Python Modules and Packages

# Import entire module
import math
print(math.sqrt(16))  # 4.0

# Import specific items
from math import sqrt, pi
print(sqrt(16))       # 4.0 (no math. prefix needed)
print(pi)             # 3.14159...

# Import with alias
import numpy as np
from datetime import datetime as dt

# Import everything (DON'T do this — namespace pollution!)
from math import *

# mymodule.py
"""A helpful utilities module."""

__version__ = "1.0.0"

def greet(name: str) -> str:
    """Return a greeting message."""
    return f"Hello, {name}!"

class Timer:
    """Simple timer context manager."""
    def __enter__(self):
        import time
        self.start = time.time()
        return self
    
    def __exit__(self, *args):
        import time
        self.elapsed = time.time() - self.start

# main.py
from mymodule import greet, Timer
import mymodule

print(greet("Alice"))
print(mymodule.__version__)

with Timer() as t:
    sum(range(1000000))
print(f"Elapsed: {t.elapsed:.4f}s")

# collections — specialized containers
from collections import Counter, defaultdict, namedtuple

# Counter — count occurrences
words = ["apple", "banana", "apple", "cherry", "banana", "apple"]
word_counts = Counter(words)
print(word_counts.most_common(2))  # [('apple', 3), ('banana', 2)]

# defaultdict — dict with default values
dd = defaultdict(list)
dd["fruits"].append("apple")
dd["fruits"].append("banana")
print(dd)  # {'fruits': ['apple', 'banana']}

# datetime — date and time
from datetime import datetime, timedelta
now = datetime.now()
tomorrow = now + timedelta(days=1)
print(now.strftime("%Y-%m-%d %H:%M:%S"))  # "2024-01-15 14:30:00"

# json — JSON parsing
import json
data = '{"name": "Alice", "age": 28}'
parsed = json.loads(data)
print(parsed["name"])  # "Alice"

# re — regular expressions
import re
email = "user@example.com"
if re.match(r'^[\w.-]+@[\w.-]+\.\w+$', email):
    print("Valid email!")

# itertools — iterator tools
import itertools
for combo in itertools.combinations([1, 2, 3, 4], 2):
    print(combo)  # (1,2), (1,3), (1,4), (2,3), (2,4), (3,4)

# Create a virtual environment
python3 -m venv myenv

# Activate
source myenv/bin/activate    # macOS/Linux
myenv\Scripts\activate       # Windows

# Install packages (only in this environment)
pip install requests flask

# Save dependencies
pip freeze > requirements.txt

# Install from requirements
pip install -r requirements.txt

# Deactivate
deactivate

# Search for packages
pip search requests  # (deprecated, use pypi.org instead)

# Install packages
pip install flask
pip install flask==2.3.0       # Specific version
pip install flask>=2.0,<3.0   # Version range

# Upgrade packages
pip install --upgrade flask

# Uninstall
pip uninstall flask

# List installed packages
pip list
pip show flask