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July 15, 2021 07:24 pm GMT

Python Cheatsheet

Cheatsheets are good when you want to revise some of the concepts, but an idle way to start learning

I would recommend you to learn in depth from this course: Udemy Course

If you can self learn Repository: Github

Index

Theory

  • Python is a scripting language.
  • Scripting vs Compiled ? - Language like c++/java's code needs to compiled by its compiler, after compilation it is just machine level code.Where as in a scripting language its interpreter will be run the code one the spot one line at a time.

VSCODE extension:

  • Python
  • Python for vscode
  • Magic Python
  • Arepl

Making a virtual env:

Why Virtual env?

Because managing python dependencies is a mess, this will install dependencies for that project only instead of globally.

  • python -m venv venv this will create a venv folder in your directory.
  • source ./venv/bin/activate this will activate this virtual env.

Comments in python

  • single line use #
# single line comments
  • multiline use '''. often called as docstring, as it is just to document function/classes.
'''This is a example of Multiline comment.'''

Data Types:

  • int # Whole Numbers
  • float # Number with decimals.
  • str # String
  • list # ordered sequence of object
  • dict # unordered key-value pairs.
  • tup # ordered immutable seq. of objects.
  • set # Unordered collection of unique objs.
  • bool # Logical value True / False.
  • None # no value

Naming conventions:

  • Use underscore for variables.
  • variables cannot Start with a number.
  • Avoid special meaning keywords.
  • Use snake case for functions.
  • Use CamelCase for Classes names.

Printing in Python:

print("")

Numbers in Python:

print(1+2) # Additionprint(3-2) # Subtractionprint(3*2) # Multiplicationprint(3/2) # Divisionprint(3%2) # Mod.print(3**2) # Powerprint((3 + 10) * 15) # Using Braces.

Using with variables:

a = 10print(a)# TO check the type:print(type(a))

Variables makes it easy to understands the code.

my_income = 100tax_rate = 0.1my_taxes = my_income*tax_rateprint("My income tax is",my_taxes)

Strings in Python:

String is nothing but ordered seq. of characters.
Note: Strings are immutable

Using directly with print:

print("Simple String")print('Add quotes inside the  "string" by using single quote.')print("concat string "+"like this")

Taking Input

greeting = "Hello"name = input("Enter your name: ")print(greeting + ' ' + name)

Use , instead of + This will auto seprate them by spaces.

print(greeting,name)

Escape characters in python:

print("Hello
World")# orprint(""" ---Helloworld\Yeahh!!"Quotes"'Single quote' ---""")

Check the length of string:

print(len("Hey"))

String indexing

a = "Hello"a[0] Will return H

String Slicing

a[start:end:step]a[0:2] # Start from 0th index till 2(excluding)a[::2] # Step will be 2.# We can use this to print a string backwardss[::-1]

String methods:

# Multiply Stringsa = 'H' * 10
# Upper Case a strings.upper()
# Lower cases.lower()
# Splittings.split('W')s.split() # split via whitespace.

Formatting strings:

  • .format()
  • f"" F-string
print('The {2} {1} {0}'.format('fox','brown','quick'))print('First Object: {a}, Second Object: {b}, Third Object: {c}'.format(a=1,b='Two',c=12.3))num = 23.45print("My 10 character, four decimal number is:{0:10.4f}".format(num))print(f"My 10 character, four decimal number is:{num:10.4f}")

String Print alignment

left_alignment = "Left Text"center_alignment = "Centered Text"right_alignment = "Right Text"print(f"{left_alignment : <20}|{center_alignment : ^15}|{right_alignment : >20}")More about this: https://pyformat.info/

Lists in Python:

Basic Usage

# Supports dynamic types, as it is python :)my_list = [100,2.5,"Mohit"]# len(my_list) for length# Change objs:my_list[0]=1000

Slicing is same as String slicing

Concat

a = [1,2,3]b = [4,5]c = a + b

Append the list

my_list.append(10.8)

Poping objs

my_list.pop(index) # default index is -1, returns popped element.

Sorting

a = [1,2,3]a.sort() # in-place sort, it will modify the list, returns None# Tip: use sorted(a) it will return the value instead of in-place

Reverse

a = [1,2,3]a.reverse() # also in-place

Nested list

a = [1, 2, 3, [4,5,]]print(a[3][1]) # Returns 5.

Dictionaries in Python

Unordered key-value mappings, basically you can have custom keys
Think it like, you can use this to make dynamic variables, where key will be the variable name.
Like list value can be any data type.

