Strings files

1. Text Processing: String Methods

Strings are immutable sequences of text characters in Python. To process and clean text data (such as user inputs or database strings), Python provides powerful built-in string methods:

• strip(): Removes leading and trailing whitespace (spaces, tabs, newlines).
• lower() / upper(): Converts all characters to lowercase or uppercase.
• replace(old, new): Replaces occurrences of a substring with another.
• split(separator): Divides a string into a list of substrings based on a separator (e.g., split by commas).
• join(iterable): Combines elements of a list into a single string with a separator.

raw_input = "  alice,24,engineer \n"
clean_data = raw_input.strip()  # "alice,24,engineer"
fields = clean_data.split(",")  # ["alice", "24", "engineer"]
reconstructed = "-".join(fields)  # "alice-24-engineer"

2. String Formatting: f-strings

F-strings (formatted string literals) provide a clean, readable syntax to insert variables and expressions directly into strings. Prefix the string with an f or F, and write variables inside curly braces {}.

name = "Sam"
score = 88.5
print(f"Student {name} achieved a score of {score}%.")

3. File Operations: Context Managers

Python allows you to read from and write to physical files on your disk. Opening a file takes system resources (file handles), so files must be closed. Instead of manually calling file.close(), you should always use the with statement (a **context manager**). It guarantees the file will be closed automatically once execution exits the block, even if your code crashes.

The standard file modes are:

• "r": Read mode (default). Raises an error if the file doesn't exist.
• "w": Write mode. Overwrites the file completely or creates a new one.
• "a": Append mode. Adds new data to the end of the file.

# Writing to a file (creates or overwrites)
with open("notes.txt", "w", encoding="utf-8") as file:
    file.write("First line of notes.\n")
    file.write("Second line of notes.\n")

# Reading a file line by line
with open("notes.txt", "r", encoding="utf-8") as file:
    for line in file:
        print("Line:", line.strip())

4. Common Beginner Mistakes

Interactive Practice

Mini Practice: Line Logger

Write a script that creates a file called log.txt and writes three lines of log messages using with open(). Then, re-open the file in read mode, iterate through the lines, count how many lines it has, and print each line with its line number.

0% complete

Your code

Check when done

Uses with open('log.txt', 'w') to write content Re-opens in read mode ('r') and loops over lines Keeps track of line count and prints numbered output

Quiz

Test Your Understanding

1. Which file mode will append new content to the end of a file instead of erasing it?

"w" "a" "r"

2. Why should you always prefer using `with open(...)` over `file = open(...)`?

It makes the file read/write operations twice as fast It automatically closes the file, preventing resource leaks It bypasses operating system security permissions

3. What does string `.strip()` do?

Splits the string into a list of values Removes leading and trailing whitespace Replaces all lowercase characters with uppercase

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