Python Cheatsheet

Basics

display

print("hello world")

comments

• # is used to comment a line in Python
• You can also comment using a docstring format[Multiline comment]

Data Types

Category	Data Type
Text	str
Number	int, float, complex
Boolean	bool
Binary	bytes, bytearray, memoryview
Set	set, frozenset
Sequence	list, tuple, range
Mapping	dict
None	NoneType

• type() is used to know the data type of a variable

Data casting

Constructor function	desc
int()	constructs an integer from any form of data like string, float or integer
float()	constructs a float number from any form of data like string, float or integer
str()	constructs a string from any form of data like string, float or integer

Type Casting

In Python, Type casting or type converion is the method to convert one data-type to another data-type.
Basically there are 2 types of type casting:

1:Implicit
This method of type casting is performed by the machine or the python interpreter itself. There is no need to define externally.

2:Explicit
In this method of type casting,the user performs the changes by using the predefined functions externally.There can be a loss of data in this type of type conversion

Variables

In Python, declaring variables is not required. Means you don't need to specify whether it is an integer or string etc as Python is a dynamically typed language.

Operators

Type	Operators
Arithmetic Operators	+ - * / % ** //
Comparision Operators	== != > >= < <=
Bitwise Operators	& ^ | ^ ~ << >>
Logical Operators	&& || !
Assignment Operators	= += -= *= /= %= **= //=
Membership Operators	in, not in
Identity Operators	is, is not

Functions

# declaring a function
def function-name(parameters){ # here parameters are optional
    #code
}
function-name(parameters); # calling a function

Collections

1. List

List is ordered collection of items and are mutable [] are used to represent lists.

Example

mylist1=["iPhone","Pixel","Samsung"]
print(mylist1[0]) # prints iPhone
print(mylist1[7]) # throws IndexError : list index out of range
print(mylist1[-1]) # prints Samsung

Using List() to declare a list.

mylist2=list()
mylist2.append("iPhone") # adds iPhone to the end of list
mylist2.append("Pixel") # adds Pixel to the end of list
mylist2.append("Samsung") # adds Samsung to the end of list
print(mylist2[0]) # prints iPhone
print(mylist2[7]) # throws IndexError : list index out of range
print(mylist2[-1]) # prints Samsung

Operations

Operation	Description
lst.append(val)	add an item to list at end
lst.extend(seq)	add sequence of items to list at end
lst.insert(index,val)	insert an item at given index
lst.remove(val)	remove first item with value val
lst.pop([index])→value	remove & return item at index
lst.sort()	sort the given list items
lst.reverse()	reverse the given list items
lst.count()	Returns the number of elements with the specified value
lst.clear()	Removes all the elements from the list
lst.index()	Returns the index of the first element with the specified value
lst.copy()	Returns a copy of the list

2. Tuple

Tuple is ordered collection of items and are immutable () are used to represent Tuples.

Example

myTuple = ["iPhone","Pixel","Samsung"]
print(myTuple[0]) # prints iPhone
print(myTuple[7]) # throws IndexError: tuple index out of range
print(myTuple[-1]) # prints Samsung

3. Set

Set is unordered collection of items and it is unindexed. {} are used to represent sets.
Note- to create an empty set use the built-in set() function. If we write new_set = {} this will create a dictionary instead of a set.

Example

mySet = {"iPhone", "Pixel", "Samsung"}
mySet.add('OnePlus')
print(mySet) # prints {'iPhone', 'Samsung', 'OnePlus', 'Pixel'}

Operations

Method	Description	Usage
add()	to add an element to the set	mySet.add('value')
clear()	to remove all the elements from the set	mySet.clear()
pop()	to remove last element from the set	mySet.pop()
remove()	to remove a specified element from the set	mySet.remove("value")
del()	to delete a set	del myset
copy()	to return a copy of the set	copySet = mySet.copy()
union()	to return a set containing the union of sets	mySet3 = mySet1.union(mySet2)
update()	to update the set with the union of this set and others	mySet1.update(mySet2)

4. Dictionary

Dictionary is a collection of key value pairs which is unordered, can be changed, and indexed. They are written in curly brackets with key - value pairs.
Note- From Python 3.6 new Dict() method implementation had made it possible for maintaining the order of the key-value pairs added in the respective order.

