Day 05: Mutation, identity, types, and comparisons

a = [1]
b = a
a.append(2)
print("case 1:", a, b, a is b)
a = [1]
b = a
a = a + [2]
print("case 2:", a, b, a is b)

Inspect Reference Solution & Trace:

Output (identical on Python 3.13 and 3.14):

case 1: [1, 2] [1, 2] True
case 2: [1, 2] [1] False

Explanation:
- Case 1: b = a makes both names refer to one list. append mutates that list in place, so both names show [1, 2] and a is b is True.
- Case 2: a + [2] creates a new list [1, 2] and the assignment rebinds a to it. b still refers to the original [1], so a is b is False.

Rule: mutation changes an object, and every name that refers to it sees the change; rebinding changes which object one name refers to, and other names are unaffected.

print(1 == True, 0 == False)
print([1] == [1], [1] is [1])
raw = "3"
print(raw == 3, int(raw) == 3)
value = None
print(value is None)

Inspect Reference Solution & Trace:

Output (identical on Python 3.13 and 3.14):

True True
True False
False True
True

Explanation:
- 1 == True and 0 == False are True: bool is a subclass of int, so True equals 1 and False equals 0.
- [1] == [1] is True because the contents are equal; [1] is [1] is False because each list display creates a new object.
- raw == 3 is False: a str never equals an int. After explicit conversion, int(raw) == 3 is True. The input is misleading because 3 and "3" look alike when printed; repr(raw) and type(raw) reveal the difference.
- value is None is True. There is exactly one None object, so is None checks that object directly and cannot be fooled by a class that redefines ==.

Optional work result: int(" 3 ") returns 3 (surrounding whitespace is ignored); int("3.0") raises ValueError: invalid literal for int() with base 10: '3.0'.

score = 3
if score = 3:
    print("passed")

raw = "3"
if raw == 3:
    print("passed")
else:
    print("not passed:", repr(raw), type(raw).__name__)

items = None
items.append(1)
print(items)

Inspect Reference Solution & Trace:

Worked solution. Outputs are identical on Python 3.13 and 3.14. The File line shows your own path; the caret (^) and suggestion lines may differ on older Python versions, so match the final error line.

1. starter_syntax.py — syntax error. Python rejects the whole file before running any line:

File "starter_syntax.py", line 2
    if score = 3:
       ^^^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

Fix: = binds a name; a condition needs the comparison ==.

score = 3
-if score = 3:
+if score == 3:
     print("passed")

Output after the fix:

passed

2. starter_compare.py — type/value mismatch. There is no error, but the wrong branch runs, because the str "3" never equals the int 3:

not passed: '3' str

Fix: convert the text to an integer at the input boundary, then compare integers.

raw = "3"
-if raw == 3:
+score = int(raw)
+if score == 3:
     print("passed")
 else:
-    print("not passed:", repr(raw), type(raw).__name__)
+    print("not passed:", repr(score), type(score).__name__)

Output after the fix:

passed

Edge case raw = "4":

not passed: 4 int

Edge case raw = "three":

Traceback (most recent call last):
  File "fixed_compare.py", line 2, in <module>
    score = int(raw)
ValueError: invalid literal for int() with base 10: 'three'

Input contract: the fixed program accepts text that contains a whole number (int() also ignores surrounding spaces); non-numeric text raises ValueError, which the optional work turns into a clear message.

3. starter_none.py — invalid operation on None. None means “no list yet” and has no append method:

Traceback (most recent call last):
  File "starter_none.py", line 2, in <module>
    items.append(1)
    ^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'append'

Fix: create an empty list before appending to it.

-items = None
+items = []
 items.append(1)
 print(items)

Output after the fix:

[1]

Assistance declaration example: “Agent: , read-only access to the three starter files; I ran every file and reviewed every diff myself.” — or “No agent used.”

a = [1]
b = a
a.append(2)
print("mutation:", a, b, a is b)
a = [1]
b = a
a = a + [2]
print("rebinding:", a, b, a is b)
print(1 == True)
print([1] == [1])
print([1] is [1])
print("3" == 3)

Inspect Reference Solution & Trace:

Worked solution.

Full trace of the single combined program in a fresh script. The mutation column covers lines 1–4; the rebinding column continues at lines 5–8 without resetting b. Lines 9–12 are comparison checkpoints in that same run. L1, L2, and L3 are distinct list-object labels, not Python variables or memory addresses; a → L1 [1] records both the binding and that object's current contents. UNBOUND means the name has not been assigned.

| Checkpoint / source line just executed | a binding and object contents | b binding and object contents | a is b |
| --- | --- | --- | --- |
| Initial state, before line 1 | UNBOUND | UNBOUND | Not evaluable: NameError for a |
| 1. a = [1] | → L1 [1] | UNBOUND | Not evaluable: NameError for b |
| 2. b = a | → L1 [1] | → L1 [1] | True |
| 3. a.append(2) | → L1 [1, 2] | → L1 [1, 2] | True |
| 4. print("mutation:", a, b, a is b) | → L1 [1, 2] | → L1 [1, 2] | True |
| 5. a = [1] | → L2 [1] | → L1 [1, 2] | False |
| 6. b = a | → L2 [1] | → L2 [1] | True |
| 7. a = a + [2] | → L3 [1, 2] | → L2 [1] | False |
| 8. print("rebinding:", a, b, a is b) | → L3 [1, 2] | → L2 [1] | False |
| 9. print(1 == True) | → L3 [1, 2] | → L2 [1] | False |
| 10. print([1] == [1]) | → L3 [1, 2] | → L2 [1] | False |
| 11. print([1] is [1]) | → L3 [1, 2] | → L2 [1] | False |
| 12. print("3" == 3) | → L3 [1, 2] | → L2 [1] | False |

The initial and line-1 NameError entries describe what an added a is b probe would raise; the published program does not evaluate it there, so execution continues normally. Line 3 mutates the one shared L1 object. At line 5, b still refers to that mutated L1 while a refers to a new L2. Line 6 rebinds b to L2; L1 is no longer referenced by either name. Line 7 creates L3 and rebinds only a; L2 stays unchanged. Printing does not change either binding. The temporary lists in lines 10–11 are not a or b; those comparison results are shown in the preserved output below.

Output (identical on Python 3.13 and 3.14):

mutation: [1, 2] [1, 2] True
rebinding: [1, 2] [1] False
True
True
False
False

Comparisons: 1 == True is True (bool is a subclass of int); [1] == [1] is True (equal contents); [1] is [1] is False (two separate objects); "3" == 3 is False (str never equals int).

Prevention rules:
1. Syntax error — when testing equality, use ==, not assignment =. In this clinic, if score == 3: tests equality; score = 3 binds a name. Conditions need not test equality: truthiness (if items:), ordering (if score < 3:), identity (if items is None:), and other predicates are valid in if and while. Evidence: SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
2. str versus int — convert text input with int(raw) at the input boundary before comparing it with numbers, and print repr() and type() when a comparison is unexpectedly False. Evidence: "3" == 3 printed False, and the clinic printed not passed: '3' str.
3. None — create a collection (items = []) before calling its methods, and when None is possible, check if items is None: first. Evidence: AttributeError: 'NoneType' object has no attribute 'append'.