Using break to Exit Loops Early

A Filter Inside the Loop

A loop may examine many input lines even though only some of them matter. The continue statement lets the loop discard an uninteresting line immediately. It stops the current iteration and sends control back to the top of the loop, where the next line can be examined.

Tracing the Skip Path

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

The loop first tests whether the current line starts with From:. The condition uses not, so a line that does not start with From: is identified as uninteresting. For that line, continue runs immediately. The print statement is below continue, so it is skipped for that iteration. A line that does start with From: does not enter the continue path and reaches print instead.

The Search Pattern

The pattern if not condition: continue is useful because it describes the lines to reject first. If the condition is true, the line is interesting and the code below can process it. If the condition is false, not condition is true, so continue skips the line. This keeps the processing code focused on matching lines rather than nesting it inside another conditional branch.

A Three-Line Trace

Use a small input to trace the loop rather than trying to reason about every line at once. The sample contains Subject: Meeting, From: alice@example.com, and Date: 2024-01-15. Only the second line begins with From:.

From: alice@example.com

Compact Conditional Syntax

if not line.startswith("From:"): continue

Python allows an if statement and its action to appear on one line. In this contracted form, the condition comes first, followed by a colon and the action. The one-line form functions identically to the indented multi-line form.

Continue or Break

• Treating continue as if it ended the entire loop.

  continue skips only the rest of the current iteration and then returns control to the loop start.

  Fix: Use continue when the loop should move to the next item. Use break when the loop should end immediately.

• Expecting code after continue to run for the same line.

  Python immediately stops executing the rest of that iteration after continue.

  Fix: Place code below continue only when it should process lines that pass the filter.

• Forgetting that not reverses the test.

  The pattern tests for the lines to skip, not the lines to process.

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

Try the Trace

1. Read the current input line.
2. Ask whether it starts with From:.
3. If it does not, follow continue and move to the next iteration.
4. If it does, let the line reach the processing statement.
5. Repeat the trace for every input line.

Key Takeaways

1. continue skips the rest of the current loop iteration and starts the next iteration.
2. The pattern if not condition: continue filters out uninteresting items before the processing code.
3. Only lines that avoid continue reach statements placed below it.
4. continue differs from break because break exits the entire loop.
5. A contracted if places the action on the same line as the condition and behaves like the equivalent multi-line form.