Python #5 — Modules, Error Handling & File I/O

# Import the whole module
import math
print(math.sqrt(16))   # 4.0
print(math.pi)         # 3.141592...

# Import specific names
from math import sqrt, pi, ceil
print(sqrt(25))        # 5.0

# Import with alias
import numpy as np          # common convention
import pandas as pd
from datetime import datetime as dt

# Import everything (avoid — pollutes namespace)
from math import *

# utils.py
def clamp(value: float, min_val: float, max_val: float) -> float:
    """Clamp value to [min_val, max_val]."""
    return max(min_val, min(max_val, value))

GOLDEN_RATIO = 1.618033988749895

# main.py
from utils import clamp, GOLDEN_RATIO

print(clamp(150, 0, 100))   # 100.0
print(GOLDEN_RATIO)          # 1.618...

# script.py
def process(data):
    return [x * 2 for x in data]

# This block runs only when the file is executed directly,
# NOT when it's imported as a module
if __name__ == "__main__":
    result = process([1, 2, 3])
    print(result)

# Basic try/except
try:
    result = 10 / 0
except ZeroDivisionError:
    print("Can't divide by zero!")

# Catch multiple exceptions
try:
    data = int(input("Enter a number: "))
except ValueError:
    print("That's not a number")
except (TypeError, OverflowError) as e:
    print(f"Type/overflow error: {e}")
except Exception as e:          # catch-all (use sparingly)
    print(f"Unexpected: {e}")
else:
    print(f"Result: {data * 2}")   # runs if NO exception
finally:
    print("This always runs")      # cleanup

def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Denominator cannot be zero")
    return a / b

def get_user(user_id: int):
    if user_id <= 0:
        raise ValueError(f"Invalid user ID: {user_id}")
    # ... look up user ...
    user = None
    if user is None:
        raise LookupError(f"User {user_id} not found")
    return user

# Re-raise after logging
try:
    result = divide(10, 0)
except ValueError as e:
    print(f"Logging error: {e}")
    raise   # re-raise the same exception

class AppError(Exception):
    """Base exception for this application."""
    pass

class ValidationError(AppError):
    def __init__(self, field: str, message: str):
        self.field = field
        self.message = message
        super().__init__(f"{field}: {message}")

class NotFoundError(AppError):
    def __init__(self, resource: str, id):
        super().__init__(f"{resource} with id {id!r} not found")

# Usage
try:
    raise ValidationError("email", "must be a valid email address")
except ValidationError as e:
    print(f"Validation failed on field '{e.field}': {e.message}")

try:
    raise NotFoundError("Product", 42)
except NotFoundError as e:
    print(e)  # Product with id 42 not found

from pathlib import Path

# Read text
content = Path("readme.txt").read_text(encoding="utf-8")

# Read lines
lines = Path("data.txt").read_text().splitlines()

# Write text (overwrites)
Path("output.txt").write_text("Hello, file!\n", encoding="utf-8")

# Append to file
with open("log.txt", "a", encoding="utf-8") as f:
    f.write("New log entry\n")

# The with statement — always use it for files
# It guarantees the file is closed even if an exception occurs
with open("data.txt", "r", encoding="utf-8") as f:
    for line in f:                    # read line by line (memory efficient)
        print(line.strip())

import json
from pathlib import Path

# Write JSON
data = {
    "users": [
        {"id": 1, "name": "Alice", "active": True},
        {"id": 2, "name": "Bob",   "active": False},
    ]
}
Path("users.json").write_text(json.dumps(data, indent=2))

# Read JSON
loaded = json.loads(Path("users.json").read_text())
for user in loaded["users"]:
    status = "active" if user["active"] else "inactive"
    print(f"{user['name']} is {status}")

from pathlib import Path

p = Path("/home/rupa/projects/myapp/src/main.py")

print(p.name)        # main.py
print(p.stem)        # main
print(p.suffix)      # .py
print(p.parent)      # /home/rupa/projects/myapp/src
print(p.parts)       # ('/', 'home', 'rupa', ...)

# Build paths safely (works on all OS)
data_dir = Path.home() / "projects" / "myapp" / "data"
data_dir.mkdir(parents=True, exist_ok=True)

# Find files
src = Path("src")
py_files = list(src.rglob("*.py"))     # recursive glob
txt_files = list(src.glob("*.txt"))    # non-recursive

# Check existence
print(p.exists())     # True/False
print(p.is_file())    # True
print(p.is_dir())     # False

# datetime
from datetime import datetime, timedelta

now = datetime.now()
print(now.strftime("%Y-%m-%d %H:%M:%S"))   # 2025-04-09 10:30:00

one_week_later = now + timedelta(weeks=1)
diff = one_week_later - now
print(diff.days)   # 7

# os — environment, processes
import os
home = os.environ.get("HOME", "/tmp")
print(os.getcwd())                    # current working directory
os.makedirs("logs/2025", exist_ok=True)

# random
import random
print(random.randint(1, 10))          # random int 1-10
print(random.choice(["a", "b", "c"])) # random item
random.shuffle([1, 2, 3, 4, 5])      # in-place shuffle

# itertools
from itertools import chain, islice, groupby

flat = list(chain([1, 2], [3, 4], [5]))   # [1, 2, 3, 4, 5]
first5 = list(islice(range(1000), 5))     # [0, 1, 2, 3, 4]