Python Design

Python Design for CLI Scripts

Design patterns and principles for writing maintainable Python CLI tools and utilities. Based on A Philosophy of Software Design (Ousterhout), adapted for scripting contexts.

When to Activate

• Writing or modifying Python files
• Planning module decomposition
• Code review of Python changes
• Refactoring scripts that feel "messy"
• Adding a new subcommand or utility function

Core Thesis

The central challenge is managing complexity, not adding features.

Complexity is anything that makes code hard to understand or modify. It has three symptoms:

1. Change Amplification — A small change requires edits in many places
2. Cognitive Load — You must hold too much context to make a safe change
3. Unknown Unknowns — You don't know what you don't know (the most dangerous)

Complexity is incremental. It accumulates through hundreds of small decisions, not one catastrophic mistake. Therefore: sweat the small stuff.

Principle 1: Deep Modules

A module's value is the ratio of functionality hidden vs. interface exposed.

Deep module (good):          Shallow module (bad):
┌──────────┐                 ┌──────────────────────────┐
│ simple   │                 │ complex interface        │
│ interface│                 │ many params, many methods │
├──────────┤                 ├──────────────────────────┤
│          │                 │                          │
│  rich    │                 │  thin implementation     │
│  impl    │                 │                          │
│          │                 └──────────────────────────┘
│          │
└──────────┘

Practical test: If a caller must understand how the module works internally to use it correctly, the module is too shallow.

Example: Task Data Access

# Shallow — caller must know JSON structure, file paths, error handling
def _read_json_file(path: Path) -> dict:
    with open(path, encoding="utf-8") as f:
        return json.load(f)

# Every caller does this independently:
task_path = tasks_dir / name / "task.json"
data = _read_json_file(task_path)
title = data.get("title") or data.get("name", "")
status = data.get("status", "planning")
assignee = data.get("assignee", "")

# Deep — caller gets what they need, module hides JSON/path/parsing
@dataclass(frozen=True)
class TaskInfo:
    name: str
    title: str
    status: str
    assignee: str
    priority: str
    directory: Path

def load_task(tasks_dir: Path, name: str) -> TaskInfo | None:
    """Load task by directory name. Returns None if not found."""
    ...

def list_active_tasks(tasks_dir: Path) -> list[TaskInfo]:
    """List all non-archived tasks, sorted by priority."""
    ...

The deep version absorbs complexity: JSON parsing, field defaults, directory scanning, archive filtering. Callers just work with typed data.

Principle 2: Type-First Development

Types define contracts before implementation. This workflow catches design problems early:

1. Define data shapes — dataclass or TypedDict first
2. Define function signatures — parameter and return types
3. Implement to satisfy types — let the type checker guide completeness
4. Validate at boundaries — runtime checks only where data enters the system

Frozen Dataclasses for Internal Data

from dataclasses import dataclass
from typing import Literal

@dataclass(frozen=True)
class AgentRecord:
    agent_id: str
    task_name: str
    worktree_path: Path
    platform: Literal["claude", "codex", "cursor"]
    status: Literal["running", "done", "failed"]
    branch: str

Frozen dataclasses are immutable — no accidental mutation, safe to pass around.

TypedDict for External JSON Shapes

When the data comes from a file (task.json, config.yaml, registry.json), use TypedDict to document the expected shape:

from typing import TypedDict, Required, NotRequired

class TaskData(TypedDict):
    title: Required[str]
    status: Required[str]
    assignee: NotRequired[str]
    priority: NotRequired[str]
    parent: NotRequired[str]
    children: NotRequired[list[str]]

This eliminates scattered .get("field", default) calls — the shape is documented once.

