version: "1.0.3" name: designing-python-apis description: Designs intuitive Python library APIs following principles of simplicity, consistency, and discoverability. Handles public re-exports, API evolution, deprecation, breaking changes, and error handling. Use when designing or reviewing a library API, exposing a symbol from a package __init__, or managing API versioning and deprecations.

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?"
)

Fail Loud, Not Silent

The most expensive bugs are the ones where failure is indistinguishable from success. A caller who gets no error assumes everything worked. These patterns all turned a real failure into a silent wrong answer in shipped code — guard against every one.

Don't write the success sentinel on the failure path. An except block that sets the same state a successful run would makes failed jobs look complete.

# BAD — any failure is reported to the user as a finished result.
try:
    result = run_job()
    status = "READY"
except Exception:
    status = "READY"        # failure now looks identical to success
# GOOD — distinct terminal states; the UI/caller can react.
try:
    result = run_job()
    status = "READY"
except Exception:
    log.exception("job failed")
    status = "ERROR"

Don't swallow distinct failures into one generic message. A broad except Exception that returns "error: something went wrong" (or worse, an empty result) collapses parse errors, missing files, and bugs into the same opaque string — undebuggable and often mistaken for "no problems found." Catch the specific exceptions you can handle; let the rest propagate.

An empty/partial result is not an error signal. Returning [], an empty DataFrame, or "what I fetched before the connection dropped" looks like a valid answer. Pagination that returns partial pages on a mid-stream RequestError, then gets aggregated as if complete, produces silently wrong analytics. Either raise, or return an explicit "incomplete" marker the caller must check — never let truncation masquerade as the full set.

Meter external work where it actually happens. A quota or request budget charged once around list_all_items() undercounts whenever that helper follows pagination internally: one budget unit can hide ten HTTP requests. Spend at the lowest shared request boundary so every page, retry, and detail fetch is counted.

# BAD — the helper may issue an unbounded number of requests.
budget.spend()
items = client.list_all_items()
# GOOD — pagination cannot bypass the meter.
def request(method, url, *, budget, **kwargs):
    budget.spend()
    return http.request(method, url, **kwargs)

Keep the budget above the raw HTTP call if cache hits should be free, and below retry logic if every retry consumes provider quota. Test with a fake paginated transport that returns multiple pages and assert both the result values and the exact number of budget spends; a one-page fake cannot prove this contract.

A no-op on unexpected input is a silent corruption. Code that skips columns of the wrong type, ignores a key it doesn't recognize, or continues past a file it can't parse — with no error and no report — leaves the caller believing the operation applied. If you can't act on an input, say so (raise, warn, or return a per-item error list); don't quietly do nothing.

Validation errors must not leave partial mutations behind. An inplace=True API that renames caller-owned columns and only then validates another argument can raise the right exception while still corrupting the caller's next operation. Validate every parameter and precondition before the first mutation. When work cannot be validated up front, stage it on a copy and commit only after success.

# BAD — invalid max_fraction still changes the caller's dataframe.
def detect(data, *, max_fraction=0.1, inplace=True):
    target = data if inplace else data.copy()
    target.rename(columns={"time": "timestamp"}, inplace=True)
    if not 0 < max_fraction < 0.5:
        raise ValueError("max_fraction must be between 0 and 0.5")
    return analyze(target)
# GOOD — the error path is side-effect free.
def detect(data, *, max_fraction=0.1, inplace=True):
    if not 0 < max_fraction < 0.5:
        raise ValueError("max_fraction must be between 0 and 0.5")
    target = data if inplace else data.copy()
    target.rename(columns={"time": "timestamp"}, inplace=True)
    return analyze(target)

Pin the contract in tests: snapshot caller-owned state, trigger a late validation error with inplace=True, and assert the object is unchanged. Testing only the exception type misses the damaging half of this bug.

before = frame.copy(deep=True)
with pytest.raises(ValueError, match="max_fraction"):
    detect(frame, max_fraction=0.5, inplace=True)
pd.testing.assert_frame_equal(frame, before)

Filtering down to empty must never read as "all clear." When you narrow a rule set, check set, or work list and the filter yields nothing, an "evaluate all → 0 problems" path reports a perfect score while actually checking nothing. Guard the empty case explicitly:

selected = [r for r in rules if r.category in requested]
if not selected:                       # empty filter ≠ everything passed
    raise ValueError(f"no rules match {requested!r}")

Never fabricate a fallback that looks real. Substituting sample/random data when a fetch fails (so the UI "has something to show") presents invented numbers as genuine. Surface the failure instead; a visible error beats a plausible lie.

Don't discard the real output on a non-zero exit. A subprocess wrapper that returns f"Error: {stderr}" whenever returncode != 0 loses the answer for tools that exit non-zero by design and write results to stdout — reporting a successful run as an empty "Error: ". Inspect stdout and the actual exit semantics before deciding it failed.

Test the public import path, not only the implementation module

A package can expose two functions with the same name and only one can work:

# mylib/__init__.py
def initialize_parser() -> None:
    pass                         # stale placeholder
# mylib/parser.py
def initialize_parser() -> None:
    global _parser
    _parser = build_parser()     # real implementation

Internal tests that import mylib.parser.initialize_parser all pass, while the documented from mylib import initialize_parser call does nothing. This is an API break even though neither definition is individually invalid.

Re-export the canonical object instead of wrapping or copying it:

# mylib/__init__.py
from .parser import initialize_parser
__all__ = ["initialize_parser"]

Then test through the path users are told to import. An identity assertion is a cheap drift guard, and one behavior assertion proves the public route performs the required initialization:

import mylib
from mylib import parser
def test_public_initializer_is_canonical():
    assert mylib.initialize_parser is parser.initialize_parser
    mylib.initialize_parser()
    assert parser.is_initialized()

Apply the behavior check to convenience exports, compatibility adapters, console entry-point targets, and documented import snippets. Use identity only when the public name is intended to be a direct alias. Testing only the leaf module proves the implementation; it says nothing about whether the public API reaches it.

Deprecation

Deprecate gracefully: warn now, document the removal version, and remove only in a major release. The warning mechanics (including deprecating parameters, classes, and modules) and the migration-guide template are owned by the managing-python-releases skill — see [../release-management/MIGRATION.md](../release-management/MIGRATION.md).

Anti-Patterns

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

The mutable-default-argument trap (def f(x: list = [])) is covered by the improving-python-code-quality skill.

For detailed patterns, see:

• [PATTERNS.md](PATTERNS.md) - Builder, factory, and advanced patterns
• [EVOLUTION.md](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
 
Exports:
- [ ] Direct package exports reference the canonical implementation, not a stale stub
- [ ] Tests import through the documented public path and assert real behavior
 
Errors:
- [ ] Custom exceptions with context
- [ ] Helpful error messages
- [ ] Documented in docstrings
- [ ] Failures fail loud — no success sentinel on the error path
- [ ] Empty/partial results never masquerade as a complete answer
- [ ] No silent no-ops or fabricated fallback data
- [ ] Validation failures leave caller-owned inputs unchanged

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