10 Common Python Mistakes Every Developer Makes (And How to Avoid Them in 2026)
10 Common Python Mistakes Every Developer Makes (And How to Avoid Them in 2026) Even experienced developers make small mistakes that lead to bugs, slow performance, or unreadable code. Below are 10 common Python mistakes—and the simple, correct patterns you should use instead. 1. Using Mutable Default Arguments Defining a function with a mutable default (like a list or dict) can cause the same object to persist across calls. def add_item(item, items=[]): items.append(item) return items # BAD: same list reused across calls Use None as the default and create a new object inside the function. def add_item(item, items=None): if items is None: items = [] items.append(item) return items # Good: fresh list each call 2. Misunderstanding Shallow vs Deep Copy Assigning one list to another does not copy it; both names reference the same object. list2 = list1 # Not a copy — both reference the same list Use copy() for a shallow copy o...
