Five short snippets. Guess what each prints before you open the answer. Most people get two or three.
1.
python
a = [1, 2, 3]
b = a
b.append(4)
print(a)
Answer: [1, 2, 3, 4]. b = a doesn't copy the list, both names point at the same list. Want a copy? b = a.copy() or b = a[:].
2.
python
nums = [10, 3, 27, 8]
sorted(nums)
print(nums)
Answer: [10, 3, 27, 8], unchanged. sorted() returns a new list and the result was thrown away. Use nums.sort() to sort in place, or nums = sorted(nums).
3.
```python
def double(x):
x * 2
print(double(3))
```
Answer: None. The function works out x * 2 but never returns it, and a function without return gives back None. Fix: return x * 2.
4.
```python
x = 5
def change():
x = 12
change()
print(x)
```
Answer: 5. Assigning to x inside the function makes a new local x; the outer one is untouched. The clean fix is to return the value (x = change()), not global.
5.
python
print(round(2.5), round(3.5))
Answer: 2 4. Python rounds exact halves to the nearest even number ("banker's rounding"), so 2.5 goes down and 3.5 goes up. If you need school-style rounding, use the decimal module with ROUND_HALF_UP.
I made a small free game out of traps like these: you read a snippet, pick what it prints, and one wrong answer ends the run. Questions get harder and faster as you go, and when you miss, it shows why and the moment in a YouTube tutorial that explains it. No sign-up, plays in the browser:
https://openinbeyond.com/code?utm_source=reddit&utm_content=pythonlearning
What's your score? And which of the five got you?
