🐍 5 Python Mistakes Every Junior Dev Makes (And How to Fix Them)

Introduction If you're a junior developer writing Python, chances are you've made at least one of these mistakes — and you didn't even know it. These aren't syntax errors that your IDE catches. They're subtle bugs and bad habits that slip through code reviews and slow down your growth. Let's fix all five. Mistake #1: Mutable Default Arguments The problem: def add_item ( item , lst = []): lst . append ( item ) return lst This looks harmless, but Python creates the default list once at function definition — not on every call. So every call shares the same list. The fix: def add_item ( item , lst = None ): if lst is None : lst = [] lst . append ( item ) return lst Always use None as your default for mutable arguments, then create the object inside the function. Mistake #2: Skipping List Comprehensions The problem: squares = [] for i in range ( 10 ): squares . append ( i ** 2 ) The fix: squares = [ i ** 2 for i in range ( 10 )] Cleaner, faster, and more Pythonic. Senior devs will notice im