9 Python Mistakes Everyone Makes

9 Python Mistakes Everyone Makes Write cleaner, faster, more Pythonic code 01 MISTAKE Mutable Default Args DON'T def add(val, items=[]): items.append(val) return items DO THIS def add(val, items=None): items = [] items.append(val) return items Default [] is shared across every call and mutates silently. WHY Python creates the default once at function definition, not per call. 02 MISTAKE Bare except: Clauses DON'T try: process(data) except: pass DO THIS try: process(data) except ValueError as e: logger.error(e) Bare except: catches everything, including Ctrl+C. WHY It swallows SystemExit, KeyboardInterrupt, and hides real bugs. 03 MISTAKE Checking == None DON'T if user == None: return "anonymous" DO THIS if user is None: return "anonymous" Use is None for identity checks, not equality. WHY == calls __eq__, which can be overridden. is checks object identity. 04 MISTAKE Skipping with Statements DON'T f = open("data.csv") data = f.read() f.close() DO THIS with open("data.csv") as f: data = f.read() Context managers guarantee cleanup, even on exceptions. WHY If an exception fires before .close(), the file handle leaks. 05 MISTAKE String += In Loops DON'T result = "" for w in words: result += w DO THIS result = "".join(words) Each += copies the entire string. That's O(n²). WHY Strings are immutable. join() allocates once, += copies every time. 06 MISTAKE The range(len()) Habit DON'T for i in range(len(items)): print(items[i]) DO THIS for i, val in enumerate(items): print(val) enumerate() gives index + value with zero overhead. WHY Cleaner, less error-prone, and the Pythonic way to iterate. 07 MISTAKE Wildcard import * DON'T from os.path import * DO THIS from os.path import join, exists Dumps every name into your namespace blindly. WHY Causes silent name collisions and makes code impossible to trace. 08 MISTAKE Not Using f-strings DON'T msg = "Hi " + name + ", " + "age " + str(age) DO THIS msg = f"Hi {name}, age {age}" f-strings are faster, shorter, and easier to scan. WHY Concat is error-prone and slow. f-strings inline expressions directly. 09 MISTAKE type() For Type Checks DON'T if type(x) == int: total += x DO THIS if isinstance(x, int): total += x isinstance() respects inheritance. type() does not. WHY A bool is an int subclass. type(True) == int is False, isinstance works. Follow For More Python & Coding Content