When defining a dictionary in Python, which of the following will give you an error?

Question

BlackTom AI · Accepted Answer

Dictionary keys in Python must be immutable and hashable, so lists (which are mutable and unhashable) cannot be used as keys. Immutable built-in types like strings, numbers, and tuples (provided their contents are hashable) are valid dictionary keys.

1) Relevant knowledge points and concepts
- A dictionary maps keys to values. Keys must be hashable (implement a stable __hash__ and __eq__). Mutable types like list and dict are unhashable; attempting to use them as keys raises TypeError: unhashable type: 'list'.
- Tuples are immutable and hashable if all their elements are hashable. Strings and integers are hashable.

2) Step-by-step reasoning for each option
- Option 1: my_phonebook = {('Lisa','Lee'):'(123)456-7890', ('Jennie', 'Kim'):'(321)654-0987'}
  - Keys are tuples containing strings. Tuples are immutable and their string elements are hashable → valid.
- Option 2: my_phonebook = {'Lisa':1234567890, 'Jennie':3216540987}
  - Keys are strings (hashable) and values are integers → valid.
- Option 3 (claimed correct error): my_grade = {['A']:5, ['B']:3, ['C']:1}
  - Keys are lists. Lists are mutable and unhashable → this will raise TypeError when Python tries to hash a list to use as a dict key.
- Option 4: my_grade = {'A':5, 'B':3, 'C':1}
  - Keys are strings → valid.
- Option 5: None of the above
  - False because option 3 is invalid.

3) Intermediate “calculation” / demonstration
- If you try to execute option 3 in Python you get:
  TypeError: unhashable type: 'list'
  (This occurs because Python calls hash(key) internally and lists do not provide a hash value.)

4) Why the correct answer is correct
- Because each key in that option is a list, and lists cannot be hashed, Python raises an error. All other options use hashable key types.

5) Why other options are incorrect
- Options 1, 2, and 4 use only hashable key types (tuples, strings) so they are valid dictionary definitions. Option 5 is incorrect because there is indeed an option that causes an error.

6) Conclusion
- The dictionary that will give an error is: my_grade = {['A']:5, ['B']:3, ['C']:1}, which matches the provided correct answer.

When defining a dictionary in Python, which of the following will give you an error?单项选择题

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