common_errors.md

Common Mistakes

= vs ==

x = 5      # assignment
x == 5     # comparison

Off-by-one with range

range(5)   # 0, 1, 2, 3, 4

Forgetting indentation

if condition:
    print("Indented")

Calling vs referring to a function

len(items)   # call
len          # function object

Mutable list methods often return None

cities.sort()        # changes cities in place
sorted(cities)       # returns a new sorted list

Dictionary access

person["age"]        # KeyError if missing
person.get("age")    # returns None if missing