15. Top Pitfalls

Common mistakes that cause subtle bugs—recognize and avoid them quickly.

• Mutable default arguments

  Bad creates shared state across calls; use None and create inside.

  def add_item(x, bag=[]):  # BAD
      bag.append(x)
      return bag
  
  def add_item_safe(x, bag=None):  # GOOD
      if bag is None:
          bag = []
      bag.append(x)
      return bag
  

• List multiplication with inner lists

  grid = [[0]*3]*3      # BAD: rows share the same list
  grid[0][0] = 1        # mutates all rows
  grid = [[0 for _ in range(3)] for _ in range(3)]  # GOOD
  

• Floating point equality

  0.1 + 0.2 == 0.3      # False
  import math
  math.isclose(0.1 + 0.2, 0.3, rel_tol=1e-9)  # True
  

• Late binding in closures (loop variables)

  funcs = [lambda i=i: i*i for i in range(3)]  # default arg captures value
  [f() for f in funcs]  # [0, 1, 4]
  

• Shadowing built-ins

  Avoid names like list, dict, id, type for your variables.

• Overbroad exception handling

  try:
      risky()
  except Exception:       # BAD: hides bugs
      pass
  
  try:
      risky()
  except (ValueError, KeyError) as e:  # GOOD: handle expected cases
      handle(e)
  

• File encodings/newlines

  Always set encoding="utf-8". For CSV on Windows, pass newline="" to avoid blank lines.

• Off-by-one slicing

  Remember end index is exclusive: s[a:b] includes a, excludes b.

• Using is for value comparison

  Use == for equality, is only for identity (None, singletons).

• HTTP without timeouts

  requests.get(url, timeout=5)  # always set a timeout