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name: modern-python-patterns description: | Modern Python 3.10+ patterns and idioms. Use this skill when writing new Python code, refactoring legacy code, or enforcing consistent style. Covers type hints, dataclasses, pattern matching, walrus operator, f-strings, pathlib, and context managers.

Modern Python Patterns

Modern Python patterns for 3.10+ emphasizing readability, type safety, and idiomaticity.

Core Principles

1. Type hints everywhere - Function signatures, class attributes, variables when unclear
2. Dataclasses over dicts - Structured data with validation
3. Pathlib over os.path - Object-oriented file paths
4. f-strings over .format() - Readable string formatting

Type Hints

Always annotate function signatures

from typing import Optional, Union
from collections.abc import Sequence

def process_items(
    items: Sequence[str],
    threshold: float = 0.5,
    max_count: Optional[int] = None
) -> list[str]:
    """Process items with optional limit."""
    return [item for item in items if len(item) > threshold]

Modern patterns (3.10+):

• Use list[str] not List[str] (PEP 585)
• Use str | None not Optional[str] (PEP 604)
• Use collections.abc not typing for abstract types

See type-hints-examples.md for:

• Generic types with TypeVar
• Protocol classes for structural typing
• Literal types and type narrowing
• typing.cast() and type guards

Dataclasses

Use dataclasses for structured data

from dataclasses import dataclass, field

@dataclass
class Config:
    host: str
    port: int = 8080
    debug: bool = False
    tags: list[str] = field(default_factory=list)

Benefits:

• Auto-generated __init__, __repr__, __eq__
• Type validation with type hints
• Immutable with frozen=True
• Post-init processing with __post_init__

See dataclass-examples.md for:

• Validation patterns
• Inheritance and composition
• Integration with Pydantic
• Slots for memory optimization

Pattern Matching (3.10+)

Use match/case for complex conditionals

def process_response(response: dict) -> str:
    match response:
        case {"status": "success", "data": data}:
            return f"Success: {data}"
        case {"status": "error", "code": code, "message": msg}:
            return f"Error {code}: {msg}"
        case {"status": status}:
            return f"Unknown status: {status}"
        case _:
            return "Invalid response"

See pattern-matching-examples.md for:

• Sequence patterns
• Class patterns
• Guard clauses
• Structural pattern matching vs if/elif

Modern File Operations

Use pathlib instead of os.path

from pathlib import Path

# Reading files
config_path = Path("config") / "settings.json"
data = config_path.read_text()

# Writing files
output_path = Path("output") / "results.csv"
output_path.write_text(data)

# Iterating
for path in Path("data").glob("*.csv"):
    print(path.stem)  # filename without extension

See pathlib-patterns.md for:

• Path manipulation
• File operations
• Glob patterns
• Cross-platform paths

Context Managers

Use context managers for resource management

from contextlib import contextmanager
from typing import Generator

@contextmanager
def timer(name: str) -> Generator[None, None, None]:
    """Context manager for timing operations."""
    start = time.perf_counter()
    try:
        yield
    finally:
        elapsed = time.perf_counter() - start
        print(f"{name}: {elapsed:.3f}s")

# Usage
with timer("data processing"):
    process_data()

See context-manager-examples.md for:

• Custom context managers
• contextlib utilities
• Async context managers
• Error handling patterns

Walrus Operator (3.8+)

Use := for assignment expressions

# In conditionals
if (n := len(data)) > 100:
    print(f"Large dataset: {n} items")

# In list comprehensions
[clean for item in items if (clean := item.strip())]

# In while loops
while (line := file.readline()):
    process(line)

Use sparingly - improves readability when avoiding duplication.

F-strings

Use f-strings for all formatting

name = "Alice"
score = 95.5

# Basic
message = f"Hello, {name}!"

# Expressions
result = f"Score: {score:.1f}%"

# Multi-line
report = f"""
Name: {name}
Score: {score:.2f}
Status: {'Pass' if score >= 60 else 'Fail'}
"""

# Debug (3.8+)
print(f"{name=}, {score=}")  # name='Alice', score=95.5

See f-string-patterns.md for:

• Format specifications
• Nested expressions
• Date formatting
• Alignment and padding

Anti-Patterns to Avoid

Avoid	Use Instead
os.path.join(a, b)	Path(a) / b
"{}".format(x)	f"{x}"
Optional[str] (3.10+)	str | None
List[int] (3.10+)	list[int]
Complex if/elif chains	match/case
dict for structured data	dataclass

Style Guidelines

• Follow PEP 8 - Use tools like ruff or black
• Type hints on all public APIs - Internal helpers optional
• Docstrings for modules, classes, functions - Use Google or NumPy style
• Import order: stdlib, third-party, local (use isort)

See style-guide.md for complete conventions.

source: Modern Python best practices