Reading Files Line by Line

A Filtered Search

When a program reads a file line by line, it may need to process only lines that match a particular condition. A search loop can test each line, skip lines that are uninteresting, and allow matching lines to reach the processing code below. The continue statement is the control-flow tool that performs this skip.

What continue Changes

When Python encounters continue inside a loop, it immediately stops executing the rest of the current iteration. Execution jumps back to the top of the loop, where the next iteration begins. Any statements below continue are skipped for that item. This differs from break: continue moves to the next iteration, while break exits the loop entirely.

for line in file:
    if not line.startswith("From:"):
        continue
    print(line)

In this search loop, the condition checks whether a line does not start with From:. Such a line is treated as uninteresting and is skipped. A line that does start with From: does not execute continue, so it reaches the print statement.

Tracing Three Lines

Use a small input to make the control flow visible. Imagine that the file contains these three lines: Subject: Meeting, From: alice@example.com, and Date: 2024-01-15. The loop examines them in order. The first and third lines do not begin with From:, so they are skipped. The middle line begins with From:, so it reaches the print statement.

Filtering with Inverted Logic

A search loop is often clearer when it identifies the lines to skip rather than describing all the work to perform. The pattern if not condition: continue reverses the useful condition. If the desired condition is that a line starts with From:, then not line.startswith("From:") identifies every line that should be discarded.

for line in file: if not line.startswith("From:"): continue print(line)

Contracted If Syntax

Python allows an if statement and its action to appear on one line. This is called the contracted form in the source material. In a search loop, it places the skip condition and continue action together, which keeps the filtering rule compact.

Mistakes in Search Loops

• Treating continue as a way to exit the entire loop

  continue skips only the rest of the current iteration and then begins the next iteration.

  Fix: Use continue to discard one line. Remember that break, not continue, exits the loop entirely.

• Putting processing code before the skip condition

  Code before continue has already executed, so the line was not filtered before processing.

  Fix: Place the if not condition: continue test before the statements that should run only for interesting lines.

• Reversing the condition

  The condition determines which lines reach continue. Reversing it changes which lines are discarded.

  Fix: Read the condition as a sentence: if this line is uninteresting, continue.

• Assuming the one-line form changes the behavior

  The contracted form places the same action on the same line; it does not change the operation.

  Fix: Treat the contracted and multi-line forms as equivalent forms of the same if statement.

Practice the Trace

Key Takeaways

1. continue skips the rest of the current loop iteration and starts the next one.
2. The pattern if not condition: continue filters out uninteresting file lines before later processing.
3. Only lines that do not trigger continue reach the statements below it.
4. The contracted if form places the condition and action on one line and functions like the equivalent multi-line form.
5. Tracing each input line makes it possible to verify exactly which lines are skipped and which are processed.