\w' == '[a-zA-Z0-9_]'   # Alphanumeric