New submission from Royce Mitchell <roy...@gmail.com>:
Dear devs,
I have a small change request to make to a built-in Python file.
I'm currently running python 3.9.5
The file is copy.py
I would like to propose changing line 264 (in _reconstruct) from this:
y = func(*args)
to something like this:
try:
y = func(*args)
except TypeError as e:
raise TypeError(
f'calling {func.__module__}.{func.__qualname__}: {e.args[0]}',
*e.args[1:]
).with_traceback(e.__traceback__) from None
All the change does it inject the module and qualified-name of the function
trying to be
created onto the front-end of the error.
It makes this:
TypeError: __init__() missing 1 required positional argument: 'delta'
into this:
TypeError: calling datetime.datetime: calling mytz.Tzoffset: __init__() missing
1 required positional argument: 'delta'
Here's a summary of the situation that led to this difficulty:
I have a project that is a couple years old and I'm no longer intimately aware
of every single
thing the program is doing. I went to make some enhancements and noticed the
unit tests
hadn't been touched since early in the project and decided I wanted to start
using it.
I got stuck trying to prettyprint an object and getting a TypeError from the
line above
because it was trying to call a function but was missing a required argument.
The traceback was unhelpful because I didn't know what object it was trying to
copy,
which was very complicated with lots of data and sub-objects.
It turns out that a dataclass (named TransDetail) I was trying to prettyprint
had a list of another dataclass (named Billing) which had a datetime.datetime
object
with a custom tzinfo object that I had never tried to deepcopy before.
(The custom tzinfo object was adapted from examples on StackOverflow)
Trying to google the issue, I found many others experiencing the same problem.
The fix was to define a default datetime.timedelta value for that custom tzinfo
object,
but I had to make the changes to copy.py in order to efficiently figure out
that this
was the problem.
----------
components: Library (Lib)
messages: 413594
nosy: remdragon
priority: normal
severity: normal
status: open
title: copy.deepcopy can fail with unhelpful diagnostics
type: enhancement
versions: Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46809>
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