Extracting Substrings Using String Slicing in Python
What is the result of the expression 'Hello'[1:3]?
⚠ Common exam trap
Python Institute often tests the off-by-one error in slice stop indices, where candidates mistakenly think the stop index is inclusive and select 'ell' (option D) instead of the correct 'el'.
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
✓
'el'
In Python, string slicing uses the syntax `string[start:stop]`, where `start` is inclusive and `stop` is exclusive. For `'Hello'[1:3]`, the indices are: index 1 = 'e', index 2 = 'l', and index 3 is not included, so the slice returns 'el'. This is a fundamental string slicing behavior defined in Python's sequence protocol.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
'el'
Why this is correct
Python slicing uses a half-open interval: the start index is included, but the stop index is excluded. In 'hello', the indices are h=0, e=1, l=2, l=3, o=4, so slice [1:3] selects index 1 ('e') and index 2 ('l'), producing 'el'. The character at index 3 is not part of the result because the stop boundary is exclusive.
- ✗
'lo'
Why it's wrong here
The substring 'lo' occupies indices 2 and 3 in 'hello', so it would be returned by the slice [2:4] (or [2:] for the tail), not by [1:3]. A start of 1 means the slice begins with 'e', and a stop of 3 prevents the character at index 3, 'o', from being included. Therefore 'lo' cannot be the output of this particular slice.
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'He'
Why it's wrong here
'He' corresponds to indices 0 and 1, and would require the slice [0:2] to capture the leading 'H' and the following 'e'. The slice [1:3] explicitly starts at index 1, so the initial 'H' is skipped from the start. Thus 'He' is not a possible result of this expression.
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'ell'
Why it's wrong here
'ell' consists of indices 1, 2, and 3, which would need the stopping index to be 4 because the stop position is non-inclusive. The slice [1:3] includes only indices 1 and 2, so the second 'l' at index 3 is omitted. Consequently, 'ell' is three characters long, while [1:3] can return at most two characters.
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