Find local variables in the traceback for an exception#

For sqlite-utils issue #309 I had an error that looked like this:

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OverflowError: Python int too large to convert to SQLite INTEGER
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Traceback (most recent call last):
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  File "/home/sean/.local/bin/sqlite-utils", line 8, in <module>
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    sys.exit(cli())
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  [...]
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  File "/home/sean/.local/lib/python3.8/site-packages/sqlite_utils/db.py", line 257, in execute
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    return self.conn.execute(sql, parameters)

And I wanted to display the values of the sql and parameters variables as part of a custom error message.

It turns out Python exceptions have a e.__traceback__ property which provides access to a traceback - and the local variables in that traceback are avialable on the tb.tb_frame.f_locals dictionary - or if not that one, the tb.tb_next.tb_frame.f_locals property and so on up to the top of the stack.

I wrote this function to find them:

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def _find_variables(tb, vars):
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    to_find = list(vars)
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    found = {}
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    for var in to_find:
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        if var in tb.tb_frame.f_locals:
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            vars.remove(var)
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            found[var] = tb.tb_frame.f_locals[var]
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    if vars and tb.tb_next:
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        found.update(_find_variables(tb.tb_next, vars))
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    return found

Use it like this:

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    try:
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        db[table].insert_all(
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            docs, pk=pk, batch_size=batch_size, alter=alter, **extra_kwargs
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        )
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    except Exception as e:
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        # If we can find sql= and params= arguments, show those
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        variables = _find_variables(e.__traceback__, ["sql", "params"])
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        if "sql" in variables and "params" in variables:
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            raise click.ClickException(
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                "{}\n\nsql = {}\nparams={}".format(
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                    str(e), variables["sql"], variables["params"]
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                )
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            )
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        else:
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            raise

Full commit add this to sqlite-utils.