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Comment by masklinn | Hacker News Reader

I don't know what `@show` does exactly, but Python's is only a limited convenience for print, it still not as good as Rust's dbg![0] or TFA's ic, because it does not (and can not) return its parameter unmodified, so you still need to inject an explicit print and possibly execute the expression multiple times.

It's convenient, don't get me wrong, but it's not exactly groundsbreaking. Unlike breakpoint[1].

[0] https://doc.rust-lang.org/std/macro.dbg.html

[1] https://docs.python.org/3/library/functions.html#breakpoint it's not groundsbreaking on its own but the ability to very easily hook into it and replace the breakpoint hook by an arbitrary callable? Super useful.

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Do you have any concrete usage examples of that? I’ve only used breakpoint() to stop execution while debugging.

In the docs you can look up breakpoint, it has a lot of features amongst other things you can register a custom handler. I use it in selenium tests so I can either debug on error or just print the error message and continue

Could you provide a concrete example? It's still unclear to me why one wants a custom breakpoint() handler.

You can run Python with

    PYTHONBREAKPOINT=print python

and all your breakpoint() calls will be prints.

Which is not super useful, however you can use

    PYTHONBREAKPOINT=icecream.ic

and now you get the advantages of TFA without having to import anything anywhere.

The only annoyance is that the default hook takes no arguments, so you can't trivially switch between the default and custom implementations, you may need to modify the source depending on the breakpoint hook you're using.

This is how we use it in my testing repo:

__init__.py

    os.environ.setdefault('PYTHONBREAKPOINT', 'tests.utils.raise_or_debug')

tests.utils.raise_or_debug

    def raise_or_debug(msg=''):
        if settings.BREAKPOINT_ON_ERROR:
            extype, value, tb = sys.exc_info()
            if getattr(sys, 'last_traceback', None):
                pdb.pm()
            elif tb:
                pdb.post_mortem(tb)
            else:
                pdb.set_trace()
        elif msg:
            raise AssertionError(msg)
        else:
            pdb.set_trace()

The reason is so we can leave breakpoints in the tests, and then depending on context run the tests in a context where debug is possible, or simply raise the failure again.

Selenium can be very flaky, so this has really sped up iteration and improvements. Standard functions with retries, and smart assertions and timeouts can then be fixed and resumed in dev - and give a good message on fail (for example headless chrome runner on CI).

And from our own docs:

    We have manually overwritten the breakpoint handler to a custom handler that in default mode does this:
    
      - `breakpoint()` calls trigger `pdb.set_trace()` as per usual
      - `breakpoint(<str>)` calls trigger AssertionError to fail tests
    
    When `BREAKPOINT_ON_ERROR=TRUE` is turned on:
    
      - `breakpoint()` calls trigger `pdb.set_trace()` as per usual
      - `breakpoint(<str>)` call `pdb.set_trace()` so you can try to manually work out how the code is functioning
      - if you call `breakpoint(<str>)` from an `except` block it will open the debugger at the position of the exception

Usage example:

        try:
            WebDriverWait(self.driver, 10).until(
                EC.element_to_be_clickable((By.ID, 'desktop-apply')))
        except TimeoutException:
            breakpoint('Was not able to click apply button.')

What is TFA?

icecream, the subject of this post, returns its parameter unmodified without multiple evaluations.

> What is TFA?

The fine article

> icecream, the subject of this post, returns its parameter unmodified without multiple evaluations.

That would be my point, yes.

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