Python Cheatsheet

Basics

Basic syntax from the python programming language

Showing Output To User

The print function is used to display or print output as follows

print("Content that you wanna print on screen")
Python

Taking Input From the User

The input function is used to take input as string or character from the user as follows:

var1 = input("Enter your name: ")
print("My name is: ", var1)
Python

To take input in form of other datatypes we need to typecaste them as follows:-

To take input as an integer:-

var1=int(input("enter the integer value"))
print(var1)
Python

To take input as an float:-

var1=float(input("enter the float value"))
print(var1)
Python

range Function

range function returns a sequence of numbers, eg, numbers starting from 0 to n-1 for range(0, n) 

range(int_start_value,int_stop_value,int_step_value)
Python

Here the start value and step value are by default 1 if not mentioned by the programmer. but int_stop_value is the compulsory parameter in range function

example-

#Display all even numbers between 1 to 100
for i in range(0,101,2):
    print(i)
Python

Comments

Comments are used to make the code more understandable for programmers, and they are not executed by compiler or interpreter.

Single line comment

# This is a single line comment
Python

Multi-line comment

'''This is a
multi-line
comment'''
Python

Escape Sequence

An escape sequence is a sequence of characters; it doesn't represent itself (but is translated into another character) when used inside a string literal or character. Some of the escape sequence characters are as follows:

Newline

Newline Character

print("\n")
Python

Backslash

It adds a backslash

print("\\")
Python

Single Quote

It adds a single quotation mark

print("\'")
Python

Tab

It gives a tab space

print("\t")
Python

Backspace

It adds a backspace

print("\b")
Python

Octal value

It represents the value of an octal number

print("\ooo")
Python

Hex value

It represents the value of a hex number

print("\xhh")
Python

Carriage Return

Carriage return or \r will just work as you have shifted your cursor to the beginning of the string or line.

print("\r")
Python

Strings

Python string is a sequence of characters, and each character can be individually accessed using its index.

String

You can create Strings by enclosing text in both forms of quotes - single quotes or double quotes.

variable_name = "String Data"
Python

Example:

example_str = "Shruti"
print("string is ", example_str)
Python

Indexing

The position of every character placed in the string starts from the 0th position and steps by step, it ends at the length-1 position.

Slicing

Slicing refers to obtaining a sub-string from the given string. The following code will include index 1, 2, 3, and 4 for the variable named var_name.

Slicing of the string can be obtained by the following syntax:

string_var[int_start_value:int_stop_value:int_step_value]
Python
var_name[1:5]
Python

Here start and step value are considered 0 and 1, respectively if not mentioned by the programmer.

isalnum() method

Returns True if all the characters in the string are alphanumeric, else False.

string_variable.isalnum()
Python

isalpha() method

Returns True if all the characters in the string are alphabets.

string_variable.isalpha()
Python

isdecimal() method

Returns True if all the characters in the string are decimals.

string_variable.isdecimal()
Python

isdigit() method

Returns True if all the characters in the string are digits.

string_variable.isdigit()
Python

islower() method

Returns True if all characters in the string are lowercase.

string_variable.islower()
Python

isspace() method

Returns True if all characters in the string are whitespaces.

string_variable.isspace()
Python

isupper() method

Returns True if all characters in the string are uppercase.

string_variable.isupper()
Python

lower() method

Converts a string into lowercase equivalent.

string_variable.lower()
Python

upper() method

Converts a string into uppercase equivalent.

string_variable.upper()
Python

strip() method

It removes leading and trailing spaces in the string.

string_variable.strip()
Python

List

A List in Python represents a list of comma-separated values of any data type between square brackets.

var_name = [element1, element2, ...]
Python

These elements can be of different data types.

Indexing

The position of every element placed in the list starts from the 0th position and step by step, it ends at length-1 position.

List is ordered, indexed, mutable, and the most flexible and dynamic collection of elements in Python.

Empty List

This method allows you to create an empty list.

my_list = []
Python

index method

Returns the index of the first element with the specified value.

list.index(element)
Python

append method

Adds an element at the end of the list.

list.append(element)
Python

extend method

Add the elements of a given list (or any iterable) to the end of the current list.

list.extend(iterable)
Python

insert method

Adds an element at the specified position.

list.insert(position, element)
Python

pop method

Removes the element at the specified position and returns it.

list.pop(position)
Python

remove method

The remove() method removes the first occurrence of a given item from the list.

list.remove(element)
Python

clear method

Removes all the elements from the list.

list.clear()
Python

count method

Returns the number of elements with the specified value.

list.count(value)
Python

reverse method

Reverses the order of the list.

list.reverse()
Python

sort method

Sorts the list.

list.sort(reverse=True|False)
Python

Tuples

Tuples are represented as comma-separated values of any data type within parentheses.

variable_name = (element1, element2, ...)
Python

These elements can be of different data types.

Indexing

The position of every element placed in the tuple starts from the 0th position and step by step, it ends at length-1 position.

