How to Restart a Loop in Python

Python does not have a built-in keyword like restart to jump back to the beginning of the same loop execution. However, you can achieve the effect of restarting a loop by structuring your code differently, typically using nested loops and control flow statements.

Achieving the "Restart" Effect with Nested Loops

Based on the concept described in the reference, the primary method to effectively "restart" a loop from its beginning is to nest that loop inside another loop.

Here's the principle:

1. You have the loop you want to be able to "restart" (let's call this the inner loop).
2. You place this inner loop inside an outer loop.
3. Inside the inner loop, you include a condition or mechanism that, when met, causes the inner loop to terminate its current execution.
4. When the inner loop terminates this way, the control flow returns to the outer loop.
5. The outer loop then proceeds to its next iteration. As part of starting a new iteration of the outer loop, the code re-enters and re-starts the inner loop from its beginning.

The reference mentions setting specific variables like continueRoutine to False and an inner loop object's finished attribute to True. While the exact variable names (continueRoutine, name_of_your_inner_loop.finished) might be specific to a particular framework or context (like PsychoPy, where these terms are common), the underlying concept is using internal signals or variables within the inner loop to trigger its termination, which then allows the outer loop to initiate a new run of the inner loop.

How Control Flow Works

Standard Python loop control statements help manage this process:

Action	Effect on Loop	How Achieved in Python
break	Immediately exits the current loop entirely.	break keyword
continue	Skips the rest of the current iteration and moves to the next iteration of the same loop.	continue keyword
"Restart"	Exits the current run of the inner loop, allowing an outer loop to begin it again from the start in a subsequent iteration.	Nesting the inner loop, using break or flags within the inner loop to exit, and relying on the outer loop's iteration structure.

Using break inside the inner loop is a common way to exit its current run and return control to the outer loop.

Example: Simulating a Restart

Let's illustrate this with a conceptual example where we might want to "restart" processing an item under certain conditions.

# Outer loop: Controls how many times we can potentially "restart" the inner process
outer_attempts = 0
max_outer_attempts = 3

while outer_attempts < max_outer_attempts:
    outer_attempts += 1
    print(f"\n--- Outer Loop Iteration: {outer_attempts} ---")

    # Flag to signal if we need to restart the inner process
    restart_needed = False

    # Inner loop: The process we want to be able to restart
    inner_attempts = 0

    # We use 'while True' or a condition that makes sense for the inner loop's duration
    # and rely on 'break' or flags to exit it.
    while inner_attempts < 2: # Example inner loop condition
        inner_attempts += 1
        print(f"  Inner Loop Iteration: {inner_attempts}")

        # Simulate some work or checks inside the inner loop
        # ... perform task ...

        # --- Condition to trigger a "restart" ---
        # If a certain condition is met, we decide to exit this inner loop run
        # and let the outer loop start it over.
        if outer_attempts == 1 and inner_attempts == 1: # Example condition for restarting
             print("  Condition met! Signalling restart...")
             restart_needed = True # Set flag
             break # Exit the current inner loop execution
        # --- End of restart condition ---

        print("  Inner loop iteration completed normally.")

    # Check the flag after the inner loop exits
    if restart_needed:
        print("Restart signalled. Outer loop will continue to next iteration, restarting inner process.")
        # 'continue' here is implicit; the while outer_attempts loop naturally goes to the next iteration
        # if not explicitly broken out of. We just ensure we don't proceed with post-inner-loop code
        # that shouldn't run if a restart occurred.
        continue # Skip the rest of the outer loop body and go to the next outer iteration

    # This part only runs if the inner loop finished without signaling a restart
    print("Inner loop finished without needing a restart.")
    # Break the outer loop if the inner process completed successfully
    break # Exit the outer loop after successful inner completion

print("\n--- Loop process finished ---")

In this example, when restart_needed is set to True and break is called in the inner loop, the inner loop's current execution is terminated. The code then checks the restart_needed flag. If it's True, the continue statement in the outer loop causes the outer loop to immediately proceed to its next iteration, effectively restarting the inner loop process from the beginning. If the inner loop finishes without setting restart_needed, the break statement after the inner loop exits both loops, signaling completion.

Summary

To "restart" a loop in Python in the sense of breaking out of its current run and starting it over from the beginning, you must:

• Nest the loop you want to restart inside another loop.
• Use control flow (like the break statement) or status flags (similar to finished or continueRoutine mentioned in the reference) within the inner loop to signal its termination when a restart condition is met.
• The outer loop's natural iteration process will then re-enter the inner loop, effectively "restarting" it.

This pattern allows you to control the execution flow and re-run a block of code (the inner loop) multiple times based on conditions encountered during its execution.