Example

mydict = {
    "brand" :"iPhone",
    "model": "iPhone 11"
}
val = mydict["brand"]
print(val) # prints iPhone

Operation	Description
d[key]=value	To add a new key-value pair to dictionary or to change it's value if key is existing
d.copy()	Returns a copy of the dictionary
d.keys()	Returns a list containing all the dictionary's keys
d.values()	Returns a list of all the values in the dictionary
d.items()	Removes the element with the specified key
d.clear()	To empty the dictionary items.
del d[key]	To remove an item from a dictionary.
d.pop(key)	To remove an item from a dictionary.
d.popitem()	removes the item that was last inserted into the dictionary
d.get(key)	Returns the value of the specified key
d.setdefault(key)	Returns the value of the specified key. If the key does not exist then returns the default value provided
d.fromkeys(key,value)	Returns a dictionary with specified keys and values

Conditional Statements

1. If

if conditional-expression :
    #code

2. If-else

if conditional-expression :
    #code
else :
    #code

3. If-elif-else Ladder

if conditional-expression :
    #code
elif conditional-expression :
    #code
else :
    #code

4. Try Except

The try block lets you test a block of code for errors. The except block lets you handle the error.

try:
    print(1/0)
except:
    print("You can't divide by zero!")

The finally block lets you execute code, regardless of the result of the try- and except blocks.

try:
    print(x)
except:
    print("Variable x is not defined")
finally:
    print("The program is finished.")

You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error.

a = input("Digit a number: ")
try:
    b = [i for i in range(int(a))]
    print(b[3])
except ValueError:
    print("You didn't digit a number.")
except IndexError:
    print("Your list have less than 4 numbers.")
finally:
    print("Python is cool.")

Loops

1. For

For loop is used to iterate over arrays(list, tuple, set, dictionary) or strings.

Syntax

for variable in arrays :
    #code

2. While

while condition  
    #code 

Useful String Methods

str.strip()	str.lower()	str.upper()
str.replace("str to be replaced","new string to replace")	str.split("seperator")	len(str)
+ for concatenation	str.count(substr)	str.find(substr)
str.index(substr, start, end)	str.join(array)	str.partition(substr)
str.zfill(len)	str.swapcase()	str.isdecimal()
str.isdigit()	str.islower()	str.isupper()
str.endswith(value, start, end)	str.startswith(value, start, end)	str.isspace()

Python with Mongodb

Install a python driver pymongo to connect with MongoDB.

Create Database

import pymongo

db = pymongo.MongoClient("mongodb://localhost:27017/")

mydb = db["sample"]

Create collection

import pymongo
db = pymongo.MongoClient("mongodb://localhost:27017/")
mydb = db["sample"]

mycln = mydb["details"]

Insert

#insert a single document
doc = mycln.insert_one(mydict)
# insert multiple documents
mylist =[
    {"name": "foo", "age": 20},
    {"name": "bar", "age": 25},
    {"name": "apple", "age": 30}
]
doc1 = mycln.insert_many(mylist)

Read

#to return all the documents
for doc in mycln.find():
  print(doc)

# to return first occurence
doc1=mycln.find_one()
print(doc1)

Update

# to update a single document
mycln.update_one({"name" : "foo"}, {"$set":{"age": 23}})

Delete

# to delete a single document
doc1 = mycln.delete_one({"name" : "foo"})

Python with Files

Create

Use open() function with c or a or w as mode.

file = open("myfile.txt","c")

Read

Use open() function with r as mode.

file = open("myfile.txt","r")
print(file.read())

Update or Append

Use open() function with a or w as mode.

file = open("myfile.txt","a")
file.write("Happy learning!!")
file.close()

Delete

For deleting files, you must import os module and use os.remove() function.

import os
os.remove(filename)

nested list --list within list

<!-- easily accessible elements using indexes -->

L=['a',['bb','cc'],'d']
L[1][1]=0
print(L)

<!-- prints ['a',['bb',0],'d'] -->