Python Strings

Python Strings: Immutability, Slicing, & Encoding

Strings in Python are immutable sequences of Unicode code points (str). Once created, a string's memory contents cannot be changed in place; operations that modify strings return a newly created string object.

String Memory & Slicing Indexing

flowchart LR
    subgraph Slicing Index Range
        i0["Index  0 : 'P'"]
        i1["Index  1 : 'y'"]
        i2["Index  2 : 't'"]
        i3["Index  3 : 'h'"]
        i4["Index  4 : 'o'"]
        i5["Index  5 : 'n'"]
    end
    subgraph Negative Indexing
        n0["Index -6"]
        n1["Index -5"]
        n2["Index -4"]
        n3["Index -3"]
        n4["Index -2"]
        n5["Index -1"]
    end

String Slicing Syntax: string[start:stop:step]

• start: Inclusive start index (defaults to 0).
• stop: Exclusive end index (defaults to len(string)).
• step: Increment step size (negative step reverses direction).

Practical Code Example

from typing import List

def process_text_data(raw_text: str) -> None:
    # String Slicing & Reversal
    word = "Python"
    prefix = word[:2]        # 'Py'
    suffix = word[2:]        # 'thon'
    reversed_word = word[::-1] # 'nohtyP'
    print(f"Original: {word} | Prefix: {prefix} | Reversed: {reversed_word}")

    # Standard String Methods
    cleaned_text = raw_text.strip().lower()
    tokens: List[str] = cleaned_text.split(",")
    formatted_output = " | ".join([t.strip().capitalize() for t in tokens])
    
    print(f"Raw Input: '{raw_text}'")
    print(f"Processed Output: '{formatted_output}'")

    # UTF-8 Encoding & Decoding
    encoded_bytes: bytes = word.encode("utf-8")
    decoded_str: str = encoded_bytes.decode("utf-8")
    print(f"UTF-8 Bytes: {encoded_bytes} -> Decoded: {decoded_str}")

if __name__ == "__main__":
    sample_csv = "  apple , banana, CHERRY , date  "
    process_text_data(sample_csv)

Best Practices & Gotchas

• Use ''.join(list) for Efficient Concatenation: Avoid looping with s += item inside loops, as creating new strings in each iteration causes $O(N^2)$ memory overhead. Use str.join() for $O(N)$ speed.
• F-Strings for Readability: Use f-strings (f"User: {username}") instead of string concatenation or % formatting.
• Strings are Immutable: Calling s.replace("a", "b") does not alter s; you must reassign the returned result (s = s.replace(...)).

Self-Check Challenge

Write a function is_palindrome(text: str) -> bool that strips spaces, converts text to lowercase, and checks if it equals its reverse slice (text[::-1]).