5 Python mistakes every beginner makes (and how to fix them in 2026)
Most Python beginners hit the same five walls. Once you know they're coming, you avoid them entirely. Here they are, with the fix for each.
Mistake 1: Mutable default arguments
The trap:
The list default is created once at function definition time, not each call. Every call mutates the same list.
The fix:
Rule: never use mutable objects ([], {}, set()) as default arguments. Use None and create them inside the function.
Mistake 2: Confusing copy and reference
The trap:
The fix:
For nested structures (lists of lists, dicts of lists), .copy() is not enough. You need copy.deepcopy().
Mistake 3: Using == instead of is (or vice versa)
The trap:
The fix: use is for None, True, False. Use == for everything else.
is checks memory identity. == checks logical equality. x is 0 happens to work in current CPython for small ints (interned), but you're relying on an implementation detail. Don't.
Mistake 4: Mixing for i in range(len(items)) with iteration
The trap (un-Pythonic):
The fix (Pythonic):
enumerate exists exactly so you don't need range(len(...)). The Pythonic version is also faster and clearer.
Mistake 5: Catching all exceptions silently
The trap:
The fix:
Rule: catch the specific exception you expect. If you must catch everything, at least log it. Bare except: pass is the #1 reason beginner code has invisible bugs that show up six months later in production.
Bonus: forgetting that strings are immutable
The fix:
This catches almost every beginner the first time they try to modify a string. Strings are immutable. You create a new one.
How to internalize these
Reading is not enough. Open a Python REPL (or our interactive lessons) and reproduce each trap. Run the broken code, see the bug, write the fix, run it again. That muscle memory is what makes you actually avoid these in your own code.
Our Common Mistakes track covers all five of these plus 15 more in interactive, runnable form — with an AI tutor that explains why each one breaks.