NewType for Domain Primitives

When two strings mean different things, make the type system enforce it:

from typing import NewType

TaskName = NewType("TaskName", str)    # directory name like "03-10-v040"
BranchName = NewType("BranchName", str)  # git branch like "feat/v0.4.0"

def create_branch(task: TaskName) -> BranchName:
    return BranchName(f"task/{task}")

Discriminated Unions for State

When an entity can be in distinct states with different data:

@dataclass(frozen=True)
class Pending:
    status: Literal["pending"] = "pending"

@dataclass(frozen=True)
class Running:
    status: Literal["running"] = "running"
    pid: int
    worktree: Path

@dataclass(frozen=True)
class Completed:
    status: Literal["completed"] = "completed"
    branch: str
    commit: str

AgentState = Pending | Running | Completed

def handle(state: AgentState) -> None:
    match state:
        case Running(pid=pid, worktree=wt):
            check_process(pid)
        case Completed(branch=br):
            create_pr(br)
        case Pending():
            pass

The type checker ensures every state is handled. No more if data.get("status") == "running" with forgotten branches.

Principle 3: Information Hiding

Each module should encapsulate design decisions. When the same knowledge appears in multiple modules, information has leaked.

Common Leakage Patterns in Scripts

JSON schema knowledge scattered everywhere:

# BAD — 9 files all know how to iterate tasks and parse task.json
for d in sorted(tasks_dir.iterdir()):
    if d.name == "archive" or not d.is_dir():
        continue
    task_json = d / "task.json"
    if task_json.exists():
        data = json.loads(task_json.read_text())
        title = data.get("title") or data.get("name", "")
        ...

# GOOD — one module owns task iteration
# common/tasks.py
def iter_active_tasks(tasks_dir: Path) -> Iterator[TaskInfo]:
    """Yield all active (non-archived) tasks."""
    for d in sorted(tasks_dir.iterdir()):
        if d.name == "archive" or not d.is_dir():
            continue
        info = _load_task_json(d)
        if info:
            yield info

File format details leaking through layers:

# BAD — caller knows it's JSON, knows the path convention
registry_path = trellis_dir / "registry.json"
data = json.loads(registry_path.read_text())
data["agents"][agent_id] = {...}
registry_path.write_text(json.dumps(data, indent=2))

# GOOD — module hides storage format
registry = AgentRegistry(trellis_dir)
registry.add(agent_id, task=task_name, platform="claude")

Principle 4: Pull Complexity Downward

When complexity is unavoidable, the module should absorb it internally rather than pushing it to callers. A module has few developers but many users — it's better for the module author to handle complexity once than for every caller to handle it independently.

# BAD — pushes complexity to every caller
def run_git(args: list[str]) -> subprocess.CompletedProcess:
    return subprocess.run(["git"] + args, capture_output=True, text=True)

# Every caller must: check returncode, decode stderr, handle encoding,
# strip whitespace, handle repo not found, etc.

# GOOD — absorbs complexity
def run_git(args: list[str], *, cwd: Path | None = None) -> str:
    """Run git command, return stdout. Raises GitError on failure."""
    result = subprocess.run(
        ["git"] + args,
        capture_output=True, text=True, encoding="utf-8",
        errors="replace", cwd=cwd,
    )
    if result.returncode != 0:
        raise GitError(args[0], result.stderr.strip())
    return result.stdout.strip()

Anti-patterns of Pushing Complexity Up

• Returning raw subprocess.CompletedProcess and letting callers check .returncode
• Raising generic exceptions that callers must parse
• Using configuration parameters to avoid making decisions
• Returning dict when a typed object would let callers skip validation

Principle 5: Define Errors Out of Existence

Exception handling is a major source of complexity. The best strategy is to design semantics so error conditions simply aren't errors.

# BAD — raises if key doesn't exist
def remove_agent(registry: dict, agent_id: str) -> None:
    if agent_id not in registry["agents"]:
        raise KeyError(f"Agent {agent_id} not found")
    del registry["agents"][agent_id]

# GOOD — guarantees postcondition: agent is not in registry
def remove_agent(registry: dict, agent_id: str) -> None:
    """Ensure agent_id is not in the registry after this call."""
    registry["agents"].pop(agent_id, None)

# BAD — raises if directory already exists
def init_workspace(path: Path) -> None:
    if path.exists():
        raise FileExistsError(f"{path} already exists")
    path.mkdir()

# GOOD — guarantees postcondition: directory exists
def ensure_workspace(path: Path) -> Path:
    """Ensure workspace directory exists. Returns the path."""
    path.mkdir(parents=True, exist_ok=True)
    return path

The key insight: define the operation by its postcondition ("after this call, X is true") rather than its precondition ("X must be true before calling").

