200-901 Software Development and Design Practice Question
Which Python exception would be raised by the following code?
my_dict = {'a': 1} value = my_dict['b']
⚠ Common exam trap
Cisco often tests the distinction between KeyError and IndexError, trapping candidates who confuse dictionary key access with list index access, especially when the code uses square brackets in both contexts.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
KeyError
Accessing a dictionary key that does not exist raises a KeyError in Python. In the code, my_dict['b'] attempts to retrieve the value for key 'b', which is not present in the dictionary {'a': 1}, so Python raises KeyError.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
KeyError
Why this is correct
Accessing a missing key in a dictionary raises KeyError, because Python's dict lookup requires the key to exist. The code requests 'b' from a dict containing only 'a', so the lookup fails immediately and KeyError propagates.
- ✗
ValueError
Why it's wrong here
A missing dictionary key raises KeyError, not ValueError. ValueError arises when a built-in operation receives an argument of the right type but an invalid value, such as int('abc'), so it would fit a failed type conversion rather than a failed lookup.
- ✗
IndexError
Why it's wrong here
IndexError arises from sequence types such as lists or tuples when a positional subscript exceeds their bounds; dictionaries are mappings keyed by hash, so a missing key raises KeyError instead. It tempts because both are lookup failures on subscripting, and IndexError would be correct for `[1, 2][5]`.
- ✗
AttributeError
Why it's wrong here
AttributeError occurs when an attribute or method does not exist on an object, for example calling 'abc'.append(). Subscripting a dictionary with a missing key raises KeyError, so AttributeError would apply to a misspelled method name, not a lookup.
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Written by Johnson Ajibi, MSc IT Security
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