Questions: String Operations and Methods

Question 1

What does the following code print? s = "hello" s.upper() print(s)

• A. "HELLO"
• B. "hello"
• C. An error, because upper() is not a valid string method
• D. Nothing — print() requires an argument

Answer: (B)

Strings are immutable in Python. The method s.upper() returns a new string "HELLO" but does not modify s in place. Because the return value is discarded (not assigned to any variable), s still holds "hello" when print(s) runs. To actually change s, you would need to write s = s.upper(). This is the most common beginner mistake with string methods: calling them without capturing their return value and then wondering why nothing changed.

Question 2

What does s[1:4] return when s = "python"?

• A. "yth"
• B. "pyt"
• C. "ytho"
• D. "ython"

Answer: (A)

Python string slicing uses a half-open interval: s[start:stop] returns characters at indices start, start+1, ..., stop-1 — not including stop. For s = "python", the indices are p=0, y=1, t=2, h=3, o=4, n=5. So s[1:4] returns characters at indices 1, 2, and 3: 'y', 't', 'h' → "yth". A common error is to think s[1:4] includes index 4 (giving "ytho"), but the stop index is always excluded.

Question 3

The expression s[::-1] reverses the string s.

• T. True
• F. False

Answer: True

The slice s[start:stop:step] with step=-1 traverses the string backwards. Omitting start and stop means 'from the end to the beginning,' and step=-1 means 'move one character backward at each step.' Together, s[::-1] produces a new string with all characters in reverse order. This is a common Python idiom for reversing strings and works because slicing never modifies the original string — it always returns a new one.

Question 4

Calling s.strip() on a string removes its leading and trailing whitespace by modifying the original string in place.

• T. True
• F. False

Answer: False

Strings are immutable in Python — no string method modifies the original string in place. s.strip() returns a new string with leading and trailing whitespace removed, but s itself is unchanged. To use the stripped version, you must capture the result: s = s.strip() or cleaned = s.strip(). This applies to all string methods: upper(), lower(), replace(), split(), and so on all return new strings without touching the original.

Question 5

Why must you write s = s.upper() rather than just s.upper() if you want s to hold the uppercased version of the string?

Model answer: Strings are immutable — they cannot be changed in place. Every string method returns a new string object with the transformation applied. Calling s.upper() produces that new string but immediately discards it if you don't assign it to a variable. The original string s remains unchanged. By writing s = s.upper(), you reassign the variable s to point to the newly created uppercase string. The same rule applies to every string method: strip(), replace(), split(), and others all return new strings rather than modifying the original.

This is one of the most frequent bugs beginners write: they call s.strip() or s.replace() at the top of a function, see no error, and then wonder why the string is still dirty or unmodified later. The fix is always the same — capture the return value. Understanding immutability explains the behavior: strings are fixed objects, and methods create new ones.