Basic Usage

prices = {"apple":10, "orange":20.5}print(prices)print(prices["apple"])print(prices.keys()) # get keysprint(prices.values()) # get valuesprint(prices.items()) # get items, return tuples with keys and valuesprint(prices.pop("apple")) # Pop the objectprint(prices)print(prices.clear()) # Clear Allprint(prices)print(prices.get("banana")) # it will check if it is present, return None if not.print(prices.__contains__("apple")) # Returns true/false

Tuples in Python

Same as list, but immutable.

Basic Usage

a = (1,2,2,4)print(a)# Interesting Fact: tuple supports only two methods:a.count(2) # This can be use with list as well.a.index(3) # This can be use with list as well.

Sets in Python

Sets are an unordered collection of unique elements.

a = set()a.add(1)a.add(1)a.add(1)print(a) # {1}

Convert list to set

a = [1,1,2,2,2,3,3,3]a = set(a)

File IO with Python

init file obj

file = open("file.txt")

read contents

contents = file.read()print(contents)

move the cursor/pointer

file.seek(0)

read line by line

contents = file.readlines() # returns list of lines.

close the file

file.close()

Using context manager

with open("file.txt") as file:    print(file.read())

different modes for file

  • r: Read
  • r+: Read and Write
  • w: Write (will override the file)
  • w+: Write + Read (will override the file)
  • a: Append the file

Chaining comparison operators:

To chain ==, != <, >, >=, <= and is these operators, we have these logical operators

  • and
  • or
  • not
if 2 > 3 and 2 > 5:    print("I am inevitable")if "hello" is "world" or "india" is "country":    print("Yeah!!")if not 10 == 10:    print("Tough luck!" )

Python Statements:

Indentation is important in the python.

if, elif, else

loc = 'Bank'if loc == 'Auto Shop':    print('Welcome to the Auto Shop!')elif loc == 'Bank':    print('Welcome to the bank!')else:    print('Where are you?')

for loops

# iterate list/string/tuplel = [1,2,3]for item in l:    print(item)# extraction made easylist2 = [(2,4),(6,8),(10,12)]for t1,t2 in list2: # Same as dict. t1 will be key, t2 will be value.    print(t1) # will print 2,6,10# Protip about dict: use .value() and .keys() for looping Values/keys.

white loops

in python we can use python with else statement.

x = 1while x < 3:    print(f"x is {x}")    x = x + 1else:    print("Uhhoo! x > 3")

Statement in python

  • break: Breaks out of the current closest enclosing loop.
  • continue: Goes to the top of the closest enclosing loop.
  • pass: Does nothing at all, Programmers use this for placeholder.
def future_method():    # Todo: implement it later.    passwhile True:    breakfor i in range(10):    if i == 5:        #omit        continue     print(i)

Some useful operators

range()

range is a generator.

Syntax: range(start,end,step)


Use directly with loops for iteration.

a = list(range(0,11,2)) # returns 0,2,4,..10

enumerate()

with help of this we can keep track of index and value.

a = [20,100,5,3,6]for index,value in enumerate(a):    print(f"{index}{value}")

zip()

zip multiple lists.a = [1,2,3]b = [4,5,6]for item in zip(a,b):    print(item)

in operator:

a = [1,2,3]print(3 in a) # True

min and max:

a = [1,2,3]print(min(a)) # 1print(max(a)) # 3

List Comprehensions

Quicker and unique way to create lists.

# Grab every letter in stringlst = [x for x in 'word']# Square numbers in range and turn into listlst = [x**2 for x in range(0,11)]# Check for even numbers in a rangelst = [x for x in range(11) if x % 2 == 0]

help function in python

if you are lazy like me, want to learn documentation about specific inbuilt method via terminal, you can use help()

a = [1,2,3]help(a.insert) # will print info about this method

Functions in python

Basic function with argument and default value

# def keyword to define functions.def say_hello(name="world"):    print(f"Hello {name}!")    # or return f"Hello {name}!" if you want to return it.

