Breakpoint on continue in if statement not hit

ghz 11hours ago ⋅ 3 views

In the following code, both a and b are outputs of generator functions, and could evaluate to None or have a value.

def testBehaviour(self):
  a = None
  b = 5

  while True:
    if not a or not b:
      continue
    print('blaat')

If I put breakpoints (using Visual Studio Code) on the line with the continue statement, and the line with the print statement, neither is hit. The print statement is not called, and the loop just keeps running indefinitely as expected, but I would expect that breakpoints are hit.

If I change the code to one of the following:

def testBehaviour(self):
  a = None
  b = 5

  while True:
    if not a:
      continue
    print('blaat')

or:

def testBehaviour(self):
  a = None
  b = 5

  while True:
    if not a or not b:
      c = 'dummy'
      continue
    print('blaat')

And again place breakpoints on the lines with the continue and print statements, the breakpoints are hit.

Can anyone tell me why the breakpoints are not hit? It seems to not happen only in Visual Studio Code, because our code-coverage tool also states that the continue statement was not called.

This is on python 2.7 on Windows 7, 32 bit.

Answer

It sounds like you're encountering an issue with breakpoints not being hit when debugging in Visual Studio Code, and you've noticed a discrepancy in behavior based on the structure of your code. This is an interesting scenario, and there are a few potential reasons why this might be happening:

1. Optimizer and Debugger Interaction

Python's interpreter might optimize certain loops or statements in ways that prevent the debugger from properly interacting with the code. Specifically, in your original example:

if not a or not b:
   continue
  • The condition if not a or not b: may be evaluated in a way that is bypassing the debugging mechanism in Visual Studio Code, especially if a and b are being evaluated as generators or are changing in a non-obvious way. Debuggers like the one in VS Code may not be able to step into or stop at certain conditions if the code execution does not explicitly pass through the conditional branches due to optimizations or delays.

2. Python 2.7 Debugger Issues

Python 2.7, especially on Windows, has known issues with the debugging experience. The built-in debugger (pdb) or other third-party debuggers, especially when integrated into IDEs like Visual Studio Code, may not always function as expected. These issues can manifest in cases where breakpoints aren't hit, particularly in loops with complex conditions or when using generators, as the state of the variables may be harder to track.

Given that Python 2.7 is no longer officially supported, you might be hitting a bug or limitation in the Python 2.7 debugger when debugging certain control flow structures.

3. Changes in the Debugger Behavior Based on Code Structure

In the examples where you do hit breakpoints:

if not a:
    continue

or:

if not a or not b:
    c = 'dummy'
    continue

The behavior may be more predictable because:

  • The condition if not a: is simple, so the debugger can follow it easily.
  • The addition of c = 'dummy' may introduce a side effect that makes the code execution path more straightforward for the debugger to track.

The introduction of a side effect like setting a variable (c = 'dummy') can sometimes cause the debugger to recognize the execution flow better, especially if the line with the continue is being optimized away or skipped without such an effect.

4. Workaround and Debugging Strategy

To work around this issue, you can try a few approaches:

  • Add Debug Prints: Sometimes, adding print statements before the continue or print statements can help make the debugger behave more predictably. This can also help verify whether the loop is behaving as expected.

    if not a or not b:
    print("Skipping loop iteration due to condition")
    continue

  • Use a Different Debugger: Since you're using Visual Studio Code with Python 2.7, you might want to try using a different debugger such as ipdb, which is often more robust for certain debugging tasks compared to the default pdb. You can install it with pip install ipdb and then use it in your code like so:

    import ipdb; ipdb.set_trace()

  • Upgrade Python Version: If possible, consider upgrading to Python 3, as Python 2.7 is no longer officially supported, and you'll have a better experience with modern debugging tools and enhanced debugging capabilities.

  • Isolate the Generator: If a or b are generators, it might be helpful to isolate their values before entering the loop. Sometimes, the debugger struggles with dynamic changes in values that are generated during execution.

    a_value = next(a, None)  # or similar
b_value = b  # If b is static
if not a_value or not b_value:
    continue

Conclusion:

The issue you're seeing is likely related to a combination of Python 2.7's debugger limitations, how the condition is evaluated in the loop, and potential optimizations in Python’s execution. Try debugging with print statements, switching to ipdb, or considering Python 3 if possible.

#py