Python anti patterns

Python Anti-Patterns

Prescriptive module-organization rules learned in practice. These are enforced engineering standards, not style preferences: apply them on every Python file touched.

Activation Contract

Apply when writing or reviewing any Python module: checking import order, file size, module cohesion, hardcoded values, or where constants live. Flag every violation below; do not wait to be asked.

Hard Rules

• Import order (custom convention): first all import x statements (alphabetical), then from x import ... for stdlib/third-party, then a blank line, then first-party from core... imports. No blank line between the import block and the non-local from block. See references/import-order.md. NOTE: ruff's isort (I) rule reverts this: disable I in the linter config or it overwrites the skill.
• No reflexive from __future__ import annotations: do not add it by default; in most modules it is noise. Keep it ONLY when load-bearing: annotations that reference TYPE_CHECKING-guarded imports (to break import cycles) or forward references you would otherwise have to quote. Remove it everywhere else.
• File length: ~300 lines is a hard smell. Split by responsibility before that. A long file almost always hides multiple concerns.
• Module cohesion: functions with no relationship to the module's purpose must be extracted to their own module. One module = one cohesive concern.
• No hardcoded values: never inline literals (URLs, paths, thresholds, column names, magic numbers/strings) in logic. Name them.
• Separate constants from logic (discriminant): domain/config constants (geometry, thresholds, URLs, column names, labels) must NOT sit atop a logic module, even if few or single-use. Move them to a dedicated constants.py, or to a constants/ subpackage with one module per category (constants/layout.py, constants/routing.py) when they are many or span categories. EXCEPTION: a value meaningful only inside one algorithm (a private sentinel, an internal state marker) stays local: model it as a local Enum or a _private constant in that module, never hoisted to the shared constants surface. The deciding test: would this constant mean anything to another module? Yes, centralize it. No (algorithm-internal), keep it local and prefer an Enum.

Decision Gates

Symptom	Fix
Imports mixed/misordered	Reorder per the custom convention above
from __future__ import annotations with no TYPE_CHECKING/forward-ref need	Remove it
File approaching ~300 lines	Split by responsibility into separate modules
Function unrelated to module purpose	Extract to its own module
Literal inside logic	Replace with a named constant
Domain/config constant atop a logic module	Move to constants.py (or constants/<category>.py), import it
Many constants spanning categories	constants/ subpackage, one module per category
Private algorithm-internal value (sentinel/state)	Keep local; prefer an Enum over a bare string

Execution Steps

1. Scan the module against each gate; list every hit with file:line.
2. Fix imports first (cheap, mechanical), then move domain/config constants to constants.py (or a constants/<category>.py subpackage), keeping algorithm-internal values local, then split oversized/incohesive files.
3. Replace each hardcoded literal with an imported named constant.
4. Confirm each remaining module has a single cohesive concern.

# Bad: hardcoded + local constant + mixed imports
from core.utils.logger import get_logger
import requests
TIMEOUT = 30
def fetch(): requests.get("https://api.inegi.org.mx/data", timeout=TIMEOUT)

# Good: named, centralized, ordered
# constants.py
INEGI_DATA_URL = "https://api.inegi.org.mx/data"
REQUEST_TIMEOUT_SECONDS = 30

# module.py
import requests

from core.constants import INEGI_DATA_URL, REQUEST_TIMEOUT_SECONDS
from core.utils.logger import get_logger

def fetch():
    return requests.get(INEGI_DATA_URL, timeout=REQUEST_TIMEOUT_SECONDS)

Output Contract

Return the corrected module(s) with imports reordered, domain/config constants moved to constants.py (or a constants/<category>.py subpackage) while algorithm-internal values stay local as Enums, hardcoded values named, and oversized/incohesive files split. Cite each violation by file:line.

References

• references/import-order.md: the custom import-ordering convention with full examples.