Python Cheat Sheet: The Basics

A quick reference for Python fundamentals, ideal for Data Science, AI, and Development learners.

Python Data Types

String

A sequence of characters stored as text.

my_string = "Hello" 

Common operations:

my_string.upper()      # Convert to uppercase

len(my_string)         # Get length

my_string.find('l')    # Find index of first 'l'

my_string.replace('H', 'C')  # Replace 'H' with 'C'

Integer

Whole numbers.

my_integer = 12321 

Float

Decimal numbers.

my_decimal = 3.14 

Boolean

True or False values.

a = True b = False 

Dictionary

Changeable collection of key-value pairs.

my_dictionary = {'banana': 1, 12: 'laptop', (0,0): 'center'}

my_dictionary['banana']      # Access value

my_dictionary.keys()         # List of keys

my_dictionary.values()       # List of values

Tuple

Unchangeable collection of objects.

tup = (1, 3.12, False, "Hi") 

List

Changeable collection of objects.

my_collection = [1, 1, 3.12, False, "Hi"]

len(my_collection)           # Length

my_collection.extend(["More", "Items"])  # Add multiple items

my_collection.append("Single")           # Add single item

del(my_collection[2])        # Delete item at index 2

clone = my_collection[:]     # Clone list

my_collection_3 = my_collection + ["a", "b", "c"]  # Concatenate

sum([1,2,3,4.5])             # Sum numbers

item in my_collection        # Check existence

item not in my_collection    # Check non-existence

Set

Unordered collection of unique objects.

a = {100, 3.12, False, "Bye"}

b = {100, 3.12, "Welcome"}

my_set = set([1,1,2,3])      # Convert list to set

a.add(4)                     # Add item

a.remove("Bye")              # Remove item

a.difference(b)              # Set difference

a.intersection(b)            # Set intersection

a.union(b)                   # Set union

a.issubset(b)                # Subset check

a.issuperset(b)              # Superset check

Indexing and Slicing

Access elements by position:

my_string[0] my_collection[1] my_tup[2] 

Access a range of elements:

my_string[1:4] my_collection[0:3] my_tup[1:3] 

Operators

Comparison Operators

• a == b : Equal
• a < b : Less Than
• a > b : Greater Than
• a >= b : Greater Than or Equal
• a <= b : Less Than or Equal
• a != b : Not Equal

Arithmetic Operators

• + : Addition
• - : Subtraction
• * : Multiplication
• / : Division
• // : Integer Division

Conditional Operators

• a and b : True if both are true
• a or b : True if either is true
• not a : Opposite of a

Control Flow

Loops

for x in range(5):           # Loop 5 times

    print(x)

for item in iterable:        # Loop through iterable

    print(item)

while condition:             # Loop while condition is true

    # code

Conditional Statements

if condition1:

    # code

elif condition2:

    # code

else:

    # code

Try/Except

try:

    # code

except ExceptionType:

    # handle error

else:

    # code if no error

Common Error Types

• IndexError : Index out of range
• NameError : Variable name not found
• SyntaxError : Code syntax error
• ZeroDivisionError : Division by zero

Range

Create sequences of numbers:

range(5) # 0,1,2,3,4 range(2, 10, 2) # 2,4,6,8 

Webscraping

Using BeautifulSoup:

from bs4 import BeautifulSoup

soup = BeautifulSoup(html, 'html5lib')

soup.prettify()

soup.find('tag')

soup.find_all('tag')

Requests

Using the requests library:

import requests

response = requests.get(url, params)

response.status_code

response.text

response.json()

requests.post(url, data)

Functions

Define and call functions:

def my_function(param1, param2):

    # code

    return result

output = my_function(arg1, arg2)

Working with Files

Reading:

file = open(file_name, "r")

content = file.read()

file.close()

Writing:

file = open(file_name, "w")

file.write(content)

file.close()

Objects and Classes

Define classes and create objects:

class MyClass:

    def init(self, param1, param2):

        self.attr1 = param1

        self.attr2 = param2
def method(self, param):
    return param

obj = MyClass(val1, val2)

obj.method(val3)

List Comprehensions:

squares = [x*x for x in range(10)] 

Lambda Functions

add = lambda x, y: x + y 

Importing Modules

import math from collections import Counter 

Exception Handling: finally

try: # code finally: # always runs 

With Statement (Context Managers)

with open('file.txt', 'r') as f: data = f.read() 

Type Conversion

int('123')

str(123)

float('3.14')

list('abc')

Basic Numpy/Pandas Usage (for data science focus)

import numpy as np

import pandas as pd

arr = np.array([1,2,3])

df = pd.DataFrame({'a':[1,2], 'b':[3,4]})

Virtual Environments

python -m venv myenv source myenv/bin/activate 

Package Installation

pip install requests 

Docstrings

def foo(): """This is a docstring.""" pass 

Comments

[IO_Node]

Dope stuff, thanks a lot :)

[Garvit Khamesra]

Thanks man