*args and **kwargs

  • *args: N number of arguments, returns tuple.
  • **kwargs: N number of keyword arguments, returns dict.
def total_income(*args, **kwargs):    print(f"Income for month, {kwargs['month']} is : {sum(args)}")total_income(10,20,300,month="July")

lamda, filter and map

#mapdef square(num):    return num**2my_nums = [1,2,3,4,5]map(square,my_nums) # 1, 4, 9, 16, 25# filterdef check_even(num):    return num % 2 == 0nums = [0,1,2,3,4,5,6,7,8,9,10]filter(check_even, nums) # 0, 2, 4, 6, 8, 10# lets convert each of the above function to lambda.map(lambda num:num**2,my_nums)filter(lambda num:num%2==0, nums) 

Classes in python

Basic implementation

class Circle:    pi = 3.14    # Circle gets instantiated with a radius (default is 1)    def __init__(self, radius=1):        self.radius = radius         self.area = radius * radius * Circle.pi    # Method for resetting Radius    def setRadius(self, new_radius):        self.radius = new_radius        self.area = new_radius * new_radius * self.pi    # Method for getting Circumference    def getCircumference(self):        return self.radius * self.pi * 2c = Circle()print('Radius is: ',c.radius)print('Area is: ',c.area)print('Circumference is: ',c.getCircumference())

Inheritance

class Animal:    def __init__(self):        print("Animal created")    def whoAmI(self):        print("Animal")    def eat(self):        print("Eating")class Dog(Animal):    def __init__(self):        Animal.__init__(self)        print("Dog created")    def whoAmI(self):        print("Dog")    def bark(self):        print("Woof!")

Polymorphism

class Animal:    def __init__(self, name):    # Constructor of the class        self.name = name    def speak(self):              # Abstract method, defined by convention only        raise NotImplementedError("Subclass must implement abstract method")class Dog(Animal):    def speak(self):        return self.name+' says Woof!'class Cat(Animal):    def speak(self):        return self.name+' says Meow!'fido = Dog('Fido')isis = Cat('Isis')print(fido.speak())print(isis.speak())

Using Special methods

Just like __init__ we have more special methods.

class Book:    def __init__(self, title, author, pages):        print("A book is created")        self.title = title        self.author = author        self.pages = pages    def __str__(self):        return "Title: %s, author: %s, pages: %s" %(self.title, self.author, self.pages)    def __len__(self):        return self.pages    def __del__(self):        print("A book is destroyed")book = Book("Python Rocks!", "Jose Portilla", 159)#Special Methodsprint(book)print(len(book))del book

Exception Handling

try, except, finally, and else.

def askint():    while True:        try:            val = int(input("Please enter an integer: "))        except:            # You can also expect specific error like TypeError or generic type Exception            print("Looks like you did not enter an integer!")            continue        else:            print("Yep that's an integer!")            break        finally:            print("Finally, I executed!")        print(val)

Decorators

def new_decorator(func):    def wrap_func():        print("Code would be here, before executing the func")        func()        print("Code here will execute after the func()")    return wrap_func@new_decoratordef func_needs_decorator():    print("This function is in need of a Decorator")func_needs_decorator()# Code would be here, before executing the func# This function is in need of a Decorator# Code here will execute after the func()

Generators

# Without generatordef get_me_cubes(n):    output_list = []    for i in range(n):        output_list.append(i**3)    return output_listprint(get_me_cubes(10))# With generatordef generate_cubes(n):    for i in range(n):        yield i**3print(generate_cubes(10))

Useful Python modules you should look.

  • collections
  • os
  • shutil
  • datetime
  • math
  • random
  • pdb
  • re
  • timeit
  • zipfile

Working with CSVs in python

# don't forget to install csvimport csvdata = open('example.csv',encoding="utf-8")# passing encoding is important otherwise you will get the Unicode error.csv_data = csv.reader(data)# readingdata_lines = list(csv_data)# writing file_to_output = open('to_save_file.csv','w',newline='')# use 'a' for appendcsv_writer = csv.writer(file_to_output,delimiter=',')csv_writer.writerow(['a','b','c'])file_to_output.close()

Working with pdfs in python

# don't forget to use PyPDF2import PyPDF2f = open('Working_Business_Proposal.pdf','rb')# we need to pass rb for binary files.pdf_text = []pdf_reader = PyPDF2.PdfFileReader(f)for p in range(pdf_reader.numPages):    page = pdf_reader.getPage(p)    pdf_text.append(page.extractText())

Sending Emails with python

import smtplibsmtp_object = smtplib.SMTP('smtp.gmail.com',587)email = "[email protected]"password = "yourpassword"# Tip: search about how you generate app passwords.smtp_object.login(email,password)from_address = "[email protected]"to_address = "[email protected]"subject = "Subject"message = "Message"msg = "Subject: " + subject + '
' + messagesmtp_object.sendmail(from_address,to_address,msg)smtp_object.quit()

Original Link: https://dev.to/tanwanimohit/python-cheatsheet-25a3

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