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Basics

Table of Contents

Python Standard Libraries

Category Libraries Description
Data Processing csv, json, xml File format parsing and handling
Files & OS os, pathlib, shutil, glob File operations, paths, system interaction
Date & Time datetime, time, calendar Date, time manipulation and formatting
Mathematics math, random, statistics Mathematical functions, random number generation
Networking socket, urllib, http Low-level networking, HTTP requests
Concurrency threading, multiprocessing, asyncio Multi-threading, parallelism, async operations
Data Structures collections, array, heapq Specialized container datatypes
Testing unittest, doctest Unit testing frameworks
Category Libraries Description
Data Science numpy, pandas, scipy Numerical computing, data analysis
Machine Learning scikit-learn, tensorflow, pytorch ML algorithms, deep learning
Visualization matplotlib, seaborn, plotly Charts, graphs, interactive plots
Web Development django, flask, fastapi Web frameworks and APIs
HTTP requests, httpx HTTP clients
CLI Tools click, typer, argparse Command-line interfaces
DevOps docker, kubernetes, ansible Infrastructure automation
Testing pytest, selenium Testing frameworks
Utilities pillow, beautifulsoup4, sqlalchemy Image processing, web scraping, ORM

Commonly used buildin keywords

Keyword Description Keyword Description
and, or, not Logical operators for boolean operations if, elif, else Conditional statement keywords
for, while Loop creation keywords break, continue Loop control keywords
def, return Function definition and return value lambda Anonymous function creation
class Class definition keyword import, from, as Module import keywords
try, except, finally Exception handling keywords raise Raises an exception explicitly
with Context manager for resource management in Membership testing operator
is Identity operator (object comparison) pass Null operation placeholder
global, nonlocal Variable scope declaration True, False, None Built-in constants
yield Generator function keyword assert Debugging check that raises exceptions
del Removes object references or items async, await Asynchronous programming keywords
match, case Pattern matching (Python 3.10+) type Type annotations context (Python 3.10+)

yield

Core Idea: Creating Iterators the Easy Way

At its heart, a function that contains the yield keyword is called a generator function. When you call a generator function, it doesn't execute the function body immediately. Instead, it returns a special object called a generator object (or simply generator).

This generator object is an iterator. Iterators are objects that you can loop over (like lists or tuples), but they produce their items one at a time and only when requested.

  • How yield Works vs. return

    • return: When a regular function hits a return statement, it sends a value back to the caller and exits completely. All its local variables are destroyed, and its execution state is lost. If you call the function again, it starts from the beginning.

    • yield: When a generator function hits a yield statement: It pauses its execution right at that point. It sends the value specified after yield back to the caller (in C, it voluntarily yields thread to CPU and allow others to run). Crucially, it remembers its state (local variables, instruction pointer, etc.). The next time the caller asks for a value from the generator (usually via a for loop or the next() function), the generator function resumes execution immediately after the yield statement where it paused.

def count_up_to(n):
    count = 1
    while count <= n:
        yield count  # Yield the current count and pause execution
        count += 1   # Increment the count

for number in count_up_to(5):
    print(number)  # Prints numbers from 1 to 5, one at a time

assert

Assert is a debugging aid that tests a condition. used for internal sanity checks in your code.

Throw an AssertionError if the condition is false. It is not meant to be used for data validation or user input validation.

DO NOT USE assert for data validation or user input validation. It is not a substitute for proper error handling.

assert can be disabled globally with the -O (optimize) flag when running Python scripts. This means that any assertions in your code will be ignored

  • assert <condition>
  • assert <condition>, <optional_error_message>
def calculate_discount(price, discount_percentage):
    # Ensure discount is reasonable before proceeding
    assert 0 <= discount_percentage <= 1, f"Discount percentage must be between 0 and 1, but got {discount_percentage}"
    assert price >= 0, f"Price cannot be negative, but got {price}"

try:
    calculate_discount(100, 1.1) # Discount > 1 (110%)
except AssertionError as e:
    print(f"Assertion Error: {e}")

try:
    calculate_discount(-50, 0.2) # Negative price
except AssertionError as e:
    print(f"Assertion Error: {e}")

try:
    calculate_discount(100, -0.1) # Negative discount
except AssertionError as e:
    print(f"Assertion Error: {e}")

Assertion Error: Discount percentage must be between 0 and 1, but got 1.1
Assertion Error: Price cannot be negative, but got -50
Assertion Error: Discount percentage must be between 0 and 1, but got -0.1

