Python

Write reliable Python avoiding mutable defaults, import traps, and common runtime surprises.

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Last updated: April 23, 2026

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Quick Reference

Topic	File
Dynamic typing, type hints, duck typing	`types.md`
List/dict/set gotchas, comprehensions	`collections.md`
Args/kwargs, closures, decorators, generators	`functions.md`
Inheritance, descriptors, metaclasses	`classes.md`
GIL, threading, asyncio, multiprocessing	`concurrency.md`
Circular imports, packages, init.py	`imports.md`
Pytest, mocking, fixtures	`testing.md`

Critical Rules

def f(items=[])

shares list across all calls — use

items=None

then

items = items or []

```
is
```
checks identity,
```
==
```
checks equality —
```
"a" * 100 is "a" * 100
```
may be False
Modifying list while iterating skips elements — iterate over copy:
```
for x in list(items):
```
GIL prevents true parallel Python threads — use multiprocessing for CPU-bound

Bare

except:

catches

SystemExit

and

KeyboardInterrupt

— use

except Exception:

```
UnboundLocalError
```
when assigning to outer scope variable — use
```
nonlocal
```
or
```
global
```
```
open()
```
without context manager leaks handles — always use
```
with open():
```
Circular imports fail silently or partially — import inside function to break cycle
```
0.1 + 0.2 != 0.3
```
— floating point, use
```
decimal.Decimal
```
for money
Generator exhausted after one iteration — can't reuse, recreate or use
```
itertools.tee
```
Class attributes with mutables shared across instances — define in
```
__init__
```
instead
```
__init__
```
is not constructor —
```
__new__
```
creates instance,
```
__init__
```
initializes
Default encoding is platform-dependent — always specify
```
encoding='utf-8'
```

Python

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Critical Rules

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