Tuples are ordered, indexed, immutable, and the most secured collection of elements.

Lets talk about some of the tuple methods:

count method

It returns the number of times a specified value occurs in a tuple.

tuple.count(value)
Python

index method

It searches the tuple for a specified value and returns the position.

tuple.index(value)
Python

Sets

A set is a collection of multiple values which is both unordered and unindexed. It is written in curly brackets.

Set Creation: Way 1

var_name = {element1, element2, ...}
Python

Set Creation: Way 2

var_name = set([element1, element2, ...])
Python

Set is an unordered, immutable, non-indexed type of collection. Duplicate elements are not allowed in sets.

Set Methods

add() method

Adds an element to a set.

set.add(element)
Python

clear() method

Remove all elements from a set.

set.clear()
Python

discard() method

Removes the specified item from the set.

set.discard(value)
Python

intersection() method

Returns intersection of two or more sets.

set.intersection(set1, set2, ... etc)
Python

issubset() method

Checks if a set is a subset of another set.

set.issubset(set)
Python

pop() method

Removes an element from the set.

set.pop()
Python

remove() method

Removes the specified element from the set.

set.remove(item)
Python

union() method

Returns the union of two or more sets.

set.union(set1, set2, ...)
Python

Dictionaries

The dictionary is an unordered set of comma-separated key:value pairs, within {}, with the requirement that within a dictionary, no two keys can be the same.

Dictionary

<dictionary-name> = {<key>: value, <key>: value ...}
Python

Dictionary is an ordered and mutable collection of elements. Dictionary allows duplicate values but not duplicate keys.

Empty Dictionary

By putting two curly braces, you can create a blank dictionary.

mydict = {}
Python

Adding Element to a Dictionary

By this method, one can add new elements to the dictionary.

<dictionary>[<key>] = <value>
Python

Updating Element in a Dictionary

If a specified key already exists, then its value will get updated.

<dictionary>[<key>] = <value>
Python

Deleting an Element from a Dictionary

The `del` keyword is used to delete a specified key:value pair from the dictionary as follows:

del <dictionary>[<key>]
Python

Dictionary Functions & Methods

Below are some of the methods of dictionaries.

len() method

It returns the length of the dictionary, i.e., the count of elements (key: value pairs) in the dictionary.

len(dictionary)
Python

clear() method

Removes all the elements from the dictionary.

dictionary.clear()
Python

get() method

Returns the value of the specified key.

dictionary.get(keyname)
Python

items() method

Returns a list containing a tuple for each key-value pair.

dictionary.items()
Python

keys() method

Returns a list containing the dictionary's keys.

dictionary.keys()
Python

values() method

Returns a list of all the values in the dictionary.

dictionary.values()
Python

update() method

Updates the dictionary with the specified key-value pairs.

dictionary.update(iterable)
Python

Indentation

In Python, indentation means the code is written with some spaces or tabs into many different blocks of code to indent it so that the interpreter can easily execute the Python code.

Indentation is applied on conditional statements and loop control statements. Indent specifies the block of code that is to be executed depending on the conditions.

Conditional Statements

The if, elif, and else statements are the conditional statements in Python, and these implement selection constructs (decision constructs).

if Statement

if (conditional expression):
    statements
Python

if-else Statement

if (conditional expression):
    statements
else:
    statements
Python

if-elif Statement

if (conditional expression):
    statements
elif (conditional expression):
    statements
else:
    statements
Python

Nested if-else Statement

if (conditional expression):
    if (conditional expression):
        statements
    else:
        statements
else:
    statements
Python

Example:

a = 15
b = 20
c = 12
if (a > b and a > c):
    print(a, "is greatest")
elif (b > c and b > a):
    print(b, " is greatest")
else:
    print(c, "is greatest")
Python

Loops in Python

A loop or iteration statement repeatedly executes a statement, known as the loop body, until the controlling expression is false (0).

For Loop

for <variable> in <sequence>:
    statements_to_repeat
Python

Example:

for i in range(1, 101, 1):
    print(i)
Python

While Loop

while <logical-expression>:
    loop-body
Python

Example:

i = 1
while (i <= 100):
    print(i)
    i = i + 1
Python

Break Statement

The break statement enables a program to skip over a part of the code. A break statement terminates the very loop it lies within.

for <var> in <sequence>:
    statement1
    if <condition>:
        break
    statement2
statement_after_loop
Python

Example:

for i in range(1, 101, 1):
    print(i, end=" ")
    if (i == 50):
        break
    else:
        print("Mississippi")
print("Thank you")
Python

Continue Statement

The continue statement skips the rest of the loop statements and causes the next iteration to occur.

for <var> in <sequence>:
    statement1
    if <condition>:
        continue
    statement2
    statement3
    statement4
Python

Example:

for i in [2,3,4,6,8,0]:
    if (i%2!=0):
        continue
    print(i)
Python

Functions

A function is a block of code that performs a specific task. You can pass parameters into a function. It helps us to make our code more organized and manageable.