Commonly Used Built-in Functions

buildin functions

Function Description Example
abs(x) Return absolute value of a number abs(-5) → 5
all(iterable) Return True if all elements are true all([True, 1, "text"]) → True
any(iterable) Return True if any element is true any([False, 0, ""]) → False
bool(x) Convert value to Boolean bool(0) → False
dict() Create a dictionary dict(a=1, b=2) → {'a': 1, 'b': 2}
dir([object]) List names in scope or object attributes dir(str) → list of string methods
enumerate(iterable) Return index-value pairs list(enumerate(['a', 'b'])) → [(0, 'a'), (1, 'b')]
filter(function, iterable) Filter elements by function list(filter(lambda x: x>0, [-1,0,1])) → [1]
float(x) Convert to float float('3.14') → 3.14
int(x[, base]) Convert to integer int('100', 2) → 4
len(s) Return length of object len([1, 2, 3]) → 3
list([iterable]) Create a list list('abc') → ['a', 'b', 'c']
map(function, iterable) Apply function to all items list(map(lambda x: x*2, [1,2,3])) → [2,4,6]
max(iterable) Return largest item max([1, 5, 3]) → 5
min(iterable) Return smallest item min([1, 5, 3]) → 1
open(file, mode='r') Open file and return file object with open('file.txt', 'r') as f:
print(*objects) Print objects to stdout print('Hello', 'World') → Hello World
range(stop) Create sequence of numbers list(range(3)) → [0, 1, 2]
sorted(iterable) Return sorted list sorted([3, 1, 2]) → [1, 2, 3]
str(object) Convert to string str(123) → '123'
sum(iterable) Sum all items sum([1, 2, 3]) → 6
type(object) Return type of object type(5) → <class 'int'>
zip(*iterables) Combine iterables into tuples list(zip([1,2], ['a','b'])) → [(1,'a'), (2,'b')]

String Methods

Method Description Method Description
str.capitalize() Return string with first character capitalized str.casefold() Return string as casefolded for caseless matching
str.center(width) Return centered string padded with spaces str.count(sub) Count occurrences of substring
str.encode() Return encoded string as bytes str.endswith(suffix) Check if string ends with suffix
str.expandtabs() Replace tabs with spaces str.find(sub) Find substring (return index or -1)
str.format() Format string with {} placeholders str.index(sub) Find substring (raise ValueError if not found)
str.isalnum() Check if all characters are alphanumeric str.isalpha() Check if all characters are alphabetic
str.isdigit() Check if all characters are digits str.islower() Check if all characters are lowercase
str.isupper() Check if all characters are uppercase str.join(iterable) Join strings with separator
str.lower() Convert string to lowercase str.lstrip() Remove leading whitespace
str.replace(old, new) Replace substring str.split(sep) Split string at separator
str.strip() Remove leading and trailing whitespace str.title() Return titlecased version of string
str.upper() Convert string to uppercase str.zfill(width) Pad string with zeros on the left
str.partition(sep) Partition string at first separator occurrence str.rpartition(sep) Partition string at last separator occurrence
str.splitlines() Split string at line breaks str.swapcase() Swap case of all characters
str.istitle() Check if string is titlecased str.isspace() Check if string contains only whitespace
str.isnumeric() Check if all characters are numeric str.isdecimal() Check if all characters are decimal
str.rstrip() Remove trailing whitespace str.rsplit(sep) Split from right at separator
str.ljust(width) Left-justify string in field of width str.rjust(width) Right-justify string in field of width
str.startswith(prefix) Check if string starts with prefix str.translate(table) Translate string using character mapping table

Related Properties in Other Languages:

Python C# Java JavaScript
str.lower() string.ToLower() str.toLowerCase() str.toLowerCase()
str.upper() string.ToUpper() str.toUpperCase() str.toUpperCase()
str.strip() string.Trim() str.trim() str.trim()
str.startswith() string.StartsWith() str.startsWith() str.startsWith()
str.endswith() string.EndsWith() str.endsWith() str.endsWith()
len(str) string.Length str.length() str.length
str.isdigit() char.IsDigit() Character.isDigit() str.match(/^\d+$/)
str.isalpha() char.IsLetter() Character.isLetter() str.match(/^[A-Za-z]+$/)

String Indexing and Slicing

Operation Description Example Result
str[i] Access character at index i "Python"[0] 'P'
str[i:j] Slice from index i to j (excluding j) "Python"[0:2] 'Py'
str[i:j:k] Slice with step k "Python"[0:6:2] 'Pto'
str[:] Get a copy of the whole string "Python"[:] 'Python'
str[-i] Access character at index i from the end "Python"[-1] 'n'
str[-i:-j] Slice from i to j positions from the end "Python"[-3:-1] 'ho'
str[:j] Slice from start to index j "Python"[:3] 'Pyt'
str[i:] Slice from index i to the end "Python"[3:] 'hon'
str[::-1] Reverse the string "Python"[::-1] 'nohtyP'