Loops and Conditionals

From Filename to Lines

File processing becomes useful when several small programming actions work together. A program can ask the user which file to process, open that file, visit its contents one line at a time, transform each line, and select only the lines that match a pattern. Loops provide repetition, while conditionals provide decisions about individual lines.

First Pass Through the File

A for loop can process a file one line at a time. On each pass, the loop provides the next line for the program to process. The same operation can therefore be applied repeatedly without writing a separate instruction for every line. After the available lines have been processed, the loop stops.

with open("notes.txt") as file:
    for line in file:
        print(line.rstrip())

In this generated example, the with statement is used for the file operation, and the for loop receives each line in turn. The rstrip() call removes the line-ending characters before printing, which avoids the extra blank lines that can occur when a line already contains a newline character.

Transforming Each Line

A loop does not have to print a line exactly as it was read. Inside the loop, string methods can transform the current line. For example, upper() can convert the line's text to uppercase before it is displayed or used elsewhere.

with open("notes.txt") as file:
    for line in file:
        print(line.upper().rstrip())

The current line is transformed with upper() and then cleaned with rstrip() before printing. The order makes the purpose of each string operation visible: one changes the letter case, and the other handles the newline at the end of the line.

Filtering Matching Lines

A conditional lets the program decide what to do with each line. The string method startswith() can test whether a line begins with a chosen pattern. If the test matches, the program can print or transform that line; if it does not match, the program can continue to the next line.

with open("activity.log") as file:
    for line in file:
        if line.startswith("ERROR:"):
            print(line.rstrip())

Here, every line is examined, but only lines beginning with the generated pattern ERROR: are displayed. The loop still controls repetition, while the if statement controls selection within each repetition.

Complete File-Processing Pattern

The separate techniques can be combined into one end-to-end program. The input() call makes the file choice dynamic: instead of placing a fixed file name in the program, the program uses the user's response directly with open(). The loop then searches for matching lines and transforms the selected text.

filename = input("Enter a file name: ") with open(filename) as file: for line in file: if line.startswith("ERROR:"): print(line.upper().rstrip())

Mistakes in File Loops

• Using a fixed file name when the program should respond to user input

  The program always selects the same file instead of using the file name supplied by the user.

  Fix: Store input() in a variable and pass that variable directly to open().

• Printing a file line without handling its newline character

  The line already includes a newline character, and print() can add another line ending, producing extra blank lines.

  Fix: Use line.rstrip() or adjust the print() call.

• Applying the condition outside the line-processing loop

  Pattern searching must be performed on each individual line as the loop processes it.

  Fix: Place the startswith() test inside the for loop.

• Opening a file without a with statement

  The source material emphasizes using with so file operations are handled safely and the file is properly closed.

  Fix: Use with open(filename) as file: and place the processing code inside the block.

Practice: Build the Search

Key Takeaways

1. A for loop processes a file one line at a time.
2. String methods such as upper() can transform each line during iteration.
3. input() can provide a file name that is passed directly to open().
4. startswith() can select only lines that begin with a chosen pattern.
5. rstrip() helps prevent extra blank lines when printing file contents, and with helps ensure file operations are handled safely.