Function Definition

def my_function():
      # statements
Python

The `def` keyword is used before defining the function.

Function Call

my_function()
Python

Whenever we need that block of code in our program, simply call that function name whenever needed. If parameters are passed during defining the function, we have to pass the parameters while calling that function.

Example:

def add():       # function definition
    a = 10
    b = 20
    print(a + b)
add()            # function call
Python

Return statement in Python function

The function return statement returns the specified value or data item to the caller.

return [value/expression]
Python

Arguments in Python function

Arguments are the values passed inside the parenthesis of the function while defining as well as while calling.

def my_function(arg1, arg2, arg3, ... argn):
      # statements
my_function(arg1, arg2, arg3, ... argn)
Python

Example:

def add(a, b):
    return a + b
x = add(7, 8)
print(x)
Python

File Handling

File handling refers to reading or writing data from files. Python provides some functions that allow us to manipulate data in the files.

open() function

var_name = open("file name", " mode")
Python

Modes:

r - to read the content from the file

w - to write the content into the file

a - to append the existing content into the file

r+ - To read and write data into the file. The previous data in the file will be overridden.

w+ - To write and read data. It will override existing data.

a+ - To append and read data from the file. It won’t override existing data.

close() function

var_name.close()
Python

read() function

The read function contains different methods: read(), readline(), and readlines()

read()  # return one big string
It returns a list of lines

readlines()  # returns a list
It returns one line at a time

readline  # returns one line at a time
Python

write function

This function writes a sequence of strings to the file.

write()  # Used to write a fixed sequence of characters to a file
It is used to write a list of strings

writelines()
Python

Exception Handling

An exception is an unusual condition that results in an interruption in the flow of a program.

try and except

A basic try-catch block in Python. When the try block throws an error, the control goes to the except block.

try:
    # [Statement body block]
    raise Exception()
except Exceptionname:
    # [Error processing block]
Python

else

The else block is executed if the try block has not raised any exception, and the code has been running successfully.

try:
    # statements
except:
    # statements
else:
    # statements
Python

finally

The finally block will be executed even if the try block of code has been running successfully or the except block of code is executed. The finally block of code will be executed compulsorily.

try:
    # statements
except:
    # statements
else:
    # statements
finally:
    # statements
Python

Object Oriented Programming (OOPS)

It is a programming approach that primarily focuses on using objects and classes. The objects can be any real-world entities.

class

The syntax for writing a class in Python:

class class_name:
    pass  # statements
Python

Creating an object

Instantiating an object can be done as follows:

<object-name> = <class-name>(<arguments>)
Python

self parameter

The self parameter is the first parameter of any function present in the class. It can be of a different name, but this parameter is a must while defining any function in the class, as it is used to access other data members of the class.

class with a constructor

Constructor is a special function of the class used to initialize the objects. The syntax for writing a class with a constructor in Python:

class CodeWithHarry:

    # Default constructor
    def __init__(self):
        self.name = "CodeWithHarry"

    # A method for printing data members
    def print_me(self):
        print(self.name)
Python

Inheritance in Python

By using inheritance, we can create a class which uses all the properties and behavior of another class. The new class is known as a derived class or child class, and the one whose properties are acquired is known as a base class or parent class.

It provides the re-usability of the code.

class Base_class:

class Base_class:
    pass
Python

class Derived_class(Base_class):

class Derived_class(Base_class):
    pass
Python

Types of Inheritance:

  1. Single inheritance
  2. Multiple inheritance
  3. Multilevel inheritance
  4. Hierarchical inheritance

filter function

The filter function allows you to process an iterable and extract those items that satisfy a given condition.

filter(function, iterable)
Python

issubclass function

Used to find whether a class is a subclass of a given class or not, as follows:

issubclass(obj, classinfo)  # returns true if obj is a subclass of classinfo
Python

Iterators and Generators

Here are some of the advanced topics of the Python programming language like iterators and generators.

Iterator

Used to create an iterator over an iterable.

iter_list = iter(['Harry', 'Aakash', 'Rohan'])
print(next(iter_list))
print(next(iter_list))
print(next(iter_list))
Python

Generator

Used to generate values on the fly.

A simple generator function

def my_gen():
    n = 1
    print('This is printed first')
    # Generator function contains yield statements
    yield n
    n += 1
    print('This is printed second')
    yield n
    n += 1
    print('This is printed at last')
    yield n
Python

Decorators

Decorators are used to modifying the behavior of a function or a class. They are usually called before the definition of a function you want to decorate.

property Decorator (getter)

@property
def name(self):
    return self.__name
Python

setter Decorator

It is used to set the property 'name'

@name.setter
def name(self, value):
    self.__name = value
Python

deleter Decorator

It is used to delete the property 'name'

@name.deleter  # property-name.deleter decorator
def name(self, value):
    print('Deleting..')
    del self.__name
Python