Data Structure Pitfalls

This lesson is about mistakes that cause bugs in real projects.

1) Modifying a list while iterating

Bad:

nums = [1,2,3,4]
for n in nums:
    if n % 2 == 0:
        nums.remove(n)
print(nums)  # can behave unexpectedly

Good:

nums = [1,2,3,4]
nums = [n for n in nums if n % 2 != 0]
print(nums)

2) Shallow copy with nested structures

a = [[1],[2]]
b = a.copy()
b[0].append(99)
print(a)  # changed

Fix:

import copy
b = copy.deepcopy(a)

3) Assuming set has stable order

Sets are not designed for “index order”.
If you need order, use list.

4) KeyError in dict

Use get() for safe reads:

d = {"a": 1}
print(d.get("x", 0))

Graph: pitfalls map

flowchart TD
  A[Bug] --> B[Mutation during loop]
  A --> C[Shallow copy]
  A --> D[Wrong structure assumption]
  A --> E[Unsafe dict access]

Remember

• Use get(), copy(), deepcopy()
• Don’t mutate while iterating
• Choose structure that matches your needs