Principle 6: KISS and Rule of Three

KISS — Keep It Simple

Choose the simplest solution that works. Complexity must be justified by concrete (not hypothetical) requirements.

# Over-engineered — registry pattern for 3 formatters
class FormatterRegistry:
    _registry: dict[str, type] = {}
    @classmethod
    def register(cls, name: str): ...
    @classmethod
    def create(cls, name: str): ...

# Simple — just a dictionary
FORMATTERS = {"json": format_json, "text": format_text, "table": format_table}

def format_output(fmt: str, data: Any) -> str:
    formatter = FORMATTERS.get(fmt)
    if not formatter:
        raise ValueError(f"Unknown format: {fmt}")
    return formatter(data)

Rule of Three

Wait until you have three instances of a pattern before extracting an abstraction. Two is coincidence; three is a pattern. Premature abstraction is worse than duplication because:

• It couples unrelated code through a shared abstraction
• It makes each instance harder to understand independently
• It creates pressure to fit future cases into the abstraction even when they don't fit

However: when you do hit three, extract immediately. Don't let it reach nine.

Principle 7: Single Responsibility and Module Boundaries

Each module should have one reason to change. When a module grows beyond ~300 lines, check if it has multiple responsibilities.

Decomposition Signals

Split when:

• A file has multiple "sections" separated by comment headers
• You need to import only one function from a large module
• Tests for different parts of the module have no shared setup
• Changes to one responsibility don't require understanding the other

How to Split

Split by information hiding (what knowledge is encapsulated), not by execution order (what runs when).

# BAD — split by execution order (temporal decomposition)
# step1_parse_args.py, step2_validate.py, step3_execute.py
# All three must know the command structure

# GOOD — split by responsibility
# task_store.py    — owns task.json read/write, schema, iteration
# task_cli.py      — owns argparse, subcommand routing
# task_display.py  — owns formatting, colors, table output

Principle 8: Consistent Shared Infrastructure

When multiple scripts need the same capability, provide it once in common/.

Naming: If a function is used by other modules, it's public API — don't prefix it with _.

Principle 9: Structured CLI Output Parsing

When parsing output from shell commands (git, grep, etc.), respect semantic whitespace:

# BAD — .strip() destroys semantic whitespace
# git submodule status prefix: ' ' = initialized, '-' = uninitialized, '+' = changed
line = output_line.strip()  # Loses the prefix character!

# GOOD — strip only trailing newlines
line = output_line.rstrip("\n\r")
prefix = line[0] if line else " "

Always document what each field position means when parsing structured command output.

Red Flags Quick Reference

Use during code review and self-review:

Design Checklist (Before Writing Code)

1. Types first: Define the data shape before writing logic
2. Module depth check: Will the interface be simpler than the implementation?
3. Duplication scan: grep -r "pattern" . before creating new utilities
4. Responsibility check: Does this belong in an existing module?
5. Error design: Can you define the error out of existence?
6. Naming precision: Does the name convey meaning without reading the implementation?

Design Checklist (During Code Review)

1. Red flags scan: Check the table above against the diff
2. Type safety: Are new data shapes documented with types?
3. Information hiding: Does the change leak implementation details?
4. Consistency: Does it follow the existing patterns in the module?
5. Depth: Is the common path simple for callers?

Strategic Investment

Spend roughly 10-20% of each change improving surrounding design.

Working code is necessary but not sufficient. The increments of software development should be abstractions, not just features. Each change should leave the codebase slightly better than you found it.

This is not perfectionism — it's compound interest. Small design improvements accumulate into a system that's dramatically easier to work with over time.