Designing Python Apis

Python API Design

Core Principles

1. Simplicity: Simple things simple, complex things possible
2. Consistency: Similar operations work similarly
3. Least Surprise: Behave as users expect
4. Discoverability: Find via autocomplete and help

Progressive Disclosure Pattern

# Level 1: Simple functions
from mylib import encode, decode
result = encode(37.7749, -122.4194)

# Level 2: Configurable classes
from mylib import Encoder
encoder = Encoder(precision=15)

# Level 3: Low-level access
from mylib.internals import BitEncoder

Naming Conventions

# Actions: verbs
encode(), decode(), validate()

# Retrieval: get_*
get_user(), get_config()

# Boolean: is_*, has_*, can_*
is_valid(), has_permission()

# Conversion: to_*, from_*
to_dict(), from_json()

Error Handling

class MyLibError(Exception):
    """Base exception with helpful messages."""
    def __init__(self, message: str, *, hint: str = None):
        super().__init__(message)
        self.hint = hint

# Usage
raise ValidationError(
    f"Latitude must be -90 to 90, got {lat}",
    hint="Did you swap latitude and longitude?"
)

Deprecation

import warnings

def old_function():
    warnings.warn(
        "old_function() deprecated, use new_function()",
        DeprecationWarning,
        stacklevel=2,
    )
    return new_function()

Anti-Patterns

# Bad: Boolean trap
process(data, True, False, True)

# Good: Keyword arguments
process(data, validate=True, cache=False)

# Bad: Mutable default
def process(items: list = []):

# Good: None default
def process(items: list | None = None):

For detailed patterns, see:

• PATTERNS.md - Builder, factory, and advanced patterns
• EVOLUTION.md - API versioning and migration guides

Review Checklist

Naming:
- [ ] Clear, self-documenting names
- [ ] Consistent patterns throughout
- [ ] Boolean params read naturally

Parameters:
- [ ] Minimal required parameters
- [ ] Sensible defaults
- [ ] Keyword-only after positional clarity

Errors:
- [ ] Custom exceptions with context
- [ ] Helpful error messages
- [ ] Documented in docstrings

Learn More

This skill is based on the Ergonomics section of the Guide to Developing High-Quality Python Libraries by Will McGinnis. See these posts for deeper coverage:

• The Art of API Design
• Designing for Developer Joy

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