Python

Mutable Default Arguments

• def f(items=[]) shares list across all calls — use items=None then items = items or []
• Same for dicts, sets, any mutable — default evaluated once at definition, not per call
• Class attributes with mutables shared across instances — define in __init__ instead

Import Traps

• Circular imports fail silently or partially — import inside function to break cycle
• from module import * pollutes namespace — explicit imports always
• Relative imports require package context — python -m package.module not python module.py
• Import order matters for monkey patching — patch before target module imports patched module
• __init__.py runs on package import — keep it minimal

Scope Gotchas

• UnboundLocalError when assigning to variable that exists in outer scope — use nonlocal or global
• List comprehension has own scope, but loop variable leaks in Python 2 — not 3
• except Exception as e — e is deleted after except block in Python 3
• Default argument values bound at definition — closure over loop variable captures final value

String Pitfalls

• is vs ==: is checks identity, == checks equality — "a" * 100 is "a" * 100 may be False
• f-strings evaluate at runtime — f"{obj}" calls __str__ each time
• str.split() vs str.split(' ') — no arg splits on any whitespace and removes empties
• Unicode normalization: 'café' != 'café' if composed differently — use unicodedata.normalize()

Iteration Traps

• Modifying list while iterating skips elements — iterate over copy: for x in list(items):
• Dictionary size change during iteration raises RuntimeError — copy keys: for k in list(d.keys()):
• Generator exhausted after one iteration — can't reuse, recreate or use itertools.tee
• range() doesn't include end — range(5) is 0,1,2,3,4

Class Gotchas

• __init__ is not constructor — __new__ creates instance, __init__ initializes
• Method resolution order (MRO) in multiple inheritance — use super() correctly or break chain
• @property makes attribute read-only unless you add setter
• __slots__ breaks __dict__ and dynamic attributes — can't add new attributes
• Mutable class attribute shared by all instances — each instance modifying same list

Exception Handling

• Bare except: catches SystemExit and KeyboardInterrupt — use except Exception:
• except A, B: is syntax error in Python 3 — use except (A, B):
• Exception chaining: raise NewError() from original preserves context
• finally runs even on return — return in finally overrides try's return
• else in try/except runs only if no exception — often clearer than flag variable

Numeric Surprises

• 0.1 + 0.2 != 0.3 — floating point, use decimal.Decimal for money
• Integer division: // always floors toward negative infinity — -7 // 2 is -4, not -3
• bool is subclass of int — True + True == 2
• Large integers have no overflow — but operations get slow
• is fails for large integers — only small ints (-5 to 256) are cached

File and I/O

• open() without context manager leaks file handles — always use with open():
• Default encoding is platform-dependent — always specify encoding='utf-8'
• read() loads entire file to memory — use readline() or iterate for large files
• Binary mode 'rb' required for non-text — Windows line endings differ in text mode

Concurrency

• GIL prevents true parallel Python threads — use multiprocessing for CPU-bound
• Threading still useful for I/O-bound — network, file operations release GIL
• async/await is not threading — single-threaded cooperative multitasking
• multiprocessing shares nothing by default — use Queue or Manager for communication
• Daemon threads killed abruptly on exit — may corrupt data