Python — Confusions, Labs, Gotchas & Mock Interview
💡 Interview Tip
Goal: After this page, you should NEVER struggle with Python interview concepts.
Where to run: any Python 3.9+ REPL, Jupyter, or https://pythontutor.com (visualizer).
Memory Map
🧠 PYTHON MASTERY → MUTABLE-SCOPE-ITER
PYTHON MASTERYMUTABLE-SCOPE-ITER
───────────────────────────────────
MMutable vs Immutable (list/dict/set vs tuple/str/int)
UUnpacking (*, **, walrus, tuple/list)
TTruthiness ([] is falsy, None is falsy, 0 is falsy)
AArguments (default, *args, **kwargs, positional vs keyword)
BBinding (closures, late-binding, nonlocal, global)
LList vs Dict vs Set complexity (O(n) vs O(1))
EEquality (is vs → → , hash, __eq__)
SECTION 0: TOP 8 PYTHON CONFUSIONS — Cleared Forever
Confusion 1: is vs ==
python — editable
a = [1, 2, 3]
b = [1, 2, 3]
a == b # True — values are equal
a is b # False — different objects in memory!
c = a
c is a # True — same object (c is a reference)Rule:
==→ VALUES equal (uses__eq__)is→ SAME OBJECT in memory (identity check)
When to use is:
python — editable
if x is None: # ✅ correct — None is a singleton
...
if x == None: # ❌ works but discouraged (triggers __eq__)
...Gotcha — small-int caching:
python — editable
a = 256
b = 256
a is b # True (CPython caches -5 to 256)
a = 257
b = 257
a is b # False! (or True, depending on context — DO NOT RELY ON THIS)Interview answer: "Use is only for None, True, False, or intentional identity checks. Use == for value equality."
Confusion 2: Mutable Default Arguments (THE classic bug)
python — editable
def add_item(item, target=[]): # ❌ DANGEROUS
target.append(item)
return target
print(add_item(1)) # [1]
print(add_item(2)) # [1, 2] ← WAIT WHAT?
print(add_item(3)) # [1, 2, 3] ← default is SHARED across calls!Why? Default values are evaluated ONCE, when the function is DEFINED. The same list object is reus